Examples

PostgreSQL 7.3.4 Documentation
Prev | Chapter 17. Server Programming Interface | Next

# 17.5. Examples

This example of SPI usage demonstrates the visibility rule. There are more complex examples in src/test/regress/regress.c and in contrib/spi.

This is a very simple example of SPI usage. The procedure execq accepts an SQL-query in its first argument and tcount in its second, executes the query using SPI_exec and returns the number of tuples for which the query executed:

    #include "executor/spi.h"   /* this is what you need to work with SPI */

    int execq(text *sql, int cnt);

    int
    execq(text *sql, int cnt)
    {
        char *query;
        int ret;
        int proc;

        /* Convert given TEXT object to a C string */
        query = DatumGetCString(DirectFunctionCall1(textout,
                                                    PointerGetDatum(sql)));

        SPI_connect();

        ret = SPI_exec(query, cnt);

        proc = SPI_processed;
        /*
         * If this is SELECT and some tuple(s) fetched -
         * returns tuples to the caller via elog (INFO).
         */
        if ( ret == SPI_OK_SELECT && SPI_processed > 0 )
        {
            TupleDesc tupdesc = SPI_tuptable->tupdesc;
            SPITupleTable *tuptable = SPI_tuptable;
            char buf[8192];
            int i,j;

            for (j = 0; j < proc; j++)
            {
                HeapTuple tuple = tuptable->vals[j];

                for (i = 1, buf[0] = 0; i <= tupdesc->natts; i++)
                    snprintf(buf + strlen (buf), sizeof(buf) - strlen(buf)," %s%s",
                            SPI_getvalue(tuple, tupdesc, i),
                            (i == tupdesc->natts) ? " " : " |");
                elog (INFO, "EXECQ: %s", buf);
            }
        }

        SPI_finish();

        pfree(query);

        return (proc);
    }

Now, compile and create the function:

    CREATE FUNCTION execq (text, integer) RETURNS integer
        AS '...path_to_so'
        LANGUAGE C;

    vac=> SELECT execq('CREATE TABLE a (x INTEGER)', 0);
    execq
    -----
        0
    (1 row)

    vac=> INSERT INTO a VALUES (execq('INSERT INTO a VALUES (0)',0));
    INSERT 167631 1
    vac=> SELECT execq('SELECT * FROM a',0);
    INFO:  EXECQ:  0 <<< inserted by execq

    INFO:  EXECQ:  1 <<< value returned by execq and inserted by upper INSERT

    execq
    -----
        2
    (1 row)

    vac=> SELECT execq('INSERT INTO a SELECT x + 2 FROM a',1);
    execq
    -----
        1
    (1 row)

    vac=> SELECT execq('SELECT * FROM a', 10);
    INFO:  EXECQ:  0

    INFO:  EXECQ:  1

    INFO:  EXECQ:  2 <<< 0 + 2, only one tuple inserted - as specified

    execq
    -----
        3            <<< 10 is max value only, 3 is real # of tuples
    (1 row)

    vac=> DELETE FROM a;
    DELETE 3
    vac=> INSERT INTO a VALUES (execq('SELECT * FROM a', 0) + 1);
    INSERT 167712 1
    vac=> SELECT * FROM a;
    x
    -
    1                <<< no tuples in a (0) + 1
    (1 row)

    vac=> INSERT INTO a VALUES (execq('SELECT * FROM a', 0) + 1);
    INFO:  EXECQ:  0
    INSERT 167713 1
    vac=> SELECT * FROM a;
    x
    -
    1
    2                <<< there was single tuple in a + 1
    (2 rows)

    --   This demonstrates data changes visibility rule:

    vac=> INSERT INTO a SELECT execq('SELECT * FROM a', 0) * x FROM a;
    INFO:  EXECQ:  1
    INFO:  EXECQ:  2
    INFO:  EXECQ:  1
    INFO:  EXECQ:  2
    INFO:  EXECQ:  2
    INSERT 0 2
    vac=> SELECT * FROM a;
    x
    -
    1
    2
    2                <<< 2 tuples * 1 (x in first tuple)
    6                <<< 3 tuples (2 + 1 just inserted) * 2 (x in second tuple)
    (4 rows)             ^^^^^^^^
                         tuples visible to execq() in different invocations

Prev | Home | Next
Visibility of Data Changes | Up | Procedural Languages