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21.5. 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 (NOTICE).
     */
    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++)
                sprintf(buf + strlen (buf), " %s%s",
                        SPI_getvalue(tuple, tupdesc, i),
                        (i == tupdesc->natts) ? " " : " |");
            elog (NOTICE, "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);
NOTICE:EXECQ:  0 <<< inserted by execq

NOTICE: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);
NOTICE:EXECQ:  0 

NOTICE:EXECQ:  1 

NOTICE: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);
NOTICE: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;
NOTICE:EXECQ:  1 
NOTICE:EXECQ:  2 
NOTICE:EXECQ:  1 
NOTICE:EXECQ:  2 
NOTICE: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

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