# PL/pgSQL — SQL Procedural Language

## Overview

PL/pgSQL is a loadable procedural language for the PostgreSQL database system. The design goals of PL/pgSQL were to create a loadable procedural language that

- can be used to create functions, procedures, and triggers,

- adds control structures to the SQL language,

- can perform complex computations,

- inherits all user-defined types, functions, procedures, and operators,

- can be defined to be trusted by the server,

- is easy to use.

Functions created with PL/pgSQL can be used anywhere that built-in functions could be used. For example, it is possible to create complex conditional computation functions and later use them to define operators or use them in index expressions.

In PostgreSQL 9.0 and later, PL/pgSQL is installed by default. However it is still a loadable module, so especially security-conscious administrators could choose to remove it.

### Advantages of Using PL/pgSQL

SQL is the language PostgreSQL and most other relational databases use as query language. It's portable and easy to learn. But every SQL statement must be executed individually by the database server.

That means that your client application must send each query to the database server, wait for it to be processed, receive and process the results, do some computation, then send further queries to the server. All this incurs interprocess communication and will also incur network overhead if your client is on a different machine than the database server.

With PL/pgSQL you can group a block of computation and a series of queries inside the database server, thus having the power of a procedural language and the ease of use of SQL, but with considerable savings of client/server communication overhead.

- Extra round trips between client and server are eliminated

- Intermediate results that the client does not need do not have to be marshaled or transferred between server and client

- Multiple rounds of query parsing can be avoided

This can result in a considerable performance increase as compared to an application that does not use stored functions.

Also, with PL/pgSQL you can use all the data types, operators and functions of SQL.

### Supported Argument and Result Data Types

Functions written in PL/pgSQL can accept as arguments any scalar or array data type supported by the server, and they can return a result of any of these types. They can also accept or return any composite type (row type) specified by name. It is also possible to declare a PL/pgSQL function as accepting `record`, which means that any composite type will do as input, or as returning `record`, which means that the result is a row type whose columns are determined by specification in the calling query, as discussed in queries-tablefunctions.

PL/pgSQL functions can be declared to accept a variable number of arguments by using the `VARIADIC` marker. This works exactly the same way as for SQL functions, as discussed in xfunc-sql-variadic-functions.

PL/pgSQL functions can also be declared to accept and return the polymorphic types described in extend-types-polymorphic, thus allowing the actual data types handled by the function to vary from call to call. Examples appear in plpgsql-declaration-parameters.

PL/pgSQL functions can also be declared to return a set (or table) of any data type that can be returned as a single instance. Such a function generates its output by executing `RETURN NEXT` for each desired element of the result set, or by using `RETURN QUERY` to output the result of evaluating a query.

Finally, a PL/pgSQL function can be declared to return `void` if it has no useful return value. (Alternatively, it could be written as a procedure in that case.)

PL/pgSQL functions can also be declared with output parameters in place of an explicit specification of the return type. This does not add any fundamental capability to the language, but it is often convenient, especially for returning multiple values. The `RETURNS TABLE` notation can also be used in place of `RETURNS SETOF`.

Specific examples appear in plpgsql-declaration-parameters and plpgsql-statements-returning.

## Structure of PL/pgSQL

Functions written in PL/pgSQL are defined to the server by executing sql-createfunction commands. Such a command would normally look like, say,

    CREATE FUNCTION somefunc(integer, text) RETURNS integer
    AS 'function body text'
    LANGUAGE plpgsql;

The function body is simply a string literal so far as `CREATE FUNCTION` is concerned. It is often helpful to use dollar quoting (see sql-syntax-dollar-quoting) to write the function body, rather than the normal single quote syntax. Without dollar quoting, any single quotes or backslashes in the function body must be escaped by doubling them. Almost all the examples in this chapter use dollar-quoted literals for their function bodies.

PL/pgSQL is a block-structured language. The complete text of a function body must be a block. A block is defined as:

     <<label>>
     DECLARE
        declarations
    BEGIN
        statements
    END  label ;

Each declaration and each statement within a block is terminated by a semicolon. A block that appears within another block must have a semicolon after `END`, as shown above; however the final `END` that concludes a function body does not require a semicolon.

A common mistake is to write a semicolon immediately after `BEGIN`. This is incorrect and will result in a syntax error.

A `label` is only needed if you want to identify the block for use in an `EXIT` statement, or to qualify the names of the variables declared in the block. If a label is given after `END`, it must match the label at the block's beginning.

All key words are case-insensitive. Identifiers are implicitly converted to lower case unless double-quoted, just as they are in ordinary SQL commands.

Comments work the same way in PL/pgSQL code as in ordinary SQL. A double dash (`--`) starts a comment that extends to the end of the line. A `/*` starts a block comment that extends to the matching occurrence of `*/`. Block comments nest.

Any statement in the statement section of a block can be a subblock. Subblocks can be used for logical grouping or to localize variables to a small group of statements. Variables declared in a subblock mask any similarly-named variables of outer blocks for the duration of the subblock; but you can access the outer variables anyway if you qualify their names with their block's label. For example:

    CREATE FUNCTION somefunc() RETURNS integer AS $$
    << outerblock >>
    DECLARE
        quantity integer := 30;
    BEGIN
        RAISE NOTICE 'Quantity here is %', quantity;  -- Prints 30
        quantity := 50;
        --
        -- Create a subblock
        --
        DECLARE
            quantity integer := 80;
        BEGIN
            RAISE NOTICE 'Quantity here is %', quantity;  -- Prints 80
            RAISE NOTICE 'Outer quantity here is %', outerblock.quantity;  -- Prints 50
        END;

        RAISE NOTICE 'Quantity here is %', quantity;  -- Prints 50

        RETURN quantity;
    END;
    $$ LANGUAGE plpgsql;

There is actually a hidden outer block surrounding the body of any PL/pgSQL function. This block provides the declarations of the function's parameters (if any), as well as some special variables such as `FOUND` (see plpgsql-statements-diagnostics). The outer block is labeled with the function's name, meaning that parameters and special variables can be qualified with the function's name.

It is important not to confuse the use of `BEGIN`/`END` for grouping statements in PL/pgSQL with the similarly-named SQL commands for transaction control. PL/pgSQL's `BEGIN`/`END` are only for grouping; they do not start or end a transaction. See plpgsql-transactions for information on managing transactions in PL/pgSQL. Also, a block containing an `EXCEPTION` clause effectively forms a subtransaction that can be rolled back without affecting the outer transaction. For more about that see plpgsql-error-trapping.

## Declarations

All variables used in a block must be declared in the declarations section of the block. (The only exceptions are that the loop variable of a `FOR` loop iterating over a range of integer values is automatically declared as an integer variable, and likewise the loop variable of a `FOR` loop iterating over a cursor's result is automatically declared as a record variable.)

PL/pgSQL variables can have any SQL data type, such as `integer`, `varchar`, and `char`.

Here are some examples of variable declarations:

    user_id integer;
    quantity numeric(5);
    url varchar;
    myrow tablename%ROWTYPE;
    myfield tablename.columnname%TYPE;
    arow RECORD;

The general syntax of a variable declaration is:

    name  CONSTANT  type  COLLATE collation_name   NOT NULL   { DEFAULT | := | = } expression ;

The `DEFAULT` clause, if given, specifies the initial value assigned to the variable when the block is entered. If the `DEFAULT` clause is not given then the variable is initialized to the SQL null value. The `CONSTANT` option prevents the variable from being assigned to after initialization, so that its value will remain constant for the duration of the block. The `COLLATE` option specifies a collation to use for the variable (see plpgsql-declaration-collation). If `NOT NULL` is specified, an assignment of a null value results in a run-time error. All variables declared as `NOT NULL` must have a nonnull default value specified. Equal (`=`) can be used instead of PL/SQL-compliant `:=`.

A variable's default value is evaluated and assigned to the variable each time the block is entered (not just once per function call). So, for example, assigning `now()` to a variable of type `timestamp` causes the variable to have the time of the current function call, not the time when the function was precompiled.

Examples:

    quantity integer DEFAULT 32;
    url varchar := 'http://mysite.com';
    transaction_time CONSTANT timestamp with time zone := now();

Once declared, a variable's value can be used in later initialization expressions in the same block, for example:

    DECLARE
      x integer := 1;
      y integer := x + 1;

### Declaring Function Parameters

Parameters passed to functions are named with the identifiers `$1`, `$2`, etc. Optionally, aliases can be declared for `$`n`` parameter names for increased readability. Either the alias or the numeric identifier can then be used to refer to the parameter value.

There are two ways to create an alias. The preferred way is to give a name to the parameter in the `CREATE FUNCTION` command, for example:

    CREATE FUNCTION sales_tax(subtotal real) RETURNS real AS $$
    BEGIN
        RETURN subtotal * 0.06;
    END;
    $$ LANGUAGE plpgsql;

The other way is to explicitly declare an alias, using the declaration syntax

    name ALIAS FOR $n;

The same example in this style looks like:

    CREATE FUNCTION sales_tax(real) RETURNS real AS $$
    DECLARE
        subtotal ALIAS FOR $1;
    BEGIN
        RETURN subtotal * 0.06;
    END;
    $$ LANGUAGE plpgsql;

These two examples are not perfectly equivalent. In the first case, `subtotal` could be referenced as `sales_tax.subtotal`, but in the second case it could not. (Had we attached a label to the inner block, `subtotal` could be qualified with that label, instead.)

Some more examples:

    CREATE FUNCTION instr(varchar, integer) RETURNS integer AS $$
    DECLARE
        v_string ALIAS FOR $1;
        index ALIAS FOR $2;
    BEGIN
        -- some computations using v_string and index here
    END;
    $$ LANGUAGE plpgsql;

    CREATE FUNCTION concat_selected_fields(in_t sometablename) RETURNS text AS $$
    BEGIN
        RETURN in_t.f1 || in_t.f3 || in_t.f5 || in_t.f7;
    END;
    $$ LANGUAGE plpgsql;

When a PL/pgSQL function is declared with output parameters, the output parameters are given `$`n`` names and optional aliases in just the same way as the normal input parameters. An output parameter is effectively a variable that starts out NULL; it should be assigned to during the execution of the function. The final value of the parameter is what is returned. For instance, the sales-tax example could also be done this way:

    CREATE FUNCTION sales_tax(subtotal real, OUT tax real) AS $$
    BEGIN
        tax := subtotal * 0.06;
    END;
    $$ LANGUAGE plpgsql;

Notice that we omitted `RETURNS real` — we could have included it, but it would be redundant.

To call a function with `OUT` parameters, omit the output parameter(s) in the function call:

    SELECT sales_tax(100.00);

Output parameters are most useful when returning multiple values. A trivial example is:

    CREATE FUNCTION sum_n_product(x int, y int, OUT sum int, OUT prod int) AS $$
    BEGIN
        sum := x + y;
        prod := x * y;
    END;
    $$ LANGUAGE plpgsql;

    SELECT * FROM sum_n_product(2, 4);
     sum | prod
    -----+------
       6 |    8

As discussed in xfunc-output-parameters, this effectively creates an anonymous record type for the function's results. If a `RETURNS` clause is given, it must say `RETURNS record`.

This also works with procedures, for example:

    CREATE PROCEDURE sum_n_product(x int, y int, OUT sum int, OUT prod int) AS $$
    BEGIN
        sum := x + y;
        prod := x * y;
    END;
    $$ LANGUAGE plpgsql;

In a call to a procedure, all the parameters must be specified. For output parameters, `NULL` may be specified when calling the procedure from plain SQL:

    CALL sum_n_product(2, 4, NULL, NULL);
     sum | prod
    -----+------
       6 |    8

However, when calling a procedure from PL/pgSQL, you should instead write a variable for any output parameter; the variable will receive the result of the call. See plpgsql-statements-calling-procedure for details.

Another way to declare a PL/pgSQL function is with `RETURNS TABLE`, for example:

    CREATE FUNCTION extended_sales(p_itemno int)
    RETURNS TABLE(quantity int, total numeric) AS $$
    BEGIN
        RETURN QUERY SELECT s.quantity, s.quantity * s.price FROM sales AS s
                     WHERE s.itemno = p_itemno;
    END;
    $$ LANGUAGE plpgsql;

This is exactly equivalent to declaring one or more `OUT` parameters and specifying `RETURNS SETOF `sometype``.

When the return type of a PL/pgSQL function is declared as a polymorphic type (see extend-types-polymorphic), a special parameter `$0` is created. Its data type is the actual return type of the function, as deduced from the actual input types. This allows the function to access its actual return type as shown in plpgsql-declaration-type. `$0` is initialized to null and can be modified by the function, so it can be used to hold the return value if desired, though that is not required. `$0` can also be given an alias. For example, this function works on any data type that has a `+` operator:

    CREATE FUNCTION add_three_values(v1 anyelement, v2 anyelement, v3 anyelement)
    RETURNS anyelement AS $$
    DECLARE
        result ALIAS FOR $0;
    BEGIN
        result := v1 + v2 + v3;
        RETURN result;
    END;
    $$ LANGUAGE plpgsql;

The same effect can be obtained by declaring one or more output parameters as polymorphic types. In this case the special `$0` parameter is not used; the output parameters themselves serve the same purpose. For example:

    CREATE FUNCTION add_three_values(v1 anyelement, v2 anyelement, v3 anyelement,
                                     OUT sum anyelement)
    AS $$
    BEGIN
        sum := v1 + v2 + v3;
    END;
    $$ LANGUAGE plpgsql;

In practice it might be more useful to declare a polymorphic function using the `anycompatible` family of types, so that automatic promotion of the input arguments to a common type will occur. For example:

    CREATE FUNCTION add_three_values(v1 anycompatible, v2 anycompatible, v3 anycompatible)
    RETURNS anycompatible AS $$
    BEGIN
        RETURN v1 + v2 + v3;
    END;
    $$ LANGUAGE plpgsql;

With this example, a call such as

    SELECT add_three_values(1, 2, 4.7);

will work, automatically promoting the integer inputs to numeric. The function using `anyelement` would require you to cast the three inputs to the same type manually.

### `ALIAS`

    newname ALIAS FOR oldname;

The `ALIAS` syntax is more general than is suggested in the previous section: you can declare an alias for any variable, not just function parameters. The main practical use for this is to assign a different name for variables with predetermined names, such as `NEW` or `OLD` within a trigger function.

Examples:

    DECLARE
      prior ALIAS FOR old;
      updated ALIAS FOR new;

Since `ALIAS` creates two different ways to name the same object, unrestricted use can be confusing. It's best to use it only for the purpose of overriding predetermined names.

### Copying Types

    name table.column%TYPE
    name variable%TYPE

`%TYPE` provides the data type of a table column or a previously-declared PL/pgSQL variable. You can use this to declare variables that will hold database values. For example, let's say you have a column named `user_id` in your `users` table. To declare a variable with the same data type as `users.user_id` you write:

    user_id users.user_id%TYPE;

It is also possible to write array decoration after `%TYPE`, thereby creating a variable that holds an array of the referenced type:

    user_ids users.user_id%TYPE[];
    user_ids users.user_id%TYPE ARRAY[4];  -- equivalent to the above

Just as when declaring table columns that are arrays, it doesn't matter whether you write multiple bracket pairs or specific array dimensions: PostgreSQL treats all arrays of a given element type as the same type, regardless of dimensionality. (See arrays-declaration.)

By using `%TYPE` you don't need to know the data type of the structure you are referencing, and most importantly, if the data type of the referenced item changes in the future (for instance: you change the type of `user_id` from `integer` to `real`), you might not need to change your function definition.

`%TYPE` is particularly valuable in polymorphic functions, since the data types needed for internal variables can change from one call to the next. Appropriate variables can be created by applying `%TYPE` to the function's arguments or result placeholders.

### Row Types

    name table_name%ROWTYPE;
    name composite_type_name;

A variable of a composite type is called a row variable (or row-type variable). Such a variable can hold a whole row of a `SELECT` or `FOR` query result, so long as that query's column set matches the declared type of the variable. The individual fields of the row value are accessed using the usual dot notation, for example `rowvar.field`.

A row variable can be declared to have the same type as the rows of an existing table or view, by using the `table_name``%ROWTYPE` notation; or it can be declared by giving a composite type's name. (Since every table has an associated composite type of the same name, it actually does not matter in PostgreSQL whether you write `%ROWTYPE` or not. But the form with `%ROWTYPE` is more portable.)

As with `%TYPE`, `%ROWTYPE` can be followed by array decoration to declare a variable that holds an array of the referenced composite type.

Parameters to a function can be composite types (complete table rows). In that case, the corresponding identifier `$`n`` will be a row variable, and fields can be selected from it, for example `$1.user_id`.

Here is an example of using composite types. table1 and table2 are existing tables having at least the mentioned fields:

    CREATE FUNCTION merge_fields(t_row table1) RETURNS text AS $$
    DECLARE
        t2_row table2%ROWTYPE;
    BEGIN
        SELECT * INTO t2_row FROM table2 WHERE ... ;
        RETURN t_row.f1 || t2_row.f3 || t_row.f5 || t2_row.f7;
    END;
    $$ LANGUAGE plpgsql;

    SELECT merge_fields(t.*) FROM table1 t WHERE ... ;

### Record Types

    name RECORD;

Record variables are similar to row-type variables, but they have no predefined structure. They take on the actual row structure of the row they are assigned during a `SELECT` or `FOR` command. The substructure of a record variable can change each time it is assigned to. A consequence of this is that until a record variable is first assigned to, it has no substructure, and any attempt to access a field in it will draw a run-time error.

Note that `RECORD` is not a true data type, only a placeholder. One should also realize that when a PL/pgSQL function is declared to return type `record`, this is not quite the same concept as a record variable, even though such a function might use a record variable to hold its result. In both cases the actual row structure is unknown when the function is written, but for a function returning `record` the actual structure is determined when the calling query is parsed, whereas a record variable can change its row structure on-the-fly.

### Collation of PL/pgSQL Variables

When a PL/pgSQL function has one or more parameters of collatable data types, a collation is identified for each function call depending on the collations assigned to the actual arguments, as described in collation. If a collation is successfully identified (i.e., there are no conflicts of implicit collations among the arguments) then all the collatable parameters are treated as having that collation implicitly. This will affect the behavior of collation-sensitive operations within the function. For example, consider

    CREATE FUNCTION less_than(a text, b text) RETURNS boolean AS $$
    BEGIN
        RETURN a < b;
    END;
    $$ LANGUAGE plpgsql;

    SELECT less_than(text_field_1, text_field_2) FROM table1;
    SELECT less_than(text_field_1, text_field_2 COLLATE "C") FROM table1;

The first use of `less_than` will use the common collation of text_field_1 and text_field_2 for the comparison, while the second use will use `C` collation.

Furthermore, the identified collation is also assumed as the collation of any local variables that are of collatable types. Thus this function would not work any differently if it were written as

    CREATE FUNCTION less_than(a text, b text) RETURNS boolean AS $$
    DECLARE
        local_a text := a;
        local_b text := b;
    BEGIN
        RETURN local_a < local_b;
    END;
    $$ LANGUAGE plpgsql;

If there are no parameters of collatable data types, or no common collation can be identified for them, then parameters and local variables use the default collation of their data type (which is usually the database's default collation, but could be different for variables of domain types).

A local variable of a collatable data type can have a different collation associated with it by including the `COLLATE` option in its declaration, for example

    DECLARE
        local_a text COLLATE "en_US";

This option overrides the collation that would otherwise be given to the variable according to the rules above.

Also, of course explicit `COLLATE` clauses can be written inside a function if it is desired to force a particular collation to be used in a particular operation. For example,

    CREATE FUNCTION less_than_c(a text, b text) RETURNS boolean AS $$
    BEGIN
        RETURN a < b COLLATE "C";
    END;
    $$ LANGUAGE plpgsql;

This overrides the collations associated with the table columns, parameters, or local variables used in the expression, just as would happen in a plain SQL command.

## Expressions

All expressions used in PL/pgSQL statements are processed using the server's main SQL executor. For example, when you write a PL/pgSQL statement like

    IF expression THEN ...

PL/pgSQL will evaluate the expression by feeding a query like

    SELECT expression

to the main SQL engine. While forming the `SELECT` command, any occurrences of PL/pgSQL variable names are replaced by query parameters, as discussed in detail in plpgsql-var-subst. This allows the query plan for the `SELECT` to be prepared just once and then reused for subsequent evaluations with different values of the variables. Thus, what really happens on first use of an expression is essentially a `PREPARE` command. For example, if we have declared two integer variables `x` and `y`, and we write

    IF x < y THEN ...

what happens behind the scenes is equivalent to

    PREPARE statement_name(integer, integer) AS SELECT $1 < $2;

and then this prepared statement is `EXECUTE`d for each execution of the `IF` statement, with the current values of the PL/pgSQL variables supplied as parameter values. Normally these details are not important to a PL/pgSQL user, but they are useful to know when trying to diagnose a problem. More information appears in plpgsql-plan-caching.

Since an `expression` is converted to a `SELECT` command, it can contain the same clauses that an ordinary `SELECT` would, except that it cannot include a top-level `UNION`, `INTERSECT`, or `EXCEPT` clause. Thus for example one could test whether a table is non-empty with

    IF count(*) > 0 FROM my_table THEN ...

since the `expression` between `IF` and `THEN` is parsed as though it were `SELECT count(*) > 0 FROM my_table`. The `SELECT` must produce a single column, and not more than one row. (If it produces no rows, the result is taken as NULL.)

## Basic Statements

In this section and the following ones, we describe all the statement types that are explicitly understood by PL/pgSQL. Anything not recognized as one of these statement types is presumed to be an SQL command and is sent to the main database engine to execute, as described in plpgsql-statements-general-sql.

### Assignment

An assignment of a value to a PL/pgSQL variable is written as:

    variable { := | = } expression;

As explained previously, the expression in such a statement is evaluated by means of an SQL `SELECT` command sent to the main database engine. The expression must yield a single value (possibly a row value, if the variable is a row or record variable). The target variable can be a simple variable (optionally qualified with a block name), a field of a row or record target, or an element or slice of an array target. Equal (`=`) can be used instead of PL/SQL-compliant `:=`.

If the expression's result data type doesn't match the variable's data type, the value will be coerced as though by an assignment cast (see typeconv-query). If no assignment cast is known for the pair of data types involved, the PL/pgSQL interpreter will attempt to convert the result value textually, that is by applying the result type's output function followed by the variable type's input function. Note that this could result in run-time errors generated by the input function, if the string form of the result value is not acceptable to the input function.

Examples:

    tax := subtotal * 0.06;
    my_record.user_id := 20;
    my_array[j] := 20;
    my_array[1:3] := array[1,2,3];
    complex_array[n].realpart = 12.3;

### Executing SQL Commands

In general, any SQL command that does not return rows can be executed within a PL/pgSQL function just by writing the command. For example, you could create and fill a table by writing

    CREATE TABLE mytable (id int primary key, data text);
    INSERT INTO mytable VALUES (1,'one'), (2,'two');

If the command does return rows (for example `SELECT`, or `INSERT`/`UPDATE`/`DELETE`/`MERGE` with `RETURNING`), there are two ways to proceed. When the command will return at most one row, or you only care about the first row of output, write the command as usual but add an `INTO` clause to capture the output, as described in plpgsql-statements-sql-onerow. To process all of the output rows, write the command as the data source for a `FOR` loop, as described in plpgsql-records-iterating.

Usually it is not sufficient just to execute statically-defined SQL commands. Typically you'll want a command to use varying data values, or even to vary in more fundamental ways such as by using different table names at different times. Again, there are two ways to proceed depending on the situation.

PL/pgSQL variable values can be automatically inserted into optimizable SQL commands, which are `SELECT`, `INSERT`, `UPDATE`, `DELETE`, `MERGE`, and certain utility commands that incorporate one of these, such as `EXPLAIN` and `CREATE TABLE ... AS SELECT`. In these commands, any PL/pgSQL variable name appearing in the command text is replaced by a query parameter, and then the current value of the variable is provided as the parameter value at run time. This is exactly like the processing described earlier for expressions; for details see plpgsql-var-subst.

When executing an optimizable SQL command in this way, PL/pgSQL may cache and re-use the execution plan for the command, as discussed in plpgsql-plan-caching.

Non-optimizable SQL commands (also called utility commands) are not capable of accepting query parameters. So automatic substitution of PL/pgSQL variables does not work in such commands. To include non-constant text in a utility command executed from PL/pgSQL, you must build the utility command as a string and then `EXECUTE` it, as discussed in plpgsql-statements-executing-dyn.

`EXECUTE` must also be used if you want to modify the command in some other way than supplying a data value, for example by changing a table name.

Sometimes it is useful to evaluate an expression or `SELECT` query but discard the result, for example when calling a function that has side-effects but no useful result value. To do this in PL/pgSQL, use the `PERFORM` statement:

    PERFORM query;

This executes `query` and discards the result. Write the `query` the same way you would write an SQL `SELECT` command, but replace the initial keyword `SELECT` with `PERFORM`. For `WITH` queries, use `PERFORM` and then place the query in parentheses. (In this case, the query can only return one row.) PL/pgSQL variables will be substituted into the query just as described above, and the plan is cached in the same way. Also, the special variable `FOUND` is set to true if the query produced at least one row, or false if it produced no rows (see plpgsql-statements-diagnostics).

One might expect that writing `SELECT` directly would accomplish this result, but at present the only accepted way to do it is `PERFORM`. An SQL command that can return rows, such as `SELECT`, will be rejected as an error unless it has an `INTO` clause as discussed in the next section.

An example:

    PERFORM create_mv('cs_session_page_requests_mv', my_query);

### Executing a Command with a Single-Row Result

The result of an SQL command yielding a single row (possibly of multiple columns) can be assigned to a record variable, row-type variable, or list of scalar variables. This is done by writing the base SQL command and adding an `INTO` clause. For example,

    SELECT select_expressions INTO STRICT target FROM ...;
    INSERT ... RETURNING expressions INTO STRICT target;
    UPDATE ... RETURNING expressions INTO STRICT target;
    DELETE ... RETURNING expressions INTO STRICT target;
    MERGE ... RETURNING expressions INTO STRICT target;

where `target` can be a record variable, a row variable, or a comma-separated list of simple variables and record/row fields. PL/pgSQL variables will be substituted into the rest of the command (that is, everything but the `INTO` clause) just as described above, and the plan is cached in the same way. This works for `SELECT`, `INSERT`/`UPDATE`/`DELETE`/`MERGE` with `RETURNING`, and certain utility commands that return row sets, such as `EXPLAIN`. Except for the `INTO` clause, the SQL command is the same as it would be written outside PL/pgSQL.

Note that this interpretation of `SELECT` with `INTO` is quite different from PostgreSQL's regular `SELECT INTO` command, wherein the `INTO` target is a newly created table. If you want to create a table from a `SELECT` result inside a PL/pgSQL function, use the syntax `CREATE TABLE ... AS SELECT`.

If a row variable or a variable list is used as target, the command's result columns must exactly match the structure of the target as to number and data types, or else a run-time error occurs. When a record variable is the target, it automatically configures itself to the row type of the command's result columns.

The `INTO` clause can appear almost anywhere in the SQL command. Customarily it is written either just before or just after the list of `select_expressions` in a `SELECT` command, or at the end of the command for other command types. It is recommended that you follow this convention in case the PL/pgSQL parser becomes stricter in future versions.

If `STRICT` is not specified in the `INTO` clause, then `target` will be set to the first row returned by the command, or to nulls if the command returned no rows. (Note that the first row is not well-defined unless you've used `ORDER BY`.) Any result rows after the first row are discarded. You can check the special `FOUND` variable (see plpgsql-statements-diagnostics) to determine whether a row was returned:

    SELECT * INTO myrec FROM emp WHERE empname = myname;
    IF NOT FOUND THEN
        RAISE EXCEPTION 'employee % not found', myname;
    END IF;

If the `STRICT` option is specified, the command must return exactly one row or a run-time error will be reported, either `NO_DATA_FOUND` (no rows) or `TOO_MANY_ROWS` (more than one row). You can use an exception block if you wish to catch the error, for example:

    BEGIN
        SELECT * INTO STRICT myrec FROM emp WHERE empname = myname;
        EXCEPTION
            WHEN NO_DATA_FOUND THEN
                RAISE EXCEPTION 'employee % not found', myname;
            WHEN TOO_MANY_ROWS THEN
                RAISE EXCEPTION 'employee % not unique', myname;
    END;

Successful execution of a command with `STRICT` always sets `FOUND` to true.

For `INSERT`/`UPDATE`/`DELETE`/`MERGE` with `RETURNING`, PL/pgSQL reports an error for more than one returned row, even when `STRICT` is not specified. This is because there is no option such as `ORDER BY` with which to determine which affected row should be returned.

If `print_strict_params` is enabled for the function, then when an error is thrown because the requirements of `STRICT` are not met, the `DETAIL` part of the error message will include information about the parameters passed to the command. You can change the `print_strict_params` setting for all functions by setting `plpgsql.print_strict_params`, though only subsequent function compilations will be affected. You can also enable it on a per-function basis by using a compiler option, for example:

    CREATE FUNCTION get_userid(username text) RETURNS int
    AS $$
    #print_strict_params on
    DECLARE
    userid int;
    BEGIN
        SELECT users.userid INTO STRICT userid
            FROM users WHERE users.username = get_userid.username;
        RETURN userid;
    END;
    $$ LANGUAGE plpgsql;

On failure, this function might produce an error message such as

    ERROR:  query returned no rows
    DETAIL:  parameters: username = 'nosuchuser'
    CONTEXT:  PL/pgSQL function get_userid(text) line 6 at SQL statement

The `STRICT` option matches the behavior of Oracle PL/SQL's `SELECT INTO` and related statements.

### Executing Dynamic Commands

Oftentimes you will want to generate dynamic commands inside your PL/pgSQL functions, that is, commands that will involve different tables or different data types each time they are executed. PL/pgSQL's normal attempts to cache plans for commands (as discussed in plpgsql-plan-caching) will not work in such scenarios. To handle this sort of problem, the `EXECUTE` statement is provided:

    EXECUTE command-string  INTO STRICT target   USING expression , ...  ;

where `command-string` is an expression yielding a string (of type `text`) containing the command to be executed. The optional `target` is a record variable, a row variable, or a comma-separated list of simple variables and record/row fields, into which the results of the command will be stored. The optional `USING` expressions supply values to be inserted into the command.

No substitution of PL/pgSQL variables is done on the computed command string. Any required variable values must be inserted in the command string as it is constructed; or you can use parameters as described below.

Also, there is no plan caching for commands executed via `EXECUTE`. Instead, the command is always planned each time the statement is run. Thus the command string can be dynamically created within the function to perform actions on different tables and columns.

The `INTO` clause specifies where the results of an SQL command returning rows should be assigned. If a row variable or variable list is provided, it must exactly match the structure of the command's results; if a record variable is provided, it will configure itself to match the result structure automatically. If multiple rows are returned, only the first will be assigned to the `INTO` variable(s). If no rows are returned, NULL is assigned to the `INTO` variable(s). If no `INTO` clause is specified, the command results are discarded.

If the `STRICT` option is given, an error is reported unless the command produces exactly one row.

The command string can use parameter values, which are referenced in the command as `$1`, `$2`, etc. These symbols refer to values supplied in the `USING` clause. This method is often preferable to inserting data values into the command string as text: it avoids run-time overhead of converting the values to text and back, and it is much less prone to SQL-injection attacks since there is no need for quoting or escaping. An example is:

    EXECUTE 'SELECT count(*) FROM mytable WHERE inserted_by = $1 AND inserted <= $2'
       INTO c
       USING checked_user, checked_date;

Note that parameter symbols can only be used for data values — if you want to use dynamically determined table or column names, you must insert them into the command string textually. For example, if the preceding query needed to be done against a dynamically selected table, you could do this:

    EXECUTE 'SELECT count(*) FROM '
        || quote_ident(tabname)
        || ' WHERE inserted_by = $1 AND inserted <= $2'
       INTO c
       USING checked_user, checked_date;

A cleaner approach is to use `format()`'s `%I` specification to insert table or column names with automatic quoting:

    EXECUTE format('SELECT count(*) FROM %I '
       'WHERE inserted_by = $1 AND inserted <= $2', tabname)
       INTO c
       USING checked_user, checked_date;

(This example relies on the SQL rule that string literals separated by a newline are implicitly concatenated.)

Another restriction on parameter symbols is that they only work in optimizable SQL commands (`SELECT`, `INSERT`, `UPDATE`, `DELETE`, `MERGE`, and certain commands containing one of these). In other statement types (generically called utility statements), you must insert values textually even if they are just data values.

An `EXECUTE` with a simple constant command string and some `USING` parameters, as in the first example above, is functionally equivalent to just writing the command directly in PL/pgSQL and allowing replacement of PL/pgSQL variables to happen automatically. The important difference is that `EXECUTE` will re-plan the command on each execution, generating a plan that is specific to the current parameter values; whereas PL/pgSQL may otherwise create a generic plan and cache it for re-use. In situations where the best plan depends strongly on the parameter values, it can be helpful to use `EXECUTE` to positively ensure that a generic plan is not selected.

`SELECT INTO` is not currently supported within `EXECUTE`; instead, execute a plain `SELECT` command and specify `INTO` as part of the `EXECUTE` itself.

The PL/pgSQL `EXECUTE` statement is not related to the `EXECUTE` SQL statement supported by the PostgreSQL server. The server's `EXECUTE` statement cannot be used directly within PL/pgSQL functions (and is not needed).

#### Quoting Values in Dynamic Queries

When working with dynamic commands you will often have to handle escaping of single quotes. The recommended method for quoting fixed text in your function body is dollar quoting. (If you have legacy code that does not use dollar quoting, please refer to the overview in plpgsql-quote-tips, which can save you some effort when translating said code to a more reasonable scheme.)

Dynamic values require careful handling since they might contain quote characters. An example using `format()` (this assumes that you are dollar quoting the function body so quote marks need not be doubled):

    EXECUTE format('UPDATE tbl SET %I = $1 '
       'WHERE key = $2', colname) USING newvalue, keyvalue;

It is also possible to call the quoting functions directly:

    EXECUTE 'UPDATE tbl SET '
            || quote_ident(colname)
            || ' = '
            || quote_literal(newvalue)
            || ' WHERE key = '
            || quote_literal(keyvalue);

This example demonstrates the use of the `quote_ident` and `quote_literal` functions (see functions-string). For safety, expressions containing column or table identifiers should be passed through `quote_ident` before insertion in a dynamic query. Expressions containing values that should be literal strings in the constructed command should be passed through `quote_literal`. These functions take the appropriate steps to return the input text enclosed in double or single quotes respectively, with any embedded special characters properly escaped.

Because `quote_literal` is labeled `STRICT`, it will always return null when called with a null argument. In the above example, if `newvalue` or `keyvalue` were null, the entire dynamic query string would become null, leading to an error from `EXECUTE`. You can avoid this problem by using the `quote_nullable` function, which works the same as `quote_literal` except that when called with a null argument it returns the string `NULL`. For example,

    EXECUTE 'UPDATE tbl SET '
            || quote_ident(colname)
            || ' = '
            || quote_nullable(newvalue)
            || ' WHERE key = '
            || quote_nullable(keyvalue);

If you are dealing with values that might be null, you should usually use `quote_nullable` in place of `quote_literal`.

As always, care must be taken to ensure that null values in a query do not deliver unintended results. For example the `WHERE` clause

    'WHERE key = ' || quote_nullable(keyvalue)

will never succeed if `keyvalue` is null, because the result of using the equality operator `=` with a null operand is always null. If you wish null to work like an ordinary key value, you would need to rewrite the above as

    'WHERE key IS NOT DISTINCT FROM ' || quote_nullable(keyvalue)

(At present, `IS NOT DISTINCT FROM` is handled much less efficiently than `=`, so don't do this unless you must. See functions-comparison for more information on nulls and `IS DISTINCT`.)

Note that dollar quoting is only useful for quoting fixed text. It would be a very bad idea to try to write this example as:

    EXECUTE 'UPDATE tbl SET '
            || quote_ident(colname)
            || ' = $$'
            || newvalue
            || '$$ WHERE key = '
            || quote_literal(keyvalue);

because it would break if the contents of `newvalue` happened to contain `$$`. The same objection would apply to any other dollar-quoting delimiter you might pick. So, to safely quote text that is not known in advance, you must use `quote_literal`, `quote_nullable`, or `quote_ident`, as appropriate.

Dynamic SQL statements can also be safely constructed using the `format` function (see functions-string-format). For example:

    EXECUTE format('UPDATE tbl SET %I = %L '
       'WHERE key = %L', colname, newvalue, keyvalue);

`%I` is equivalent to `quote_ident`, and `%L` is equivalent to `quote_nullable`. The `format` function can be used in conjunction with the `USING` clause:

    EXECUTE format('UPDATE tbl SET %I = $1 WHERE key = $2', colname)
       USING newvalue, keyvalue;

This form is better because the variables are handled in their native data type format, rather than unconditionally converting them to text and quoting them via `%L`. It is also more efficient.

A much larger example of a dynamic command and `EXECUTE` can be seen in plpgsql-porting-ex2, which builds and executes a `CREATE FUNCTION` command to define a new function.

### Obtaining the Result Status

There are several ways to determine the effect of a command. The first method is to use the `GET DIAGNOSTICS` command, which has the form:

    GET  CURRENT  DIAGNOSTICS variable { = | := } item  , ... ;

This command allows retrieval of system status indicators. `CURRENT` is a noise word (but see also `GET STACKED DIAGNOSTICS` in plpgsql-exception-diagnostics). Each `item` is a key word identifying a status value to be assigned to the specified `variable` (which should be of the right data type to receive it). The currently available status items are shown in plpgsql-current-diagnostics-values. Colon-equal (`:=`) can be used instead of the SQL-standard `=` token. An example:

    GET DIAGNOSTICS integer_var = ROW_COUNT;

### Available Diagnostics Items

Name | Type | Description
`ROW_COUNT` | `bigint` | the number of rows processed by the most recent SQL command
`PG_CONTEXT` | `text` | line(s) of text describing the current call stack (see plpgsql-call-stack)
`PG_ROUTINE_OID` | `oid` | OID of the current function

The second method to determine the effects of a command is to check the special variable named `FOUND`, which is of type `boolean`. `FOUND` starts out false within each PL/pgSQL function call. It is set by each of the following types of statements:

- A `SELECT INTO` statement sets `FOUND` true if a row is assigned, false if no row is returned.

- A `PERFORM` statement sets `FOUND` true if it produces (and discards) one or more rows, false if no row is produced.

- `UPDATE`, `INSERT`, `DELETE`, and `MERGE` statements set `FOUND` true if at least one row is affected, false if no row is affected.

- A `FETCH` statement sets `FOUND` true if it returns a row, false if no row is returned.

- A `MOVE` statement sets `FOUND` true if it successfully repositions the cursor, false otherwise.

- A `FOR` or `FOREACH` statement sets `FOUND` true if it iterates one or more times, else false. `FOUND` is set this way when the loop exits; inside the execution of the loop, `FOUND` is not modified by the loop statement, although it might be changed by the execution of other statements within the loop body.

- `RETURN QUERY` and `RETURN QUERY EXECUTE` statements set `FOUND` true if the query returns at least one row, false if no row is returned.

Other PL/pgSQL statements do not change the state of `FOUND`. Note in particular that `EXECUTE` changes the output of `GET DIAGNOSTICS`, but does not change `FOUND`.

`FOUND` is a local variable within each PL/pgSQL function; any changes to it affect only the current function.

### Doing Nothing At All

Sometimes a placeholder statement that does nothing is useful. For example, it can indicate that one arm of an if/then/else chain is deliberately empty. For this purpose, use the `NULL` statement:

    NULL;

For example, the following two fragments of code are equivalent:

    BEGIN
        y := x / 0;
    EXCEPTION
        WHEN division_by_zero THEN
            NULL;  -- ignore the error
    END;

    BEGIN
        y := x / 0;
    EXCEPTION
        WHEN division_by_zero THEN  -- ignore the error
    END;

Which is preferable is a matter of taste.

In Oracle's PL/SQL, empty statement lists are not allowed, and so `NULL` statements are required for situations such as this. PL/pgSQL allows you to just write nothing, instead.

## Control Structures

Control structures are probably the most useful (and important) part of PL/pgSQL. With PL/pgSQL's control structures, you can manipulate PostgreSQL data in a very flexible and powerful way.

### Returning from a Function

There are two commands available that allow you to return data from a function: `RETURN` and `RETURN NEXT`.

#### `RETURN`

    RETURN expression;

`RETURN` with an expression terminates the function and returns the value of `expression` to the caller. This form is used for PL/pgSQL functions that do not return a set.

In a function that returns a scalar type, the expression's result will automatically be cast into the function's return type as described for assignments. But to return a composite (row) value, you must write an expression delivering exactly the requested column set. This may require use of explicit casting.

If you declared the function with output parameters, write just `RETURN` with no expression. The current values of the output parameter variables will be returned.

If you declared the function to return `void`, a `RETURN` statement can be used to exit the function early; but do not write an expression following `RETURN`.

The return value of a function cannot be left undefined. If control reaches the end of the top-level block of the function without hitting a `RETURN` statement, a run-time error will occur. This restriction does not apply to functions with output parameters and functions returning `void`, however. In those cases a `RETURN` statement is automatically executed if the top-level block finishes.

Some examples:

    -- functions returning a scalar type
    RETURN 1 + 2;
    RETURN scalar_var;

    -- functions returning a composite type
    RETURN composite_type_var;
    RETURN (1, 2, 'three'::text);  -- must cast columns to correct types

#### `RETURN NEXT` and `RETURN QUERY`

    RETURN NEXT expression;
    RETURN QUERY query;
    RETURN QUERY EXECUTE command-string  USING expression , ...  ;

When a PL/pgSQL function is declared to return `SETOF `sometype``, the procedure to follow is slightly different. In that case, the individual items to return are specified by a sequence of `RETURN NEXT` or `RETURN QUERY` commands, and then a final `RETURN` command with no argument is used to indicate that the function has finished executing. `RETURN NEXT` can be used with both scalar and composite data types; with a composite result type, an entire table of results will be returned. `RETURN QUERY` appends the results of executing a query to the function's result set. `RETURN NEXT` and `RETURN QUERY` can be freely intermixed in a single set-returning function, in which case their results will be concatenated.

`RETURN NEXT` and `RETURN QUERY` do not actually return from the function — they simply append zero or more rows to the function's result set. Execution then continues with the next statement in the PL/pgSQL function. As successive `RETURN NEXT` or `RETURN QUERY` commands are executed, the result set is built up. A final `RETURN`, which should have no argument, causes control to exit the function (or you can just let control reach the end of the function).

`RETURN QUERY` has a variant `RETURN QUERY EXECUTE`, which specifies the query to be executed dynamically. Parameter expressions can be inserted into the computed query string via `USING`, in just the same way as in the `EXECUTE` command.

If you declared the function with output parameters, write just `RETURN NEXT` with no expression. On each execution, the current values of the output parameter variable(s) will be saved for eventual return as a row of the result. Note that you must declare the function as returning `SETOF record` when there are multiple output parameters, or `SETOF `sometype`` when there is just one output parameter of type `sometype`, in order to create a set-returning function with output parameters.

Here is an example of a function using `RETURN NEXT`:

    CREATE TABLE foo (fooid INT, foosubid INT, fooname TEXT);
    INSERT INTO foo VALUES (1, 2, 'three');
    INSERT INTO foo VALUES (4, 5, 'six');

    CREATE OR REPLACE FUNCTION get_all_foo() RETURNS SETOF foo AS
    $BODY$
    DECLARE
        r foo%rowtype;
    BEGIN
        FOR r IN
            SELECT * FROM foo WHERE fooid > 0
        LOOP
            -- can do some processing here
            RETURN NEXT r; -- return current row of SELECT
        END LOOP;
        RETURN;
    END;
    $BODY$
    LANGUAGE plpgsql;

    SELECT * FROM get_all_foo();

Here is an example of a function using `RETURN QUERY`:

    CREATE FUNCTION get_available_flightid(date) RETURNS SETOF integer AS
    $BODY$
    BEGIN
        RETURN QUERY SELECT flightid
                       FROM flight
                      WHERE flightdate >= $1
                        AND flightdate < ($1 + 1);

        -- Since execution is not finished, we can check whether rows were returned
        -- and raise exception if not.
        IF NOT FOUND THEN
            RAISE EXCEPTION 'No flight at %.', $1;
        END IF;

        RETURN;
     END;
    $BODY$
    LANGUAGE plpgsql;

    -- Returns available flights or raises exception if there are no
    -- available flights.
    SELECT * FROM get_available_flightid(CURRENT_DATE);

The current implementation of `RETURN NEXT` and `RETURN QUERY` stores the entire result set before returning from the function, as discussed above. That means that if a PL/pgSQL function produces a very large result set, performance might be poor: data will be written to disk to avoid memory exhaustion, but the function itself will not return until the entire result set has been generated. A future version of PL/pgSQL might allow users to define set-returning functions that do not have this limitation. Currently, the point at which data begins being written to disk is controlled by the guc-work-mem configuration variable. Administrators who have sufficient memory to store larger result sets in memory should consider increasing this parameter.

### Returning from a Procedure

A procedure does not have a return value. A procedure can therefore end without a `RETURN` statement. If you wish to use a `RETURN` statement to exit the code early, write just `RETURN` with no expression.

If the procedure has output parameters, the final values of the output parameter variables will be returned to the caller.

### Calling a Procedure

A PL/pgSQL function, procedure, or `DO` block can call a procedure using `CALL`. Output parameters are handled differently from the way that `CALL` works in plain SQL. Each `OUT` or `INOUT` parameter of the procedure must correspond to a variable in the `CALL` statement, and whatever the procedure returns is assigned back to that variable after it returns. For example:

    CREATE PROCEDURE triple(INOUT x int)
    LANGUAGE plpgsql
    AS $$
    BEGIN
        x := x * 3;
    END;
    $$;

    DO $$
    DECLARE myvar int := 5;
    BEGIN
      CALL triple(myvar);
      RAISE NOTICE 'myvar = %', myvar;  -- prints 15
    END;
    $$;

The variable corresponding to an output parameter can be a simple variable or a field of a composite-type variable. Currently, it cannot be an element of an array.

### Conditionals

`IF` and `CASE` statements let you execute alternative commands based on certain conditions. PL/pgSQL has three forms of `IF`:

- `IF ... THEN ... END IF`

- `IF ... THEN ... ELSE ... END IF`

- `IF ... THEN ... ELSIF ... THEN ... ELSE ... END IF`

and two forms of `CASE`:

- `CASE ... WHEN ... THEN ... ELSE ... END CASE`

- `CASE WHEN ... THEN ... ELSE ... END CASE`

#### `IF-THEN`

    IF boolean-expression THEN
        statements
    END IF;

`IF-THEN` statements are the simplest form of `IF`. The statements between `THEN` and `END IF` will be executed if the condition is true. Otherwise, they are skipped.

Example:

    IF v_user_id <> 0 THEN
        UPDATE users SET email = v_email WHERE user_id = v_user_id;
    END IF;

#### `IF-THEN-ELSE`

    IF boolean-expression THEN
        statements
    ELSE
        statements
    END IF;

`IF-THEN-ELSE` statements add to `IF-THEN` by letting you specify an alternative set of statements that should be executed if the condition is not true. (Note this includes the case where the condition evaluates to NULL.)

Examples:

    IF parentid IS NULL OR parentid = ''
    THEN
        RETURN fullname;
    ELSE
        RETURN hp_true_filename(parentid) || '/' || fullname;
    END IF;

    IF v_count > 0 THEN
        INSERT INTO users_count (count) VALUES (v_count);
        RETURN 't';
    ELSE
        RETURN 'f';
    END IF;

#### `IF-THEN-ELSIF`

    IF boolean-expression THEN
        statements
     ELSIF boolean-expression THEN
        statements
     ELSIF boolean-expression THEN
        statements
        ...

     ELSE
        statements
    END IF;

Sometimes there are more than just two alternatives. `IF-THEN-ELSIF` provides a convenient method of checking several alternatives in turn. The `IF` conditions are tested successively until the first one that is true is found. Then the associated statement(s) are executed, after which control passes to the next statement after `END IF`. (Any subsequent `IF` conditions are not tested.) If none of the `IF` conditions is true, then the `ELSE` block (if any) is executed.

Here is an example:

    IF number = 0 THEN
        result := 'zero';
    ELSIF number > 0 THEN
        result := 'positive';
    ELSIF number < 0 THEN
        result := 'negative';
    ELSE
        -- hmm, the only other possibility is that number is null
        result := 'NULL';
    END IF;

The key word `ELSIF` can also be spelled `ELSEIF`.

An alternative way of accomplishing the same task is to nest `IF-THEN-ELSE` statements, as in the following example:

    IF demo_row.sex = 'm' THEN
        pretty_sex := 'man';
    ELSE
        IF demo_row.sex = 'f' THEN
            pretty_sex := 'woman';
        END IF;
    END IF;

However, this method requires writing a matching `END IF` for each `IF`, so it is much more cumbersome than using `ELSIF` when there are many alternatives.

#### Simple `CASE`

    CASE search-expression
        WHEN expression , expression  ...  THEN
          statements
       WHEN expression , expression  ...  THEN
          statements
        ...
       ELSE
          statements
    END CASE;

The simple form of `CASE` provides conditional execution based on equality of operands. The `search-expression` is evaluated (once) and successively compared to each `expression` in the `WHEN` clauses. If a match is found, then the corresponding `statements` are executed, and then control passes to the next statement after `END CASE`. (Subsequent `WHEN` expressions are not evaluated.) If no match is found, the `ELSE` `statements` are executed; but if `ELSE` is not present, then a `CASE_NOT_FOUND` exception is raised.

Here is a simple example:

    CASE x
        WHEN 1, 2 THEN
            msg := 'one or two';
        ELSE
            msg := 'other value than one or two';
    END CASE;

#### Searched `CASE`

    CASE
        WHEN boolean-expression THEN
          statements
       WHEN boolean-expression THEN
          statements
        ...
       ELSE
          statements
    END CASE;

The searched form of `CASE` provides conditional execution based on truth of Boolean expressions. Each `WHEN` clause's `boolean-expression` is evaluated in turn, until one is found that yields `true`. Then the corresponding `statements` are executed, and then control passes to the next statement after `END CASE`. (Subsequent `WHEN` expressions are not evaluated.) If no true result is found, the `ELSE` `statements` are executed; but if `ELSE` is not present, then a `CASE_NOT_FOUND` exception is raised.

Here is an example:

    CASE
        WHEN x BETWEEN 0 AND 10 THEN
            msg := 'value is between zero and ten';
        WHEN x BETWEEN 11 AND 20 THEN
            msg := 'value is between eleven and twenty';
    END CASE;

This form of `CASE` is entirely equivalent to `IF-THEN-ELSIF`, except for the rule that reaching an omitted `ELSE` clause results in an error rather than doing nothing.

### Simple Loops

With the `LOOP`, `EXIT`, `CONTINUE`, `WHILE`, `FOR`, and `FOREACH` statements, you can arrange for your PL/pgSQL function to repeat a series of commands.

#### `LOOP`

     <<label>>
    LOOP
        statements
    END LOOP  label ;

`LOOP` defines an unconditional loop that is repeated indefinitely until terminated by an `EXIT` or `RETURN` statement. The optional `label` can be used by `EXIT` and `CONTINUE` statements within nested loops to specify which loop those statements refer to.

#### `EXIT`

    EXIT  label   WHEN boolean-expression ;

If no `label` is given, the innermost loop is terminated and the statement following `END LOOP` is executed next. If `label` is given, it must be the label of the current or some outer level of nested loop or block. Then the named loop or block is terminated and control continues with the statement after the loop's/block's corresponding `END`.

If `WHEN` is specified, the loop exit occurs only if `boolean-expression` is true. Otherwise, control passes to the statement after `EXIT`.

`EXIT` can be used with all types of loops; it is not limited to use with unconditional loops.

When used with a `BEGIN` block, `EXIT` passes control to the next statement after the end of the block. Note that a label must be used for this purpose; an unlabeled `EXIT` is never considered to match a `BEGIN` block. (This is a change from pre-8.4 releases of PostgreSQL, which would allow an unlabeled `EXIT` to match a `BEGIN` block.)

Examples:

    LOOP
        -- some computations
        IF count > 0 THEN
            EXIT;  -- exit loop
        END IF;
    END LOOP;

    LOOP
        -- some computations
        EXIT WHEN count > 0;  -- same result as previous example
    END LOOP;

    <<ablock>>
    BEGIN
        -- some computations
        IF stocks > 100000 THEN
            EXIT ablock;  -- causes exit from the BEGIN block
        END IF;
        -- computations here will be skipped when stocks > 100000
    END;

#### `CONTINUE`

    CONTINUE  label   WHEN boolean-expression ;

If no `label` is given, the next iteration of the innermost loop is begun. That is, all statements remaining in the loop body are skipped, and control returns to the loop control expression (if any) to determine whether another loop iteration is needed. If `label` is present, it specifies the label of the loop whose execution will be continued.

If `WHEN` is specified, the next iteration of the loop is begun only if `boolean-expression` is true. Otherwise, control passes to the statement after `CONTINUE`.

`CONTINUE` can be used with all types of loops; it is not limited to use with unconditional loops.

Examples:

    LOOP
        -- some computations
        EXIT WHEN count > 100;
        CONTINUE WHEN count < 50;
        -- some computations for count IN [50 .. 100]
    END LOOP;

#### `WHILE`

     <<label>>
    WHILE boolean-expression LOOP
        statements
    END LOOP  label ;

The `WHILE` statement repeats a sequence of statements so long as the `boolean-expression` evaluates to true. The expression is checked just before each entry to the loop body.

For example:

    WHILE amount_owed > 0 AND gift_certificate_balance > 0 LOOP
        -- some computations here
    END LOOP;

    WHILE NOT done LOOP
        -- some computations here
    END LOOP;

#### `FOR` (Integer Variant)

     <<label>>
    FOR name IN  REVERSE  expression .. expression  BY expression  LOOP
        statements
    END LOOP  label ;

This form of `FOR` creates a loop that iterates over a range of integer values. The variable `name` is automatically defined as type `integer` and exists only inside the loop (any existing definition of the variable name is ignored within the loop). The two expressions giving the lower and upper bound of the range are evaluated once when entering the loop. If the `BY` clause isn't specified the iteration step is 1, otherwise it's the value specified in the `BY` clause, which again is evaluated once on loop entry. If `REVERSE` is specified then the step value is subtracted, rather than added, after each iteration.

Some examples of integer `FOR` loops:

    FOR i IN 1..10 LOOP
        -- i will take on the values 1,2,3,4,5,6,7,8,9,10 within the loop
    END LOOP;

    FOR i IN REVERSE 10..1 LOOP
        -- i will take on the values 10,9,8,7,6,5,4,3,2,1 within the loop
    END LOOP;

    FOR i IN REVERSE 10..1 BY 2 LOOP
        -- i will take on the values 10,8,6,4,2 within the loop
    END LOOP;

If the lower bound is greater than the upper bound (or less than, in the `REVERSE` case), the loop body is not executed at all. No error is raised.

If a `label` is attached to the `FOR` loop then the integer loop variable can be referenced with a qualified name, using that `label`.

### Looping through Query Results

Using a different type of `FOR` loop, you can iterate through the results of a query and manipulate that data accordingly. The syntax is:

     <<label>>
    FOR target IN query LOOP
        statements
    END LOOP  label ;

The `target` is a record variable, row variable, or comma-separated list of scalar variables. The `target` is successively assigned each row resulting from the `query` and the loop body is executed for each row. Here is an example:

    CREATE FUNCTION refresh_mviews() RETURNS integer AS $$
    DECLARE
        mviews RECORD;
    BEGIN
        RAISE NOTICE 'Refreshing all materialized views...';

        FOR mviews IN
           SELECT n.nspname AS mv_schema,
                  c.relname AS mv_name,
                  pg_catalog.pg_get_userbyid(c.relowner) AS owner
             FROM pg_catalog.pg_class c
        LEFT JOIN pg_catalog.pg_namespace n ON (n.oid = c.relnamespace)
            WHERE c.relkind = 'm'
         ORDER BY 1
        LOOP

            -- Now "mviews" has one record with information about the materialized view

            RAISE NOTICE 'Refreshing materialized view %.% (owner: %)...',
                         quote_ident(mviews.mv_schema),
                         quote_ident(mviews.mv_name),
                         quote_ident(mviews.owner);
            EXECUTE format('REFRESH MATERIALIZED VIEW %I.%I', mviews.mv_schema, mviews.mv_name);
        END LOOP;

        RAISE NOTICE 'Done refreshing materialized views.';
        RETURN 1;
    END;
    $$ LANGUAGE plpgsql;

If the loop is terminated by an `EXIT` statement, the last assigned row value is still accessible after the loop.

The `query` used in this type of `FOR` statement can be any SQL command that returns rows to the caller: `SELECT` is the most common case, but you can also use `INSERT`, `UPDATE`, `DELETE`, or `MERGE` with a `RETURNING` clause. Some utility commands such as `EXPLAIN` will work too.

PL/pgSQL variables are replaced by query parameters, and the query plan is cached for possible re-use, as discussed in detail in plpgsql-var-subst and plpgsql-plan-caching.

The `FOR-IN-EXECUTE` statement is another way to iterate over rows:

     <<label>>
    FOR target IN EXECUTE text_expression  USING expression , ...   LOOP
        statements
    END LOOP  label ;

This is like the previous form, except that the source query is specified as a string expression, which is evaluated and replanned on each entry to the `FOR` loop. This allows the programmer to choose the speed of a preplanned query or the flexibility of a dynamic query, just as with a plain `EXECUTE` statement. As with `EXECUTE`, parameter values can be inserted into the dynamic command via `USING`.

Another way to specify the query whose results should be iterated through is to declare it as a cursor. This is described in plpgsql-cursor-for-loop.

### Looping through Arrays

The `FOREACH` loop is much like a `FOR` loop, but instead of iterating through the rows returned by an SQL query, it iterates through the elements of an array value. (In general, `FOREACH` is meant for looping through components of a composite-valued expression; variants for looping through composites besides arrays may be added in future.) The `FOREACH` statement to loop over an array is:

     <<label>>
    FOREACH target  SLICE number  IN ARRAY expression LOOP
        statements
    END LOOP  label ;

Without `SLICE`, or if `SLICE 0` is specified, the loop iterates through individual elements of the array produced by evaluating the `expression`. The `target` variable is assigned each element value in sequence, and the loop body is executed for each element. Here is an example of looping through the elements of an integer array:

    CREATE FUNCTION sum(int[]) RETURNS int8 AS $$
    DECLARE
      s int8 := 0;
      x int;
    BEGIN
      FOREACH x IN ARRAY $1
      LOOP
        s := s + x;
      END LOOP;
      RETURN s;
    END;
    $$ LANGUAGE plpgsql;

The elements are visited in storage order, regardless of the number of array dimensions. Although the `target` is usually just a single variable, it can be a list of variables when looping through an array of composite values (records). In that case, for each array element, the variables are assigned from successive columns of the composite value.

With a positive `SLICE` value, `FOREACH` iterates through slices of the array rather than single elements. The `SLICE` value must be an integer constant not larger than the number of dimensions of the array. The `target` variable must be an array, and it receives successive slices of the array value, where each slice is of the number of dimensions specified by `SLICE`. Here is an example of iterating through one-dimensional slices:

    CREATE FUNCTION scan_rows(int[]) RETURNS void AS $$
    DECLARE
      x int[];
    BEGIN
      FOREACH x SLICE 1 IN ARRAY $1
      LOOP
        RAISE NOTICE 'row = %', x;
      END LOOP;
    END;
    $$ LANGUAGE plpgsql;

    SELECT scan_rows(ARRAY[[1,2,3],[4,5,6],[7,8,9],[10,11,12]]);

    NOTICE:  row = {1,2,3}
    NOTICE:  row = {4,5,6}
    NOTICE:  row = {7,8,9}
    NOTICE:  row = {10,11,12}

### Trapping Errors

By default, any error occurring in a PL/pgSQL function aborts execution of the function and the surrounding transaction. You can trap errors and recover from them by using a `BEGIN` block with an `EXCEPTION` clause. The syntax is an extension of the normal syntax for a `BEGIN` block:

     <<label>>
     DECLARE
        declarations
    BEGIN
        statements
    EXCEPTION
        WHEN condition  OR condition ...  THEN
            handler_statements
         WHEN condition  OR condition ...  THEN
              handler_statements
          ...
    END;

If no error occurs, this form of block simply executes all the `statements`, and then control passes to the next statement after `END`. But if an error occurs within the `statements`, further processing of the `statements` is abandoned, and control passes to the `EXCEPTION` list. The list is searched for the first `condition` matching the error that occurred. If a match is found, the corresponding `handler_statements` are executed, and then control passes to the next statement after `END`. If no match is found, the error propagates out as though the `EXCEPTION` clause were not there at all: the error can be caught by an enclosing block with `EXCEPTION`, or if there is none it aborts processing of the function.

The `condition` names can be any of those shown in errcodes-appendix. A category name matches any error within its category. The special condition name `OTHERS` matches every error type except `QUERY_CANCELED` and `ASSERT_FAILURE`. (It is possible, but often unwise, to trap those two error types by name.) Condition names are not case-sensitive. Also, an error condition can be specified by `SQLSTATE` code; for example these are equivalent:

    WHEN division_by_zero THEN ...
    WHEN SQLSTATE '22012' THEN ...

If a new error occurs within the selected `handler_statements`, it cannot be caught by this `EXCEPTION` clause, but is propagated out. A surrounding `EXCEPTION` clause could catch it.

When an error is caught by an `EXCEPTION` clause, the local variables of the PL/pgSQL function remain as they were when the error occurred, but all changes to persistent database state within the block are rolled back. As an example, consider this fragment:

    INSERT INTO mytab(firstname, lastname) VALUES('Tom', 'Jones');
    BEGIN
        UPDATE mytab SET firstname = 'Joe' WHERE lastname = 'Jones';
        x := x + 1;
        y := x / 0;
    EXCEPTION
        WHEN division_by_zero THEN
            RAISE NOTICE 'caught division_by_zero';
            RETURN x;
    END;

When control reaches the assignment to `y`, it will fail with a `division_by_zero` error. This will be caught by the `EXCEPTION` clause. The value returned in the `RETURN` statement will be the incremented value of `x`, but the effects of the `UPDATE` command will have been rolled back. The `INSERT` command preceding the block is not rolled back, however, so the end result is that the database contains `Tom Jones` not `Joe Jones`.

A block containing an `EXCEPTION` clause is significantly more expensive to enter and exit than a block without one. Therefore, don't use `EXCEPTION` without need.

#### Exceptions with `UPDATE`/`INSERT`

This example uses exception handling to perform either `UPDATE` or `INSERT`, as appropriate. It is recommended that applications use `INSERT` with `ON CONFLICT DO UPDATE` rather than actually using this pattern. This example serves primarily to illustrate use of PL/pgSQL control flow structures:

    CREATE TABLE db (a INT PRIMARY KEY, b TEXT);

    CREATE FUNCTION merge_db(key INT, data TEXT) RETURNS VOID AS
    $$
    BEGIN
        LOOP
            -- first try to update the key
            UPDATE db SET b = data WHERE a = key;
            IF found THEN
                RETURN;
            END IF;
            -- not there, so try to insert the key
            -- if someone else inserts the same key concurrently,
            -- we could get a unique-key failure
            BEGIN
                INSERT INTO db(a,b) VALUES (key, data);
                RETURN;
            EXCEPTION WHEN unique_violation THEN
                -- Do nothing, and loop to try the UPDATE again.
            END;
        END LOOP;
    END;
    $$
    LANGUAGE plpgsql;

    SELECT merge_db(1, 'david');
    SELECT merge_db(1, 'dennis');

This coding assumes the `unique_violation` error is caused by the `INSERT`, and not by, say, an `INSERT` in a trigger function on the table. It might also misbehave if there is more than one unique index on the table, since it will retry the operation regardless of which index caused the error. More safety could be had by using the features discussed next to check that the trapped error was the one expected.

#### Obtaining Information about an Error

Exception handlers frequently need to identify the specific error that occurred. There are two ways to get information about the current exception in PL/pgSQL: special variables and the `GET STACKED DIAGNOSTICS` command.

Within an exception handler, the special variable `SQLSTATE` contains the error code that corresponds to the exception that was raised (refer to errcodes-table for a list of possible error codes). The special variable `SQLERRM` contains the error message associated with the exception. These variables are undefined outside exception handlers.

Within an exception handler, one may also retrieve information about the current exception by using the `GET STACKED DIAGNOSTICS` command, which has the form:

    GET STACKED DIAGNOSTICS variable { = | := } item  , ... ;

Each `item` is a key word identifying a status value to be assigned to the specified `variable` (which should be of the right data type to receive it). The currently available status items are shown in plpgsql-exception-diagnostics-values.

#### Error Diagnostics Items

Name | Type | Description
`RETURNED_SQLSTATE` | `text` | the SQLSTATE error code of the exception
`COLUMN_NAME` | `text` | the name of the column related to exception
`CONSTRAINT_NAME` | `text` | the name of the constraint related to exception
`PG_DATATYPE_NAME` | `text` | the name of the data type related to exception
`MESSAGE_TEXT` | `text` | the text of the exception's primary message
`TABLE_NAME` | `text` | the name of the table related to exception
`SCHEMA_NAME` | `text` | the name of the schema related to exception
`PG_EXCEPTION_DETAIL` | `text` | the text of the exception's detail message, if any
`PG_EXCEPTION_HINT` | `text` | the text of the exception's hint message, if any
`PG_EXCEPTION_CONTEXT` | `text` | line(s) of text describing the call stack at the time of the exception (see plpgsql-call-stack)

If the exception did not set a value for an item, an empty string will be returned.

Here is an example:

    DECLARE
      text_var1 text;
      text_var2 text;
      text_var3 text;
    BEGIN
      -- some processing which might cause an exception
      ...
    EXCEPTION WHEN OTHERS THEN
      GET STACKED DIAGNOSTICS text_var1 = MESSAGE_TEXT,
                              text_var2 = PG_EXCEPTION_DETAIL,
                              text_var3 = PG_EXCEPTION_HINT;
    END;

### Obtaining Execution Location Information

The `GET DIAGNOSTICS` command, previously described in plpgsql-statements-diagnostics, retrieves information about current execution state (whereas the `GET STACKED DIAGNOSTICS` command discussed above reports information about the execution state as of a previous error). Its `PG_CONTEXT` status item is useful for identifying the current execution location. `PG_CONTEXT` returns a text string with line(s) of text describing the call stack. The first line refers to the current function and currently executing `GET DIAGNOSTICS` command. The second and any subsequent lines refer to calling functions further up the call stack. For example:

    CREATE OR REPLACE FUNCTION outer_func() RETURNS integer AS $$
    BEGIN
      RETURN inner_func();
    END;
    $$ LANGUAGE plpgsql;

    CREATE OR REPLACE FUNCTION inner_func() RETURNS integer AS $$
    DECLARE
      stack text;
    BEGIN
      GET DIAGNOSTICS stack = PG_CONTEXT;
      RAISE NOTICE E'--- Call Stack ---\n%', stack;
      RETURN 1;
    END;
    $$ LANGUAGE plpgsql;

    SELECT outer_func();

    NOTICE:  --- Call Stack ---
    PL/pgSQL function inner_func() line 5 at GET DIAGNOSTICS
    PL/pgSQL function outer_func() line 3 at RETURN
    CONTEXT:  PL/pgSQL function outer_func() line 3 at RETURN
     outer_func
     ------------
               1
    (1 row)

`GET STACKED DIAGNOSTICS ... PG_EXCEPTION_CONTEXT` returns the same sort of stack trace, but describing the location at which an error was detected, rather than the current location.

## Cursors

Rather than executing a whole query at once, it is possible to set up a cursor that encapsulates the query, and then read the query result a few rows at a time. One reason for doing this is to avoid memory overrun when the result contains a large number of rows. (However, PL/pgSQL users do not normally need to worry about that, since `FOR` loops automatically use a cursor internally to avoid memory problems.) A more interesting usage is to return a reference to a cursor that a function has created, allowing the caller to read the rows. This provides an efficient way to return large row sets from functions.

### Declaring Cursor Variables

All access to cursors in PL/pgSQL goes through cursor variables, which are always of the special data type `refcursor`. One way to create a cursor variable is just to declare it as a variable of type `refcursor`. Another way is to use the cursor declaration syntax, which in general is:

    name   NO  SCROLL  CURSOR  ( arguments )  FOR query;

(`FOR` can be replaced by `IS` for Oracle compatibility.) If `SCROLL` is specified, the cursor will be capable of scrolling backward; if `NO SCROLL` is specified, backward fetches will be rejected; if neither specification appears, it is query-dependent whether backward fetches will be allowed. `arguments`, if specified, is a comma-separated list of pairs ``name` `datatype`` that define names to be replaced by parameter values in the given query. The actual values to substitute for these names will be specified later, when the cursor is opened.

Some examples:

    DECLARE
        curs1 refcursor;
        curs2 CURSOR FOR SELECT * FROM tenk1;
        curs3 CURSOR (key integer) FOR SELECT * FROM tenk1 WHERE unique1 = key;

All three of these variables have the data type `refcursor`, but the first can be used with any query, while the second has a fully specified query already bound to it, and the last has a parameterized query bound to it. (`key` will be replaced by an integer parameter value when the cursor is opened.) The variable `curs1` is said to be unbound since it is not bound to any particular query.

The `SCROLL` option cannot be used when the cursor's query uses `FOR UPDATE/SHARE`. Also, it is best to use `NO SCROLL` with a query that involves volatile functions. The implementation of `SCROLL` assumes that re-reading the query's output will give consistent results, which a volatile function might not do.

### Opening Cursors

Before a cursor can be used to retrieve rows, it must be opened. (This is the equivalent action to the SQL command `DECLARE CURSOR`.) PL/pgSQL has three forms of the `OPEN` statement, two of which use unbound cursor variables while the third uses a bound cursor variable.

Bound cursor variables can also be used without explicitly opening the cursor, via the `FOR` statement described in plpgsql-cursor-for-loop. A `FOR` loop will open the cursor and then close it again when the loop completes.

Opening a cursor involves creating a server-internal data structure called a portal, which holds the execution state for the cursor's query. A portal has a name, which must be unique within the session for the duration of the portal's existence. By default, PL/pgSQL will assign a unique name to each portal it creates. However, if you assign a non-null string value to a cursor variable, that string will be used as its portal name. This feature can be used as described in plpgsql-cursor-returning.

#### `OPEN FOR` `query`

    OPEN unbound_cursorvar   NO  SCROLL  FOR query;

The cursor variable is opened and given the specified query to execute. The cursor cannot be open already, and it must have been declared as an unbound cursor variable (that is, as a simple `refcursor` variable). The query must be a `SELECT`, or something else that returns rows (such as `EXPLAIN`). The query is treated in the same way as other SQL commands in PL/pgSQL: PL/pgSQL variable names are substituted, and the query plan is cached for possible reuse. When a PL/pgSQL variable is substituted into the cursor query, the value that is substituted is the one it has at the time of the `OPEN`; subsequent changes to the variable will not affect the cursor's behavior. The `SCROLL` and `NO SCROLL` options have the same meanings as for a bound cursor.

An example:

    OPEN curs1 FOR SELECT * FROM foo WHERE key = mykey;

#### `OPEN FOR EXECUTE`

    OPEN unbound_cursorvar   NO  SCROLL  FOR EXECUTE query_string
                                          USING expression , ...  ;

The cursor variable is opened and given the specified query to execute. The cursor cannot be open already, and it must have been declared as an unbound cursor variable (that is, as a simple `refcursor` variable). The query is specified as a string expression, in the same way as in the `EXECUTE` command. As usual, this gives flexibility so the query plan can vary from one run to the next (see plpgsql-plan-caching), and it also means that variable substitution is not done on the command string. As with `EXECUTE`, parameter values can be inserted into the dynamic command via `format()` and `USING`. The `SCROLL` and `NO SCROLL` options have the same meanings as for a bound cursor.

An example:

    OPEN curs1 FOR EXECUTE format('SELECT * FROM %I WHERE col1 = $1',tabname) USING keyvalue;

In this example, the table name is inserted into the query via `format()`. The comparison value for `col1` is inserted via a `USING` parameter, so it needs no quoting.

#### Opening a Bound Cursor

    OPEN bound_cursorvar  (  argument_name :=  argument_value , ... ) ;

This form of `OPEN` is used to open a cursor variable whose query was bound to it when it was declared. The cursor cannot be open already. A list of actual argument value expressions must appear if and only if the cursor was declared to take arguments. These values will be substituted in the query.

The query plan for a bound cursor is always considered cacheable; there is no equivalent of `EXECUTE` in this case. Notice that `SCROLL` and `NO SCROLL` cannot be specified in `OPEN`, as the cursor's scrolling behavior was already determined.

Argument values can be passed using either positional or named notation. In positional notation, all arguments are specified in order. In named notation, each argument's name is specified using `:=` to separate it from the argument expression. Similar to calling functions, described in sql-syntax-calling-funcs, it is also allowed to mix positional and named notation.

Examples (these use the cursor declaration examples above):

    OPEN curs2;
    OPEN curs3(42);
    OPEN curs3(key := 42);

Because variable substitution is done on a bound cursor's query, there are really two ways to pass values into the cursor: either with an explicit argument to `OPEN`, or implicitly by referencing a PL/pgSQL variable in the query. However, only variables declared before the bound cursor was declared will be substituted into it. In either case the value to be passed is determined at the time of the `OPEN`. For example, another way to get the same effect as the `curs3` example above is

    DECLARE
        key integer;
        curs4 CURSOR FOR SELECT * FROM tenk1 WHERE unique1 = key;
    BEGIN
        key := 42;
        OPEN curs4;

### Using Cursors

Once a cursor has been opened, it can be manipulated with the statements described here.

These manipulations need not occur in the same function that opened the cursor to begin with. You can return a `refcursor` value out of a function and let the caller operate on the cursor. (Internally, a `refcursor` value is simply the string name of the portal containing the active query for the cursor. This name can be passed around, assigned to other `refcursor` variables, and so on, without disturbing the portal.)

All portals are implicitly closed at transaction end. Therefore a `refcursor` value is usable to reference an open cursor only until the end of the transaction.

#### `FETCH`

    FETCH  direction { FROM | IN }  cursor INTO target;

`FETCH` retrieves the next row (in the indicated direction) from the cursor into a target, which might be a row variable, a record variable, or a comma-separated list of simple variables, just like `SELECT INTO`. If there is no suitable row, the target is set to NULL(s). As with `SELECT INTO`, the special variable `FOUND` can be checked to see whether a row was obtained or not. If no row is obtained, the cursor is positioned after the last row or before the first row, depending on the movement direction.

The `direction` clause can be any of the variants allowed in the SQL sql-fetch command except the ones that can fetch more than one row; namely, it can be `NEXT`, `PRIOR`, `FIRST`, `LAST`, `ABSOLUTE` `count`, `RELATIVE` `count`, `FORWARD`, or `BACKWARD`. Omitting `direction` is the same as specifying `NEXT`. In the forms using a `count`, the `count` can be any integer-valued expression (unlike the SQL `FETCH` command, which only allows an integer constant). `direction` values that require moving backward are likely to fail unless the cursor was declared or opened with the `SCROLL` option.

`cursor` must be the name of a `refcursor` variable that references an open cursor portal.

Examples:

    FETCH curs1 INTO rowvar;
    FETCH curs2 INTO foo, bar, baz;
    FETCH LAST FROM curs3 INTO x, y;
    FETCH RELATIVE -2 FROM curs4 INTO x;

#### `MOVE`

    MOVE  direction { FROM | IN }  cursor;

`MOVE` repositions a cursor without retrieving any data. `MOVE` works like the `FETCH` command, except it only repositions the cursor and does not return the row moved to. The `direction` clause can be any of the variants allowed in the SQL sql-fetch command, including those that can fetch more than one row; the cursor is positioned to the last such row. (However, the case in which the `direction` clause is simply a `count` expression with no key word is deprecated in PL/pgSQL. That syntax is ambiguous with the case where the `direction` clause is omitted altogether, and hence it may fail if the `count` is not a constant.) As with `SELECT INTO`, the special variable `FOUND` can be checked to see whether there was a row to move to. If there is no such row, the cursor is positioned after the last row or before the first row, depending on the movement direction.

Examples:

    MOVE curs1;
    MOVE LAST FROM curs3;
    MOVE RELATIVE -2 FROM curs4;
    MOVE FORWARD 2 FROM curs4;

#### `UPDATE/DELETE WHERE CURRENT OF`

    UPDATE table SET ... WHERE CURRENT OF cursor;
    DELETE FROM table WHERE CURRENT OF cursor;

When a cursor is positioned on a table row, that row can be updated or deleted using the cursor to identify the row. There are restrictions on what the cursor's query can be (in particular, no grouping) and it's best to use `FOR UPDATE` in the cursor. For more information see the sql-declare reference page.

An example:

    UPDATE foo SET dataval = myval WHERE CURRENT OF curs1;

#### `CLOSE`

    CLOSE cursor;

`CLOSE` closes the portal underlying an open cursor. This can be used to release resources earlier than end of transaction, or to free up the cursor variable to be opened again.

An example:

    CLOSE curs1;

#### Returning Cursors

PL/pgSQL functions can return cursors to the caller. This is useful to return multiple rows or columns, especially with very large result sets. To do this, the function opens the cursor and returns the cursor name to the caller (or simply opens the cursor using a portal name specified by or otherwise known to the caller). The caller can then fetch rows from the cursor. The cursor can be closed by the caller, or it will be closed automatically when the transaction closes.

The portal name used for a cursor can be specified by the programmer or automatically generated. To specify a portal name, simply assign a string to the `refcursor` variable before opening it. The string value of the `refcursor` variable will be used by `OPEN` as the name of the underlying portal. However, if the `refcursor` variable's value is null (as it will be by default), then `OPEN` automatically generates a name that does not conflict with any existing portal, and assigns it to the `refcursor` variable.

Prior to PostgreSQL 16, bound cursor variables were initialized to contain their own names, rather than being left as null, so that the underlying portal name would be the same as the cursor variable's name by default. This was changed because it created too much risk of conflicts between similarly-named cursors in different functions.

The following example shows one way a cursor name can be supplied by the caller:

    CREATE TABLE test (col text);
    INSERT INTO test VALUES ('123');

    CREATE FUNCTION reffunc(refcursor) RETURNS refcursor AS '
    BEGIN
        OPEN $1 FOR SELECT col FROM test;
        RETURN $1;
    END;
    ' LANGUAGE plpgsql;

    BEGIN;
    SELECT reffunc('funccursor');
    FETCH ALL IN funccursor;
    COMMIT;

The following example uses automatic cursor name generation:

    CREATE FUNCTION reffunc2() RETURNS refcursor AS '
    DECLARE
        ref refcursor;
    BEGIN
        OPEN ref FOR SELECT col FROM test;
        RETURN ref;
    END;
    ' LANGUAGE plpgsql;

    -- need to be in a transaction to use cursors.
    BEGIN;
    SELECT reffunc2();

          reffunc2
    --------------------
     <unnamed cursor 1>
    (1 row)

    FETCH ALL IN "<unnamed cursor 1>";
    COMMIT;

The following example shows one way to return multiple cursors from a single function:

    CREATE FUNCTION myfunc(refcursor, refcursor) RETURNS SETOF refcursor AS $$
    BEGIN
        OPEN $1 FOR SELECT * FROM table_1;
        RETURN NEXT $1;
        OPEN $2 FOR SELECT * FROM table_2;
        RETURN NEXT $2;
    END;
    $$ LANGUAGE plpgsql;

    -- need to be in a transaction to use cursors.
    BEGIN;

    SELECT * FROM myfunc('a', 'b');

    FETCH ALL FROM a;
    FETCH ALL FROM b;
    COMMIT;

### Looping through a Cursor's Result

There is a variant of the `FOR` statement that allows iterating through the rows returned by a cursor. The syntax is:

     <<label>>
    FOR recordvar IN bound_cursorvar  (  argument_name :=  argument_value , ... )  LOOP
        statements
    END LOOP  label ;

The cursor variable must have been bound to some query when it was declared, and it cannot be open already. The `FOR` statement automatically opens the cursor, and it closes the cursor again when the loop exits. A list of actual argument value expressions must appear if and only if the cursor was declared to take arguments. These values will be substituted in the query, in just the same way as during an `OPEN` (see plpgsql-open-bound-cursor).

The variable `recordvar` is automatically defined as type `record` and exists only inside the loop (any existing definition of the variable name is ignored within the loop). Each row returned by the cursor is successively assigned to this record variable and the loop body is executed.

## Transaction Management

In procedures invoked by the `CALL` command as well as in anonymous code blocks (`DO` command), it is possible to end transactions using the commands `COMMIT` and `ROLLBACK`. A new transaction is started automatically after a transaction is ended using these commands, so there is no separate `START TRANSACTION` command. (Note that `BEGIN` and `END` have different meanings in PL/pgSQL.)

Here is a simple example:

    CREATE PROCEDURE transaction_test1()
    LANGUAGE plpgsql
    AS $$
    BEGIN
        FOR i IN 0..9 LOOP
            INSERT INTO test1 (a) VALUES (i);
            IF i % 2 = 0 THEN
                COMMIT;
            ELSE
                ROLLBACK;
            END IF;
        END LOOP;
    END;
    $$;

    CALL transaction_test1();

A new transaction starts out with default transaction characteristics such as transaction isolation level. In cases where transactions are committed in a loop, it might be desirable to start new transactions automatically with the same characteristics as the previous one. The commands `COMMIT AND CHAIN` and `ROLLBACK AND CHAIN` accomplish this.

Transaction control is only possible in `CALL` or `DO` invocations from the top level or nested `CALL` or `DO` invocations without any other intervening command. For example, if the call stack is `CALL proc1()` → `CALL proc2()` → `CALL proc3()`, then the second and third procedures can perform transaction control actions. But if the call stack is `CALL proc1()` → `SELECT func2()` → `CALL proc3()`, then the last procedure cannot do transaction control, because of the `SELECT` in between.

PL/pgSQL does not support savepoints (`SAVEPOINT`/`ROLLBACK TO SAVEPOINT`/`RELEASE SAVEPOINT` commands). Typical usage patterns for savepoints can be replaced by blocks with exception handlers (see plpgsql-error-trapping). Under the hood, a block with exception handlers forms a subtransaction, which means that transactions cannot be ended inside such a block.

Special considerations apply to cursor loops. Consider this example:

    CREATE PROCEDURE transaction_test2()
    LANGUAGE plpgsql
    AS $$
    DECLARE
        r RECORD;
    BEGIN
        FOR r IN SELECT * FROM test2 ORDER BY x LOOP
            INSERT INTO test1 (a) VALUES (r.x);
            COMMIT;
        END LOOP;
    END;
    $$;

    CALL transaction_test2();

Normally, cursors are automatically closed at transaction commit. However, a cursor created as part of a loop like this is automatically converted to a holdable cursor by the first `COMMIT` or `ROLLBACK`. That means that the cursor is fully evaluated at the first `COMMIT` or `ROLLBACK` rather than row by row. The cursor is still removed automatically after the loop, so this is mostly invisible to the user. But one must keep in mind that any table or row locks taken by the cursor's query will no longer be held after the first `COMMIT` or `ROLLBACK`.

Transaction commands are not allowed in cursor loops driven by commands that are not read-only (for example `UPDATE ... RETURNING`).

## Errors and Messages

### Reporting Errors and Messages

Use the `RAISE` statement to report messages and raise errors.

    RAISE  level  'format' , expression , ...   USING option = expression , ...  ;
    RAISE  level  condition_name  USING option = expression , ...  ;
    RAISE  level  SQLSTATE 'sqlstate'  USING option = expression , ...  ;
    RAISE  level  USING option = expression , ... ;
    RAISE ;

The `level` option specifies the error severity. Allowed levels are `DEBUG`, `LOG`, `INFO`, `NOTICE`, `WARNING`, and `EXCEPTION`, with `EXCEPTION` being the default. `EXCEPTION` raises an error (which normally aborts the current transaction); the other levels only generate messages of different priority levels. Whether messages of a particular priority are reported to the client, written to the server log, or both is controlled by the guc-log-min-messages and guc-client-min-messages configuration variables. See runtime-config for more information.

After `level` if any, you can specify a `format` string (which must be a simple string literal, not an expression). The format string specifies the error message text to be reported. The format string can be followed by optional argument expressions to be inserted into the message. Inside the format string, `%` is replaced by the string representation of the next optional argument's value. Write `%%` to emit a literal `%`. The number of arguments must match the number of `%` placeholders in the format string, or an error is raised during the compilation of the function.

In this example, the value of `v_job_id` will replace the `%` in the string:

    RAISE NOTICE 'Calling cs_create_job(%)', v_job_id;

You can attach additional information to the error report by writing `USING` followed by `option` = `expression` items. Each `expression` can be any string-valued expression. The allowed `option` key words are:

`MESSAGE`

- Sets the error message text. This option can't be used in the form of `RAISE` that includes a format string before `USING`.

`DETAIL`

- Supplies an error detail message.

`HINT`

- Supplies a hint message.

`ERRCODE`

- Specifies the error code (SQLSTATE) to report, either by condition name, as shown in errcodes-appendix, or directly as a five-character SQLSTATE code.

`COLUMN`

`CONSTRAINT`

`DATATYPE`

`TABLE`

`SCHEMA`

- Supplies the name of a related object.

This example will abort the transaction with the given error message and hint:

    RAISE EXCEPTION 'Nonexistent ID --> %', user_id
          USING HINT = 'Please check your user ID';

These two examples show equivalent ways of setting the SQLSTATE:

    RAISE 'Duplicate user ID: %', user_id USING ERRCODE = 'unique_violation';
    RAISE 'Duplicate user ID: %', user_id USING ERRCODE = '23505';

There is a second `RAISE` syntax in which the main argument is the condition name or SQLSTATE to be reported, for example:

    RAISE division_by_zero;
    RAISE SQLSTATE '22012';

In this syntax, `USING` can be used to supply a custom error message, detail, or hint. Another way to do the earlier example is

    RAISE unique_violation USING MESSAGE = 'Duplicate user ID: ' || user_id;

Still another variant is to write `RAISE USING` or `RAISE `level` USING` and put everything else into the `USING` list.

The last variant of `RAISE` has no parameters at all. This form can only be used inside a `BEGIN` block's `EXCEPTION` clause; it causes the error currently being handled to be re-thrown.

Before PostgreSQL 9.1, `RAISE` without parameters was interpreted as re-throwing the error from the block containing the active exception handler. Thus an `EXCEPTION` clause nested within that handler could not catch it, even if the `RAISE` was within the nested `EXCEPTION` clause's block. This was deemed surprising as well as being incompatible with Oracle's PL/SQL.

If no condition name nor SQLSTATE is specified in a `RAISE EXCEPTION` command, the default is to use `raise_exception` (`P0001`). If no message text is specified, the default is to use the condition name or SQLSTATE as message text.

When specifying an error code by SQLSTATE code, you are not limited to the predefined error codes, but can select any error code consisting of five digits and/or upper-case ASCII letters, other than `00000`. It is recommended that you avoid throwing error codes that end in three zeroes, because these are category codes and can only be trapped by trapping the whole category.

### Checking Assertions

The `ASSERT` statement is a convenient shorthand for inserting debugging checks into PL/pgSQL functions.

    ASSERT condition  , message ;

The `condition` is a Boolean expression that is expected to always evaluate to true; if it does, the `ASSERT` statement does nothing further. If the result is false or null, then an `ASSERT_FAILURE` exception is raised. (If an error occurs while evaluating the `condition`, it is reported as a normal error.)

If the optional `message` is provided, it is an expression whose result (if not null) replaces the default error message text assertion failed, should the `condition` fail. The `message` expression is not evaluated in the normal case where the assertion succeeds.

Testing of assertions can be enabled or disabled via the configuration parameter `plpgsql.check_asserts`, which takes a Boolean value; the default is `on`. If this parameter is `off` then `ASSERT` statements do nothing.

Note that `ASSERT` is meant for detecting program bugs, not for reporting ordinary error conditions. Use the `RAISE` statement, described above, for that.

## Trigger Functions

PL/pgSQL can be used to define trigger functions on data changes or database events. A trigger function is created with the `CREATE FUNCTION` command, declaring it as a function with no arguments and a return type of `trigger` (for data change triggers) or `event_trigger` (for database event triggers). Special local variables named `TG_`something`` are automatically defined to describe the condition that triggered the call.

### Triggers on Data Changes

A data change trigger is declared as a function with no arguments and a return type of `trigger`. Note that the function must be declared with no arguments even if it expects to receive some arguments specified in `CREATE TRIGGER` — such arguments are passed via `TG_ARGV`, as described below.

When a PL/pgSQL function is called as a trigger, several special variables are created automatically in the top-level block. They are:

`NEW` `record`

- new database row for `INSERT`/`UPDATE` operations in row-level triggers. This variable is null in statement-level triggers and for `DELETE` operations.

`OLD` `record`

- old database row for `UPDATE`/`DELETE` operations in row-level triggers. This variable is null in statement-level triggers and for `INSERT` operations.

`TG_NAME` `name`

- name of the trigger which fired.

`TG_WHEN` `text`

- `BEFORE`, `AFTER`, or `INSTEAD OF`, depending on the trigger's definition.

`TG_LEVEL` `text`

- `ROW` or `STATEMENT`, depending on the trigger's definition.

`TG_OP` `text`

- operation for which the trigger was fired: `INSERT`, `UPDATE`, `DELETE`, or `TRUNCATE`.

`TG_RELID` `oid` (references pg_class.oid)

- object ID of the table that caused the trigger invocation.

`TG_RELNAME` `name`

- table that caused the trigger invocation. This is now deprecated, and could disappear in a future release. Use `TG_TABLE_NAME` instead.

`TG_TABLE_NAME` `name`

- table that caused the trigger invocation.

`TG_TABLE_SCHEMA` `name`

- schema of the table that caused the trigger invocation.

`TG_NARGS` `integer`

- number of arguments given to the trigger function in the `CREATE TRIGGER` statement.

`TG_ARGV` `text[]`

- arguments from the `CREATE TRIGGER` statement. The index counts from 0. Invalid indexes (less than 0 or greater than or equal to `tg_nargs`) result in a null value.

A trigger function must return either NULL or a record/row value having exactly the structure of the table the trigger was fired for.

Row-level triggers fired `BEFORE` can return null to signal the trigger manager to skip the rest of the operation for this row (i.e., subsequent triggers are not fired, and the `INSERT`/`UPDATE`/`DELETE` does not occur for this row). If a nonnull value is returned then the operation proceeds with that row value. Returning a row value different from the original value of `NEW` alters the row that will be inserted or updated. Thus, if the trigger function wants the triggering action to succeed normally without altering the row value, `NEW` (or a value equal thereto) has to be returned. To alter the row to be stored, it is possible to replace single values directly in `NEW` and return the modified `NEW`, or to build a complete new record/row to return. In the case of a before-trigger on `DELETE`, the returned value has no direct effect, but it has to be nonnull to allow the trigger action to proceed. Note that `NEW` is null in `DELETE` triggers, so returning that is usually not sensible. The usual idiom in `DELETE` triggers is to return `OLD`.

`INSTEAD OF` triggers (which are always row-level triggers, and may only be used on views) can return null to signal that they did not perform any updates, and that the rest of the operation for this row should be skipped (i.e., subsequent triggers are not fired, and the row is not counted in the rows-affected status for the surrounding `INSERT`/`UPDATE`/`DELETE`). Otherwise a nonnull value should be returned, to signal that the trigger performed the requested operation. For `INSERT` and `UPDATE` operations, the return value should be `NEW`, which the trigger function may modify to support `INSERT RETURNING` and `UPDATE RETURNING` (this will also affect the row value passed to any subsequent triggers, or passed to a special `EXCLUDED` alias reference within an `INSERT` statement with an `ON CONFLICT DO UPDATE` clause). For `DELETE` operations, the return value should be `OLD`.

The return value of a row-level trigger fired `AFTER` or a statement-level trigger fired `BEFORE` or `AFTER` is always ignored; it might as well be null. However, any of these types of triggers might still abort the entire operation by raising an error.

plpgsql-trigger-example shows an example of a trigger function in PL/pgSQL.

#### A PL/pgSQL Trigger Function

This example trigger ensures that any time a row is inserted or updated in the table, the current user name and time are stamped into the row. And it checks that an employee's name is given and that the salary is a positive value.

    CREATE TABLE emp (
        empname           text,
        salary            integer,
        last_date         timestamp,
        last_user         text
    );

    CREATE FUNCTION emp_stamp() RETURNS trigger AS $emp_stamp$
        BEGIN
            -- Check that empname and salary are given
            IF NEW.empname IS NULL THEN
                RAISE EXCEPTION 'empname cannot be null';
            END IF;
            IF NEW.salary IS NULL THEN
                RAISE EXCEPTION '% cannot have null salary', NEW.empname;
            END IF;

            -- Who works for us when they must pay for it?
            IF NEW.salary < 0 THEN
                RAISE EXCEPTION '% cannot have a negative salary', NEW.empname;
            END IF;

            -- Remember who changed the payroll when
            NEW.last_date := current_timestamp;
            NEW.last_user := current_user;
            RETURN NEW;
        END;
    $emp_stamp$ LANGUAGE plpgsql;

    CREATE TRIGGER emp_stamp BEFORE INSERT OR UPDATE ON emp
        FOR EACH ROW EXECUTE FUNCTION emp_stamp();

Another way to log changes to a table involves creating a new table that holds a row for each insert, update, or delete that occurs. This approach can be thought of as auditing changes to a table. plpgsql-trigger-audit-example shows an example of an audit trigger function in PL/pgSQL.

#### A PL/pgSQL Trigger Function for Auditing

This example trigger ensures that any insert, update or delete of a row in the `emp` table is recorded (i.e., audited) in the `emp_audit` table. The current time and user name are stamped into the row, together with the type of operation performed on it.

    CREATE TABLE emp (
        empname           text NOT NULL,
        salary            integer
    );

    CREATE TABLE emp_audit(
        operation         char(1)   NOT NULL,
        stamp             timestamp NOT NULL,
        userid            text      NOT NULL,
        empname           text      NOT NULL,
        salary            integer
    );

    CREATE OR REPLACE FUNCTION process_emp_audit() RETURNS TRIGGER AS $emp_audit$
        BEGIN
            --
            -- Create a row in emp_audit to reflect the operation performed on emp,
            -- making use of the special variable TG_OP to work out the operation.
            --
            IF (TG_OP = 'DELETE') THEN
                INSERT INTO emp_audit SELECT 'D', now(), current_user, OLD.*;
            ELSIF (TG_OP = 'UPDATE') THEN
                INSERT INTO emp_audit SELECT 'U', now(), current_user, NEW.*;
            ELSIF (TG_OP = 'INSERT') THEN
                INSERT INTO emp_audit SELECT 'I', now(), current_user, NEW.*;
            END IF;
            RETURN NULL; -- result is ignored since this is an AFTER trigger
        END;
    $emp_audit$ LANGUAGE plpgsql;

    CREATE TRIGGER emp_audit
    AFTER INSERT OR UPDATE OR DELETE ON emp
        FOR EACH ROW EXECUTE FUNCTION process_emp_audit();

A variation of the previous example uses a view joining the main table to the audit table, to show when each entry was last modified. This approach still records the full audit trail of changes to the table, but also presents a simplified view of the audit trail, showing just the last modified timestamp derived from the audit trail for each entry. plpgsql-view-trigger-audit-example shows an example of an audit trigger on a view in PL/pgSQL.

#### A PL/pgSQL View Trigger Function for Auditing

This example uses a trigger on the view to make it updatable, and ensure that any insert, update or delete of a row in the view is recorded (i.e., audited) in the `emp_audit` table. The current time and user name are recorded, together with the type of operation performed, and the view displays the last modified time of each row.

    CREATE TABLE emp (
        empname           text PRIMARY KEY,
        salary            integer
    );

    CREATE TABLE emp_audit(
        operation         char(1)   NOT NULL,
        userid            text      NOT NULL,
        empname           text      NOT NULL,
        salary            integer,
        stamp             timestamp NOT NULL
    );

    CREATE VIEW emp_view AS
        SELECT e.empname,
               e.salary,
               max(ea.stamp) AS last_updated
          FROM emp e
          LEFT JOIN emp_audit ea ON ea.empname = e.empname
         GROUP BY 1, 2;

    CREATE OR REPLACE FUNCTION update_emp_view() RETURNS TRIGGER AS $$
        BEGIN
            --
            -- Perform the required operation on emp, and create a row in emp_audit
            -- to reflect the change made to emp.
            --
            IF (TG_OP = 'DELETE') THEN
                DELETE FROM emp WHERE empname = OLD.empname;
                IF NOT FOUND THEN RETURN NULL; END IF;

                OLD.last_updated = now();
                INSERT INTO emp_audit VALUES('D', current_user, OLD.*);
                RETURN OLD;
            ELSIF (TG_OP = 'UPDATE') THEN
                UPDATE emp SET salary = NEW.salary WHERE empname = OLD.empname;
                IF NOT FOUND THEN RETURN NULL; END IF;

                NEW.last_updated = now();
                INSERT INTO emp_audit VALUES('U', current_user, NEW.*);
                RETURN NEW;
            ELSIF (TG_OP = 'INSERT') THEN
                INSERT INTO emp VALUES(NEW.empname, NEW.salary);

                NEW.last_updated = now();
                INSERT INTO emp_audit VALUES('I', current_user, NEW.*);
                RETURN NEW;
            END IF;
        END;
    $$ LANGUAGE plpgsql;

    CREATE TRIGGER emp_audit
    INSTEAD OF INSERT OR UPDATE OR DELETE ON emp_view
        FOR EACH ROW EXECUTE FUNCTION update_emp_view();

One use of triggers is to maintain a summary table of another table. The resulting summary can be used in place of the original table for certain queries — often with vastly reduced run times. This technique is commonly used in Data Warehousing, where the tables of measured or observed data (called fact tables) might be extremely large. plpgsql-trigger-summary-example shows an example of a trigger function in PL/pgSQL that maintains a summary table for a fact table in a data warehouse.

#### A PL/pgSQL Trigger Function for Maintaining a Summary Table

The schema detailed here is partly based on the Grocery Store example from The Data Warehouse Toolkit by Ralph Kimball.

    --
    -- Main tables - time dimension and sales fact.
    --
    CREATE TABLE time_dimension (
        time_key                    integer NOT NULL,
        day_of_week                 integer NOT NULL,
        day_of_month                integer NOT NULL,
        month                       integer NOT NULL,
        quarter                     integer NOT NULL,
        year                        integer NOT NULL
    );
    CREATE UNIQUE INDEX time_dimension_key ON time_dimension(time_key);

    CREATE TABLE sales_fact (
        time_key                    integer NOT NULL,
        product_key                 integer NOT NULL,
        store_key                   integer NOT NULL,
        amount_sold                 numeric(12,2) NOT NULL,
        units_sold                  integer NOT NULL,
        amount_cost                 numeric(12,2) NOT NULL
    );
    CREATE INDEX sales_fact_time ON sales_fact(time_key);

    --
    -- Summary table - sales by time.
    --
    CREATE TABLE sales_summary_bytime (
        time_key                    integer NOT NULL,
        amount_sold                 numeric(15,2) NOT NULL,
        units_sold                  numeric(12) NOT NULL,
        amount_cost                 numeric(15,2) NOT NULL
    );
    CREATE UNIQUE INDEX sales_summary_bytime_key ON sales_summary_bytime(time_key);

    --
    -- Function and trigger to amend summarized column(s) on UPDATE, INSERT, DELETE.
    --
    CREATE OR REPLACE FUNCTION maint_sales_summary_bytime() RETURNS TRIGGER
    AS $maint_sales_summary_bytime$
        DECLARE
            delta_time_key          integer;
            delta_amount_sold       numeric(15,2);
            delta_units_sold        numeric(12);
            delta_amount_cost       numeric(15,2);
        BEGIN

            -- Work out the increment/decrement amount(s).
            IF (TG_OP = 'DELETE') THEN

                delta_time_key = OLD.time_key;
                delta_amount_sold = -1 * OLD.amount_sold;
                delta_units_sold = -1 * OLD.units_sold;
                delta_amount_cost = -1 * OLD.amount_cost;

            ELSIF (TG_OP = 'UPDATE') THEN

                -- forbid updates that change the time_key -
                -- (probably not too onerous, as DELETE + INSERT is how most
                -- changes will be made).
                IF ( OLD.time_key != NEW.time_key) THEN
                    RAISE EXCEPTION 'Update of time_key : % -> % not allowed',
                                                          OLD.time_key, NEW.time_key;
                END IF;

                delta_time_key = OLD.time_key;
                delta_amount_sold = NEW.amount_sold - OLD.amount_sold;
                delta_units_sold = NEW.units_sold - OLD.units_sold;
                delta_amount_cost = NEW.amount_cost - OLD.amount_cost;

            ELSIF (TG_OP = 'INSERT') THEN

                delta_time_key = NEW.time_key;
                delta_amount_sold = NEW.amount_sold;
                delta_units_sold = NEW.units_sold;
                delta_amount_cost = NEW.amount_cost;

            END IF;

            -- Insert or update the summary row with the new values.
            <<insert_update>>
            LOOP
                UPDATE sales_summary_bytime
                    SET amount_sold = amount_sold + delta_amount_sold,
                        units_sold = units_sold + delta_units_sold,
                        amount_cost = amount_cost + delta_amount_cost
                    WHERE time_key = delta_time_key;

                EXIT insert_update WHEN found;

                BEGIN
                    INSERT INTO sales_summary_bytime (
                                time_key,
                                amount_sold,
                                units_sold,
                                amount_cost)
                        VALUES (
                                delta_time_key,
                                delta_amount_sold,
                                delta_units_sold,
                                delta_amount_cost
                               );

                    EXIT insert_update;

                EXCEPTION
                    WHEN UNIQUE_VIOLATION THEN
                        -- do nothing
                END;
            END LOOP insert_update;

            RETURN NULL;

        END;
    $maint_sales_summary_bytime$ LANGUAGE plpgsql;

    CREATE TRIGGER maint_sales_summary_bytime
    AFTER INSERT OR UPDATE OR DELETE ON sales_fact
        FOR EACH ROW EXECUTE FUNCTION maint_sales_summary_bytime();

    INSERT INTO sales_fact VALUES(1,1,1,10,3,15);
    INSERT INTO sales_fact VALUES(1,2,1,20,5,35);
    INSERT INTO sales_fact VALUES(2,2,1,40,15,135);
    INSERT INTO sales_fact VALUES(2,3,1,10,1,13);
    SELECT * FROM sales_summary_bytime;
    DELETE FROM sales_fact WHERE product_key = 1;
    SELECT * FROM sales_summary_bytime;
    UPDATE sales_fact SET units_sold = units_sold * 2;
    SELECT * FROM sales_summary_bytime;

`AFTER` triggers can also make use of transition tables to inspect the entire set of rows changed by the triggering statement. The `CREATE TRIGGER` command assigns names to one or both transition tables, and then the function can refer to those names as though they were read-only temporary tables. plpgsql-trigger-audit-transition-example shows an example.

#### Auditing with Transition Tables

This example produces the same results as plpgsql-trigger-audit-example, but instead of using a trigger that fires for every row, it uses a trigger that fires once per statement, after collecting the relevant information in a transition table. This can be significantly faster than the row-trigger approach when the invoking statement has modified many rows. Notice that we must make a separate trigger declaration for each kind of event, since the `REFERENCING` clauses must be different for each case. But this does not stop us from using a single trigger function if we choose. (In practice, it might be better to use three separate functions and avoid the run-time tests on `TG_OP`.)

    CREATE TABLE emp (
        empname           text NOT NULL,
        salary            integer
    );

    CREATE TABLE emp_audit(
        operation         char(1)   NOT NULL,
        stamp             timestamp NOT NULL,
        userid            text      NOT NULL,
        empname           text      NOT NULL,
        salary            integer
    );

    CREATE OR REPLACE FUNCTION process_emp_audit() RETURNS TRIGGER AS $emp_audit$
        BEGIN
            --
            -- Create rows in emp_audit to reflect the operations performed on emp,
            -- making use of the special variable TG_OP to work out the operation.
            --
            IF (TG_OP = 'DELETE') THEN
                INSERT INTO emp_audit
                    SELECT 'D', now(), current_user, o.* FROM old_table o;
            ELSIF (TG_OP = 'UPDATE') THEN
                INSERT INTO emp_audit
                    SELECT 'U', now(), current_user, n.* FROM new_table n;
            ELSIF (TG_OP = 'INSERT') THEN
                INSERT INTO emp_audit
                    SELECT 'I', now(), current_user, n.* FROM new_table n;
            END IF;
            RETURN NULL; -- result is ignored since this is an AFTER trigger
        END;
    $emp_audit$ LANGUAGE plpgsql;

    CREATE TRIGGER emp_audit_ins
        AFTER INSERT ON emp
        REFERENCING NEW TABLE AS new_table
        FOR EACH STATEMENT EXECUTE FUNCTION process_emp_audit();
    CREATE TRIGGER emp_audit_upd
        AFTER UPDATE ON emp
        REFERENCING OLD TABLE AS old_table NEW TABLE AS new_table
        FOR EACH STATEMENT EXECUTE FUNCTION process_emp_audit();
    CREATE TRIGGER emp_audit_del
        AFTER DELETE ON emp
        REFERENCING OLD TABLE AS old_table
        FOR EACH STATEMENT EXECUTE FUNCTION process_emp_audit();

### Triggers on Events

PL/pgSQL can be used to define event triggers. PostgreSQL requires that a function that is to be called as an event trigger must be declared as a function with no arguments and a return type of `event_trigger`.

When a PL/pgSQL function is called as an event trigger, several special variables are created automatically in the top-level block. They are:

`TG_EVENT` `text`

- event the trigger is fired for.

`TG_TAG` `text`

- command tag for which the trigger is fired.

plpgsql-event-trigger-example shows an example of an event trigger function in PL/pgSQL.

#### A PL/pgSQL Event Trigger Function

This example trigger simply raises a `NOTICE` message each time a supported command is executed.

    CREATE OR REPLACE FUNCTION snitch() RETURNS event_trigger AS $$
    BEGIN
        RAISE NOTICE 'snitch: % %', tg_event, tg_tag;
    END;
    $$ LANGUAGE plpgsql;

    CREATE EVENT TRIGGER snitch ON ddl_command_start EXECUTE FUNCTION snitch();

## PL/pgSQL under the Hood

This section discusses some implementation details that are frequently important for PL/pgSQL users to know.

### Variable Substitution

SQL statements and expressions within a PL/pgSQL function can refer to variables and parameters of the function. Behind the scenes, PL/pgSQL substitutes query parameters for such references. Query parameters will only be substituted in places where they are syntactically permissible. As an extreme case, consider this example of poor programming style:

    INSERT INTO foo (foo) VALUES (foo(foo));

The first occurrence of `foo` must syntactically be a table name, so it will not be substituted, even if the function has a variable named `foo`. The second occurrence must be the name of a column of that table, so it will not be substituted either. Likewise the third occurrence must be a function name, so it also will not be substituted for. Only the last occurrence is a candidate to be a reference to a variable of the PL/pgSQL function.

Another way to understand this is that variable substitution can only insert data values into an SQL command; it cannot dynamically change which database objects are referenced by the command. (If you want to do that, you must build a command string dynamically, as explained in plpgsql-statements-executing-dyn.)

Since the names of variables are syntactically no different from the names of table columns, there can be ambiguity in statements that also refer to tables: is a given name meant to refer to a table column, or a variable? Let's change the previous example to

    INSERT INTO dest (col) SELECT foo + bar FROM src;

Here, `dest` and `src` must be table names, and `col` must be a column of `dest`, but `foo` and `bar` might reasonably be either variables of the function or columns of `src`.

By default, PL/pgSQL will report an error if a name in an SQL statement could refer to either a variable or a table column. You can fix such a problem by renaming the variable or column, or by qualifying the ambiguous reference, or by telling PL/pgSQL which interpretation to prefer.

The simplest solution is to rename the variable or column. A common coding rule is to use a different naming convention for PL/pgSQL variables than you use for column names. For example, if you consistently name function variables `v_`something`` while none of your column names start with `v_`, no conflicts will occur.

Alternatively you can qualify ambiguous references to make them clear. In the above example, `src.foo` would be an unambiguous reference to the table column. To create an unambiguous reference to a variable, declare it in a labeled block and use the block's label (see plpgsql-structure). For example,

    <<block>>
    DECLARE
        foo int;
    BEGIN
        foo := ...;
        INSERT INTO dest (col) SELECT block.foo + bar FROM src;

Here `block.foo` means the variable even if there is a column `foo` in `src`. Function parameters, as well as special variables such as `FOUND`, can be qualified by the function's name, because they are implicitly declared in an outer block labeled with the function's name.

Sometimes it is impractical to fix all the ambiguous references in a large body of PL/pgSQL code. In such cases you can specify that PL/pgSQL should resolve ambiguous references as the variable (which is compatible with PL/pgSQL's behavior before PostgreSQL 9.0), or as the table column (which is compatible with some other systems such as Oracle).

To change this behavior on a system-wide basis, set the configuration parameter `plpgsql.variable_conflict` to one of `error`, `use_variable`, or `use_column` (where `error` is the factory default). This parameter affects subsequent compilations of statements in PL/pgSQL functions, but not statements already compiled in the current session. Because changing this setting can cause unexpected changes in the behavior of PL/pgSQL functions, it can only be changed by a superuser.

You can also set the behavior on a function-by-function basis, by inserting one of these special commands at the start of the function text:

    #variable_conflict error
    #variable_conflict use_variable
    #variable_conflict use_column

These commands affect only the function they are written in, and override the setting of `plpgsql.variable_conflict`. An example is

    CREATE FUNCTION stamp_user(id int, comment text) RETURNS void AS $$
        #variable_conflict use_variable
        DECLARE
            curtime timestamp := now();
        BEGIN
            UPDATE users SET last_modified = curtime, comment = comment
              WHERE users.id = id;
        END;
    $$ LANGUAGE plpgsql;

In the `UPDATE` command, `curtime`, `comment`, and `id` will refer to the function's variable and parameters whether or not `users` has columns of those names. Notice that we had to qualify the reference to `users.id` in the `WHERE` clause to make it refer to the table column. But we did not have to qualify the reference to `comment` as a target in the `UPDATE` list, because syntactically that must be a column of `users`. We could write the same function without depending on the `variable_conflict` setting in this way:

    CREATE FUNCTION stamp_user(id int, comment text) RETURNS void AS $$
        <<fn>>
        DECLARE
            curtime timestamp := now();
        BEGIN
            UPDATE users SET last_modified = fn.curtime, comment = stamp_user.comment
              WHERE users.id = stamp_user.id;
        END;
    $$ LANGUAGE plpgsql;

Variable substitution does not happen in a command string given to `EXECUTE` or one of its variants. If you need to insert a varying value into such a command, do so as part of constructing the string value, or use `USING`, as illustrated in plpgsql-statements-executing-dyn.

Variable substitution currently works only in `SELECT`, `INSERT`, `UPDATE`, `DELETE`, `MERGE` and commands containing one of these (such as `EXPLAIN` and `CREATE TABLE ... AS SELECT`), because the main SQL engine allows query parameters only in these commands. To use a non-constant name or value in other statement types (generically called utility statements), you must construct the utility statement as a string and `EXECUTE` it.

### Plan Caching

The PL/pgSQL interpreter parses the function's source text and produces an internal binary instruction tree the first time the function is called (within each session). The instruction tree fully translates the PL/pgSQL statement structure, but individual SQL expressions and SQL commands used in the function are not translated immediately.

As each expression and SQL command is first executed in the function, the PL/pgSQL interpreter parses and analyzes the command to create a prepared statement, using the SPI manager's `SPI_prepare` function. Subsequent visits to that expression or command reuse the prepared statement. Thus, a function with conditional code paths that are seldom visited will never incur the overhead of analyzing those commands that are never executed within the current session. A disadvantage is that errors in a specific expression or command cannot be detected until that part of the function is reached in execution. (Trivial syntax errors will be detected during the initial parsing pass, but anything deeper will not be detected until execution.)

PL/pgSQL (or more precisely, the SPI manager) can furthermore attempt to cache the execution plan associated with any particular prepared statement. If a cached plan is not used, then a fresh execution plan is generated on each visit to the statement, and the current parameter values (that is, PL/pgSQL variable values) can be used to optimize the selected plan. If the statement has no parameters, or is executed many times, the SPI manager will consider creating a generic plan that is not dependent on specific parameter values, and caching that for re-use. Typically this will happen only if the execution plan is not very sensitive to the values of the PL/pgSQL variables referenced in it. If it is, generating a plan each time is a net win. See sql-prepare for more information about the behavior of prepared statements.

Because PL/pgSQL saves prepared statements and sometimes execution plans in this way, SQL commands that appear directly in a PL/pgSQL function must refer to the same tables and columns on every execution; that is, you cannot use a parameter as the name of a table or column in an SQL command. To get around this restriction, you can construct dynamic commands using the PL/pgSQL `EXECUTE` statement — at the price of performing new parse analysis and constructing a new execution plan on every execution.

The mutable nature of record variables presents another problem in this connection. When fields of a record variable are used in expressions or statements, the data types of the fields must not change from one call of the function to the next, since each expression will be analyzed using the data type that is present when the expression is first reached. `EXECUTE` can be used to get around this problem when necessary.

If the same function is used as a trigger for more than one table, PL/pgSQL prepares and caches statements independently for each such table — that is, there is a cache for each trigger function and table combination, not just for each function. This alleviates some of the problems with varying data types; for instance, a trigger function will be able to work successfully with a column named `key` even if it happens to have different types in different tables.

Likewise, functions having polymorphic argument types have a separate statement cache for each combination of actual argument types they have been invoked for, so that data type differences do not cause unexpected failures.

Statement caching can sometimes have surprising effects on the interpretation of time-sensitive values. For example there is a difference between what these two functions do:

    CREATE FUNCTION logfunc1(logtxt text) RETURNS void AS $$
        BEGIN
            INSERT INTO logtable VALUES (logtxt, 'now');
        END;
    $$ LANGUAGE plpgsql;

and:

    CREATE FUNCTION logfunc2(logtxt text) RETURNS void AS $$
        DECLARE
            curtime timestamp;
        BEGIN
            curtime := 'now';
            INSERT INTO logtable VALUES (logtxt, curtime);
        END;
    $$ LANGUAGE plpgsql;

In the case of `logfunc1`, the PostgreSQL main parser knows when analyzing the `INSERT` that the string `'now'` should be interpreted as `timestamp`, because the target column of `logtable` is of that type. Thus, `'now'` will be converted to a `timestamp` constant when the `INSERT` is analyzed, and then used in all invocations of `logfunc1` during the lifetime of the session. Needless to say, this isn't what the programmer wanted. A better idea is to use the `now()` or `current_timestamp` function.

In the case of `logfunc2`, the PostgreSQL main parser does not know what type `'now'` should become and therefore it returns a data value of type `text` containing the string `now`. During the ensuing assignment to the local variable `curtime`, the PL/pgSQL interpreter casts this string to the `timestamp` type by calling the `textout` and `timestamp_in` functions for the conversion. So, the computed time stamp is updated on each execution as the programmer expects. Even though this happens to work as expected, it's not terribly efficient, so use of the `now()` function would still be a better idea.

## Tips for Developing in PL/pgSQL

One good way to develop in PL/pgSQL is to use the text editor of your choice to create your functions, and in another window, use psql to load and test those functions. If you are doing it this way, it is a good idea to write the function using `CREATE OR REPLACE FUNCTION`. That way you can just reload the file to update the function definition. For example:

    CREATE OR REPLACE FUNCTION testfunc(integer) RETURNS integer AS $$
              ....
    $$ LANGUAGE plpgsql;

While running psql, you can load or reload such a function definition file with:

    \i filename.sql

and then immediately issue SQL commands to test the function.

Another good way to develop in PL/pgSQL is with a GUI database access tool that facilitates development in a procedural language. One example of such a tool is pgAdmin, although others exist. These tools often provide convenient features such as escaping single quotes and making it easier to recreate and debug functions.

### Handling of Quotation Marks

The code of a PL/pgSQL function is specified in `CREATE FUNCTION` as a string literal. If you write the string literal in the ordinary way with surrounding single quotes, then any single quotes inside the function body must be doubled; likewise any backslashes must be doubled (assuming escape string syntax is used). Doubling quotes is at best tedious, and in more complicated cases the code can become downright incomprehensible, because you can easily find yourself needing half a dozen or more adjacent quote marks. It's recommended that you instead write the function body as a dollar-quoted string literal (see sql-syntax-dollar-quoting). In the dollar-quoting approach, you never double any quote marks, but instead take care to choose a different dollar-quoting delimiter for each level of nesting you need. For example, you might write the `CREATE FUNCTION` command as:

    CREATE OR REPLACE FUNCTION testfunc(integer) RETURNS integer AS $PROC$
              ....
    $PROC$ LANGUAGE plpgsql;

Within this, you might use quote marks for simple literal strings in SQL commands and `$$` to delimit fragments of SQL commands that you are assembling as strings. If you need to quote text that includes `$$`, you could use `$Q$`, and so on.

The following chart shows what you have to do when writing quote marks without dollar quoting. It might be useful when translating pre-dollar quoting code into something more comprehensible.

1 quotation mark

- To begin and end the function body, for example:

      CREATE FUNCTION foo() RETURNS integer AS '
                ....
      ' LANGUAGE plpgsql;

  Anywhere within a single-quoted function body, quote marks must appear in pairs.

2 quotation marks

- For string literals inside the function body, for example:

      a_output := ''Blah'';
      SELECT * FROM users WHERE f_name=''foobar'';

  In the dollar-quoting approach, you'd just write:

      a_output := 'Blah';
      SELECT * FROM users WHERE f_name='foobar';

  which is exactly what the PL/pgSQL parser would see in either case.

4 quotation marks

- When you need a single quotation mark in a string constant inside the function body, for example:

      a_output := a_output || '' AND name LIKE ''''foobar'''' AND xyz''

  The value actually appended to `a_output` would be: `AND name LIKE 'foobar' AND xyz`.

  In the dollar-quoting approach, you'd write:

      a_output := a_output || $$ AND name LIKE 'foobar' AND xyz$$

  being careful that any dollar-quote delimiters around this are not just `$$`.

6 quotation marks

- When a single quotation mark in a string inside the function body is adjacent to the end of that string constant, for example:

      a_output := a_output || '' AND name LIKE ''''foobar''''''

  The value appended to `a_output` would then be: `AND name LIKE 'foobar'`.

  In the dollar-quoting approach, this becomes:

      a_output := a_output || $$ AND name LIKE 'foobar'$$

10 quotation marks

- When you want two single quotation marks in a string constant (which accounts for 8 quotation marks) and this is adjacent to the end of that string constant (2 more). You will probably only need that if you are writing a function that generates other functions, as in plpgsql-porting-ex2. For example:

      a_output := a_output || '' if v_'' ||
          referrer_keys.kind || '' like ''''''''''
          || referrer_keys.key_string || ''''''''''
          then return ''''''  || referrer_keys.referrer_type
          || ''''''; end if;'';

  The value of `a_output` would then be:

      if v_... like ''...'' then return ''...''; end if;

  In the dollar-quoting approach, this becomes:

      a_output := a_output || $$ if v_$$ || referrer_keys.kind || $$ like '$$
          || referrer_keys.key_string || $$'
          then return '$$  || referrer_keys.referrer_type
          || $$'; end if;$$;

  where we assume we only need to put single quote marks into `a_output`, because it will be re-quoted before use.

### Additional Compile-Time and Run-Time Checks

To aid the user in finding instances of simple but common problems before they cause harm, PL/pgSQL provides additional `checks`. When enabled, depending on the configuration, they can be used to emit either a `WARNING` or an `ERROR` during the compilation of a function. A function which has received a `WARNING` can be executed without producing further messages, so you are advised to test in a separate development environment.

Setting `plpgsql.extra_warnings`, or `plpgsql.extra_errors`, as appropriate, to `"all"` is encouraged in development and/or testing environments.

These additional checks are enabled through the configuration variables `plpgsql.extra_warnings` for warnings and `plpgsql.extra_errors` for errors. Both can be set either to a comma-separated list of checks, `"none"` or `"all"`. The default is `"none"`. Currently the list of available checks includes:

`shadowed_variables`

- Checks if a declaration shadows a previously defined variable.

`strict_multi_assignment`

- Some PL/pgSQL commands allow assigning values to more than one variable at a time, such as `SELECT INTO`. Typically, the number of target variables and the number of source variables should match, though PL/pgSQL will use `NULL` for missing values and extra variables are ignored. Enabling this check will cause PL/pgSQL to throw a `WARNING` or `ERROR` whenever the number of target variables and the number of source variables are different.

`too_many_rows`

- Enabling this check will cause PL/pgSQL to check if a given query returns more than one row when an `INTO` clause is used. As an `INTO` statement will only ever use one row, having a query return multiple rows is generally either inefficient and/or nondeterministic and therefore is likely an error.

The following example shows the effect of `plpgsql.extra_warnings` set to `shadowed_variables`:

    SET plpgsql.extra_warnings TO 'shadowed_variables';

    CREATE FUNCTION foo(f1 int) RETURNS int AS $$
    DECLARE
    f1 int;
    BEGIN
    RETURN f1;
    END;
    $$ LANGUAGE plpgsql;
    WARNING:  variable "f1" shadows a previously defined variable
    LINE 3: f1 int;
            ^
    CREATE FUNCTION

The below example shows the effects of setting `plpgsql.extra_warnings` to `strict_multi_assignment`:

    SET plpgsql.extra_warnings TO 'strict_multi_assignment';

    CREATE OR REPLACE FUNCTION public.foo()
     RETURNS void
     LANGUAGE plpgsql
    AS $$
    DECLARE
      x int;
      y int;
    BEGIN
      SELECT 1 INTO x, y;
      SELECT 1, 2 INTO x, y;
      SELECT 1, 2, 3 INTO x, y;
    END;
    $$;

    SELECT foo();
    WARNING:  number of source and target fields in assignment does not match
    DETAIL:  strict_multi_assignment check of extra_warnings is active.
    HINT:  Make sure the query returns the exact list of columns.
    WARNING:  number of source and target fields in assignment does not match
    DETAIL:  strict_multi_assignment check of extra_warnings is active.
    HINT:  Make sure the query returns the exact list of columns.

     foo
    -----

    (1 row)

## Porting from Oracle PL/SQL

This section explains differences between PostgreSQL's PL/pgSQL language and Oracle's PL/SQL language, to help developers who port applications from Oracle to PostgreSQL.

PL/pgSQL is similar to PL/SQL in many aspects. It is a block-structured, imperative language, and all variables have to be declared. Assignments, loops, and conditionals are similar. The main differences you should keep in mind when porting from PL/SQL to PL/pgSQL are:

- If a name used in an SQL command could be either a column name of a table used in the command or a reference to a variable of the function, PL/SQL treats it as a column name. By default, PL/pgSQL will throw an error complaining that the name is ambiguous. You can specify `plpgsql.variable_conflict` = `use_column` to change this behavior to match PL/SQL, as explained in plpgsql-var-subst. It's often best to avoid such ambiguities in the first place, but if you have to port a large amount of code that depends on this behavior, setting `variable_conflict` may be the best solution.

- In PostgreSQL the function body must be written as a string literal. Therefore you need to use dollar quoting or escape single quotes in the function body. (See plpgsql-quote-tips.)

- Data type names often need translation. For example, in Oracle string values are commonly declared as being of type `varchar2`, which is a non-SQL-standard type. In PostgreSQL, use type `varchar` or `text` instead. Similarly, replace type `number` with `numeric`, or use some other numeric data type if there's a more appropriate one.

- Instead of packages, use schemas to organize your functions into groups.

- Since there are no packages, there are no package-level variables either. This is somewhat annoying. You can keep per-session state in temporary tables instead.

- Integer `FOR` loops with `REVERSE` work differently: PL/SQL counts down from the second number to the first, while PL/pgSQL counts down from the first number to the second, requiring the loop bounds to be swapped when porting. This incompatibility is unfortunate but is unlikely to be changed. (See plpgsql-integer-for.)

- `FOR` loops over queries (other than cursors) also work differently: the target variable(s) must have been declared, whereas PL/SQL always declares them implicitly. An advantage of this is that the variable values are still accessible after the loop exits.

- There are various notational differences for the use of cursor variables.

### Porting Examples

pgsql-porting-ex1 shows how to port a simple function from PL/SQL to PL/pgSQL.

#### Porting a Simple Function from PL/SQL to PL/pgSQL

Here is an Oracle PL/SQL function:

    CREATE OR REPLACE FUNCTION cs_fmt_browser_version(v_name varchar2,
                                                      v_version varchar2)
    RETURN varchar2 IS
    BEGIN
        IF v_version IS NULL THEN
            RETURN v_name;
        END IF;
        RETURN v_name || '/' || v_version;
    END;
    /
    show errors;

Let's go through this function and see the differences compared to PL/pgSQL:

- The type name `varchar2` has to be changed to `varchar` or `text`. In the examples in this section, we'll use `varchar`, but `text` is often a better choice if you do not need specific string length limits.

- The `RETURN` key word in the function prototype (not the function body) becomes `RETURNS` in PostgreSQL. Also, `IS` becomes `AS`, and you need to add a `LANGUAGE` clause because PL/pgSQL is not the only possible function language.

- In PostgreSQL, the function body is considered to be a string literal, so you need to use quote marks or dollar quotes around it. This substitutes for the terminating `/` in the Oracle approach.

- The `show errors` command does not exist in PostgreSQL, and is not needed since errors are reported automatically.

This is how this function would look when ported to PostgreSQL:

    CREATE OR REPLACE FUNCTION cs_fmt_browser_version(v_name varchar,
                                                      v_version varchar)
    RETURNS varchar AS $$
    BEGIN
        IF v_version IS NULL THEN
            RETURN v_name;
        END IF;
        RETURN v_name || '/' || v_version;
    END;
    $$ LANGUAGE plpgsql;

plpgsql-porting-ex2 shows how to port a function that creates another function and how to handle the ensuing quoting problems.

#### Porting a Function that Creates Another Function from PL/SQL to PL/pgSQL

The following procedure grabs rows from a `SELECT` statement and builds a large function with the results in `IF` statements, for the sake of efficiency.

This is the Oracle version:

    CREATE OR REPLACE PROCEDURE cs_update_referrer_type_proc IS
        CURSOR referrer_keys IS
            SELECT * FROM cs_referrer_keys
            ORDER BY try_order;
        func_cmd VARCHAR(4000);
    BEGIN
        func_cmd := 'CREATE OR REPLACE FUNCTION cs_find_referrer_type(v_host IN VARCHAR2,
                     v_domain IN VARCHAR2, v_url IN VARCHAR2) RETURN VARCHAR2 IS BEGIN';

        FOR referrer_key IN referrer_keys LOOP
            func_cmd := func_cmd ||
              ' IF v_' || referrer_key.kind
              || ' LIKE ''' || referrer_key.key_string
              || ''' THEN RETURN ''' || referrer_key.referrer_type
              || '''; END IF;';
        END LOOP;

        func_cmd := func_cmd || ' RETURN NULL; END;';

        EXECUTE IMMEDIATE func_cmd;
    END;
    /
    show errors;

Here is how this function would end up in PostgreSQL:

    CREATE OR REPLACE PROCEDURE cs_update_referrer_type_proc() AS $func$
    DECLARE
        referrer_keys CURSOR IS
            SELECT * FROM cs_referrer_keys
            ORDER BY try_order;
        func_body text;
        func_cmd text;
    BEGIN
        func_body := 'BEGIN';

        FOR referrer_key IN referrer_keys LOOP
            func_body := func_body ||
              ' IF v_' || referrer_key.kind
              || ' LIKE ' || quote_literal(referrer_key.key_string)
              || ' THEN RETURN ' || quote_literal(referrer_key.referrer_type)
              || '; END IF;' ;
        END LOOP;

        func_body := func_body || ' RETURN NULL; END;';

        func_cmd :=
          'CREATE OR REPLACE FUNCTION cs_find_referrer_type(v_host varchar,
                                                            v_domain varchar,
                                                            v_url varchar)
            RETURNS varchar AS '
          || quote_literal(func_body)
          || ' LANGUAGE plpgsql;' ;

        EXECUTE func_cmd;
    END;
    $func$ LANGUAGE plpgsql;

Notice how the body of the function is built separately and passed through `quote_literal` to double any quote marks in it. This technique is needed because we cannot safely use dollar quoting for defining the new function: we do not know for sure what strings will be interpolated from the referrer_key.key_string field. (We are assuming here that referrer_key.kind can be trusted to always be `host`, `domain`, or `url`, but referrer_key.key_string might be anything, in particular it might contain dollar signs.) This function is actually an improvement on the Oracle original, because it will not generate broken code when referrer_key.key_string or referrer_key.referrer_type contain quote marks.

plpgsql-porting-ex3 shows how to port a function with `OUT` parameters and string manipulation. PostgreSQL does not have a built-in `instr` function, but you can create one using a combination of other functions. In plpgsql-porting-appendix there is a PL/pgSQL implementation of `instr` that you can use to make your porting easier.

#### Porting a Procedure With String Manipulation and `OUT` Parameters from PL/SQL to PL/pgSQL

The following Oracle PL/SQL procedure is used to parse a URL and return several elements (host, path, and query).

This is the Oracle version:

    CREATE OR REPLACE PROCEDURE cs_parse_url(
        v_url IN VARCHAR2,
        v_host OUT VARCHAR2,  -- This will be passed back
        v_path OUT VARCHAR2,  -- This one too
        v_query OUT VARCHAR2) -- And this one
    IS
        a_pos1 INTEGER;
        a_pos2 INTEGER;
    BEGIN
        v_host := NULL;
        v_path := NULL;
        v_query := NULL;
        a_pos1 := instr(v_url, '//');

        IF a_pos1 = 0 THEN
            RETURN;
        END IF;
        a_pos2 := instr(v_url, '/', a_pos1 + 2);
        IF a_pos2 = 0 THEN
            v_host := substr(v_url, a_pos1 + 2);
            v_path := '/';
            RETURN;
        END IF;

        v_host := substr(v_url, a_pos1 + 2, a_pos2 - a_pos1 - 2);
        a_pos1 := instr(v_url, '?', a_pos2 + 1);

        IF a_pos1 = 0 THEN
            v_path := substr(v_url, a_pos2);
            RETURN;
        END IF;

        v_path := substr(v_url, a_pos2, a_pos1 - a_pos2);
        v_query := substr(v_url, a_pos1 + 1);
    END;
    /
    show errors;

Here is a possible translation into PL/pgSQL:

    CREATE OR REPLACE FUNCTION cs_parse_url(
        v_url IN VARCHAR,
        v_host OUT VARCHAR,  -- This will be passed back
        v_path OUT VARCHAR,  -- This one too
        v_query OUT VARCHAR) -- And this one
    AS $$
    DECLARE
        a_pos1 INTEGER;
        a_pos2 INTEGER;
    BEGIN
        v_host := NULL;
        v_path := NULL;
        v_query := NULL;
        a_pos1 := instr(v_url, '//');

        IF a_pos1 = 0 THEN
            RETURN;
        END IF;
        a_pos2 := instr(v_url, '/', a_pos1 + 2);
        IF a_pos2 = 0 THEN
            v_host := substr(v_url, a_pos1 + 2);
            v_path := '/';
            RETURN;
        END IF;

        v_host := substr(v_url, a_pos1 + 2, a_pos2 - a_pos1 - 2);
        a_pos1 := instr(v_url, '?', a_pos2 + 1);

        IF a_pos1 = 0 THEN
            v_path := substr(v_url, a_pos2);
            RETURN;
        END IF;

        v_path := substr(v_url, a_pos2, a_pos1 - a_pos2);
        v_query := substr(v_url, a_pos1 + 1);
    END;
    $$ LANGUAGE plpgsql;

This function could be used like this:

    SELECT * FROM cs_parse_url('http://foobar.com/query.cgi?baz');

plpgsql-porting-ex4 shows how to port a procedure that uses numerous features that are specific to Oracle.

#### Porting a Procedure from PL/SQL to PL/pgSQL

The Oracle version:

    CREATE OR REPLACE PROCEDURE cs_create_job(v_job_id IN INTEGER) IS
        a_running_job_count INTEGER;
    BEGIN
        LOCK TABLE cs_jobs IN EXCLUSIVE MODE;

        SELECT count(*) INTO a_running_job_count FROM cs_jobs WHERE end_stamp IS NULL;

        IF a_running_job_count > 0 THEN
            COMMIT; -- free lock
            raise_application_error(-20000,
                     'Unable to create a new job: a job is currently running.');
        END IF;

        DELETE FROM cs_active_job;
        INSERT INTO cs_active_job(job_id) VALUES (v_job_id);

        BEGIN
            INSERT INTO cs_jobs (job_id, start_stamp) VALUES (v_job_id, now());
        EXCEPTION
            WHEN dup_val_on_index THEN NULL; -- don't worry if it already exists
        END;
        COMMIT;
    END;
    /
    show errors

This is how we could port this procedure to PL/pgSQL:

    CREATE OR REPLACE PROCEDURE cs_create_job(v_job_id integer) AS $$
    DECLARE
        a_running_job_count integer;
    BEGIN
        LOCK TABLE cs_jobs IN EXCLUSIVE MODE;

        SELECT count(*) INTO a_running_job_count FROM cs_jobs WHERE end_stamp IS NULL;

        IF a_running_job_count > 0 THEN
            COMMIT; -- free lock
            RAISE EXCEPTION 'Unable to create a new job: a job is currently running'; --
        END IF;

        DELETE FROM cs_active_job;
        INSERT INTO cs_active_job(job_id) VALUES (v_job_id);

        BEGIN
            INSERT INTO cs_jobs (job_id, start_stamp) VALUES (v_job_id, now());
        EXCEPTION
            WHEN unique_violation THEN --
                -- don't worry if it already exists
        END;
        COMMIT;
    END;
    $$ LANGUAGE plpgsql;

- The syntax of `RAISE` is considerably different from Oracle's statement, although the basic case `RAISE` `exception_name` works similarly.

- The exception names supported by PL/pgSQL are different from Oracle's. The set of built-in exception names is much larger (see errcodes-appendix). There is not currently a way to declare user-defined exception names, although you can throw user-chosen SQLSTATE values instead.

### Other Things to Watch For

This section explains a few other things to watch for when porting Oracle PL/SQL functions to PostgreSQL.

#### Implicit Rollback after Exceptions

In PL/pgSQL, when an exception is caught by an `EXCEPTION` clause, all database changes since the block's `BEGIN` are automatically rolled back. That is, the behavior is equivalent to what you'd get in Oracle with:

    BEGIN
        SAVEPOINT s1;
        ... code here ...
    EXCEPTION
        WHEN ... THEN
            ROLLBACK TO s1;
            ... code here ...
        WHEN ... THEN
            ROLLBACK TO s1;
            ... code here ...
    END;

If you are translating an Oracle procedure that uses `SAVEPOINT` and `ROLLBACK TO` in this style, your task is easy: just omit the `SAVEPOINT` and `ROLLBACK TO`. If you have a procedure that uses `SAVEPOINT` and `ROLLBACK TO` in a different way then some actual thought will be required.

#### `EXECUTE`

The PL/pgSQL version of `EXECUTE` works similarly to the PL/SQL version, but you have to remember to use `quote_literal` and `quote_ident` as described in plpgsql-statements-executing-dyn. Constructs of the type `EXECUTE 'SELECT * FROM $1';` will not work reliably unless you use these functions.

#### Optimizing PL/pgSQL Functions

PostgreSQL gives you two function creation modifiers to optimize execution: volatility (whether the function always returns the same result when given the same arguments) and strictness (whether the function returns null if any argument is null). Consult the sql-createfunction reference page for details.

When making use of these optimization attributes, your `CREATE FUNCTION` statement might look something like this:

    CREATE FUNCTION foo(...) RETURNS integer AS $$
    ...
    $$ LANGUAGE plpgsql STRICT IMMUTABLE;

### Appendix

This section contains the code for a set of Oracle-compatible `instr` functions that you can use to simplify your porting efforts.

    --
    -- instr functions that mimic Oracle's counterpart
    -- Syntax: instr(string1, string2 [, n [, m]])
    -- where [] denotes optional parameters.
    --
    -- Search string1, beginning at the nth character, for the mth occurrence
    -- of string2.  If n is negative, search backwards, starting at the abs(n)'th
    -- character from the end of string1.
    -- If n is not passed, assume 1 (search starts at first character).
    -- If m is not passed, assume 1 (find first occurrence).
    -- Returns starting index of string2 in string1, or 0 if string2 is not found.
    --

    CREATE FUNCTION instr(varchar, varchar) RETURNS integer AS $$
    BEGIN
        RETURN instr($1, $2, 1);
    END;
    $$ LANGUAGE plpgsql STRICT IMMUTABLE;

    CREATE FUNCTION instr(string varchar, string_to_search_for varchar,
                          beg_index integer)
    RETURNS integer AS $$
    DECLARE
        pos integer NOT NULL DEFAULT 0;
        temp_str varchar;
        beg integer;
        length integer;
        ss_length integer;
    BEGIN
        IF beg_index > 0 THEN
            temp_str := substring(string FROM beg_index);
            pos := position(string_to_search_for IN temp_str);

            IF pos = 0 THEN
                RETURN 0;
            ELSE
                RETURN pos + beg_index - 1;
            END IF;
        ELSIF beg_index < 0 THEN
            ss_length := char_length(string_to_search_for);
            length := char_length(string);
            beg := length + 1 + beg_index;

            WHILE beg > 0 LOOP
                temp_str := substring(string FROM beg FOR ss_length);
                IF string_to_search_for = temp_str THEN
                    RETURN beg;
                END IF;

                beg := beg - 1;
            END LOOP;

            RETURN 0;
        ELSE
            RETURN 0;
        END IF;
    END;
    $$ LANGUAGE plpgsql STRICT IMMUTABLE;

    CREATE FUNCTION instr(string varchar, string_to_search_for varchar,
                          beg_index integer, occur_index integer)
    RETURNS integer AS $$
    DECLARE
        pos integer NOT NULL DEFAULT 0;
        occur_number integer NOT NULL DEFAULT 0;
        temp_str varchar;
        beg integer;
        i integer;
        length integer;
        ss_length integer;
    BEGIN
        IF occur_index <= 0 THEN
            RAISE 'argument ''%'' is out of range', occur_index
              USING ERRCODE = '22003';
        END IF;

        IF beg_index > 0 THEN
            beg := beg_index - 1;
            FOR i IN 1..occur_index LOOP
                temp_str := substring(string FROM beg + 1);
                pos := position(string_to_search_for IN temp_str);
                IF pos = 0 THEN
                    RETURN 0;
                END IF;
                beg := beg + pos;
            END LOOP;

            RETURN beg;
        ELSIF beg_index < 0 THEN
            ss_length := char_length(string_to_search_for);
            length := char_length(string);
            beg := length + 1 + beg_index;

            WHILE beg > 0 LOOP
                temp_str := substring(string FROM beg FOR ss_length);
                IF string_to_search_for = temp_str THEN
                    occur_number := occur_number + 1;
                    IF occur_number = occur_index THEN
                        RETURN beg;
                    END IF;
                END IF;

                beg := beg - 1;
            END LOOP;

            RETURN 0;
        ELSE
            RETURN 0;
        END IF;
    END;
    $$ LANGUAGE plpgsql STRICT IMMUTABLE;