Control Structures

PostgreSQL 8.2.3 Documentation
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# 37.7. 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.

## 37.7.1. Returning From a Function

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

### 37.7.1.1. `RETURN`

    RETURN expression;

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

When returning a scalar type, any expression can be used. The expression's result will be automatically cast into the function's return type as described for assignments. To return a composite (row) value, you must write a record or row variable as the `expression`.

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.

### 37.7.1.2. `RETURN NEXT`

    RETURN NEXT 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 in `RETURN NEXT` 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 NEXT` does not actually return from the function — it simply saves away the value of the expression. Execution then continues with the next statement in the PL/pgSQL function. As successive `RETURN NEXT` 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).

If you declared the function with output parameters, write just `RETURN NEXT` with no expression. The current values of the output parameter variable(s) will be saved for eventual return. 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.

Functions that use `RETURN NEXT` should be called in the following fashion:

    SELECT * FROM some_func();

That is, the function must be used as a table source in a `FROM` clause.

> Note: The current implementation of `RETURN NEXT` for PL/pgSQL 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 may 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 may 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 work_mem configuration variable. Administrators who have sufficient memory to store larger result sets in memory should consider increasing this parameter.

## 37.7.2. Conditionals

`IF` statements let you execute commands based on certain conditions. PL/pgSQL has five forms of `IF`:

- `IF ... THEN`

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

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

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

- `IF ... THEN ... ELSEIF ... THEN ... ELSE`

### 37.7.2.1. `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;

### 37.7.2.2. `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 evaluates to false.

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;

### 37.7.2.3. `IF-THEN-ELSE IF`

`IF` statements can be nested, 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;

When you use this form, you are actually nesting an `IF` statement inside the `ELSE` part of an outer `IF` statement. Thus you need one `END IF` statement for each nested `IF` and one for the parent `IF-ELSE`. This is workable but grows tedious when there are many alternatives to be checked. Hence the next form.

### 37.7.2.4. `IF-THEN-ELSIF-ELSE`

    IF boolean-expression THEN
        statements
    [ ELSIF boolean-expression THEN
        statements
    [ ELSIF boolean-expression THEN
        statements
        ...]]
    [ ELSE
        statements ]
    END IF;

`IF-THEN-ELSIF-ELSE` provides a more convenient method of checking many alternatives in one statement. Formally it is equivalent to nested `IF-THEN-ELSE-IF-THEN` commands, but only one `END IF` is needed.

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;

### 37.7.2.5. `IF-THEN-ELSEIF-ELSE`

`ELSEIF` is an alias for `ELSIF`.

## 37.7.3. Simple Loops

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

### 37.7.3.1. `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 in nested loops to specify which loop the statement should be applied to.

### 37.7.3.2. `EXIT`

    EXIT [ label ] [ WHEN 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 `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.

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;

    BEGIN
        -- some computations
        IF stocks > 100000 THEN
            EXIT;  -- causes exit from the BEGIN block
        END IF;
    END;

### 37.7.3.3. `CONTINUE`

    CONTINUE [ label ] [ WHEN expression ];

If no `label` is given, the next iteration of the innermost loop is begun. That is, control is passed back to the loop control expression (if any), and the body of the loop is re-evaluated. 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 `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;

### 37.7.3.4. `WHILE`

    [ <<label>> ]
    WHILE expression LOOP
        statements
    END LOOP [ label ];

The `WHILE` statement repeats a sequence of statements so long as the condition expression evaluates to true. The condition 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 boolean_expression LOOP
        -- some computations here
    END LOOP;

### 37.7.3.5. `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. If `REVERSE` is specified then the step value is considered negative.

Some examples of integer `FOR` loops:

    FOR i IN 1..10 LOOP
        -- some computations here
        RAISE NOTICE 'i is %', i;
    END LOOP;

    FOR i IN REVERSE 10..1 LOOP
        -- some computations here
    END LOOP;

    FOR i IN REVERSE 10..1 BY 2 LOOP
        -- some computations here
        RAISE NOTICE 'i is %', i;
    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.

## 37.7.4. 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 cs_refresh_mviews() RETURNS integer AS $$
    DECLARE
        mviews RECORD;
    BEGIN
        PERFORM cs_log('Refreshing materialized views...');

        FOR mviews IN SELECT * FROM cs_materialized_views ORDER BY sort_key LOOP

            -- Now "mviews" has one record from cs_materialized_views

            PERFORM cs_log('Refreshing materialized view ' || quote_ident(mviews.mv_name) || ' ...');
            EXECUTE 'TRUNCATE TABLE ' || quote_ident(mviews.mv_name);
            EXECUTE 'INSERT INTO ' || quote_ident(mviews.mv_name) || ' ' || mviews.mv_query;
        END LOOP;

        PERFORM cs_log('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`, or `DELETE` with a `RETURNING` clause. Some utility commands such as `EXPLAIN` will work too.

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

    [ <<label>> ]
    FOR target IN EXECUTE text_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.

> Note: The PL/pgSQL parser presently distinguishes the two kinds of `FOR` loops (integer or query result) by checking whether `..` appears outside any parentheses between `IN` and `LOOP`. If `..` is not seen then the loop is presumed to be a loop over rows. Mistyping the `..` is thus likely to lead to a complaint along the lines of "loop variable of loop over rows must be a record or row variable or list of scalar variables", rather than the simple syntax error one might expect to get.

## 37.7.5. Trapping Errors

By default, any error occurring in a PL/pgSQL function aborts execution of the function, and indeed of the surrounding transaction as well. 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 Appendix A. A category name matches any error within its category. The special condition name `OTHERS` matches every error type except `QUERY_CANCELED`. (It is possible, but often unwise, to trap `QUERY_CANCELED` by name.) Condition names are not case-sensitive.

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`.

> Tip: 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.

Within an exception handler, the `SQLSTATE` variable contains the error code that corresponds to the exception that was raised (refer to Table A-1 for a list of possible error codes). The `SQLERRM` variable contains the error message associated with the exception. These variables are undefined outside exception handlers.

Example 37-1. Exceptions with `UPDATE`/`INSERT`

This example uses exception handling to perform either `UPDATE` or `INSERT`, as appropriate.

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

    CREATE FUNCTION merge_db(key INT, data TEXT) RETURNS VOID AS
    $$
    BEGIN
        LOOP
            UPDATE db SET b = data WHERE a = key;
            IF found THEN
                RETURN;
            END IF;

            BEGIN
                INSERT INTO db(a,b) VALUES (key, data);
                RETURN;
            EXCEPTION WHEN unique_violation THEN
                -- do nothing
            END;
        END LOOP;
    END;
    $$
    LANGUAGE plpgsql;

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

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