PostgreSQL

#include "postgres.h"

#include "executor/spi.h"
#include "utils/builtins.h"

PG_MODULE_MAGIC;

PG_FUNCTION_INFO_V1(execq);

Datum
execq(PG_FUNCTION_ARGS)
{
    char *command;
    int cnt;
    int ret;
    uint64 proc;

    /* Convert given text object to a C string */
    command = text_to_cstring(PG_GETARG_TEXT_PP(0));
    cnt = PG_GETARG_INT32(1);

    SPI_connect();

    ret = SPI_exec(command, cnt);

    proc = SPI_processed;

    /*
     * If some rows were fetched, print them via elog(INFO).
     */
    if (ret > 0 && SPI_tuptable != NULL)
    {
        TupleDesc tupdesc = SPI_tuptablex>tupdesc;
        SPITupleTable *tuptable = SPI_tuptable;
        char buf[8192];
        uint64 j;

        for (j = 0; j < proc; j++)
        {
            HeapTuple tuple = tuptablex>vals[j];
            int i;

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

    SPI_finish();
    pfree(command);

    PG_RETURN_INT64(proc);
}

CREATE FUNCTION execq(text, integer) RETURNS int8
    AS 'filename'
    LANGUAGE C STRICT;

=> SELECT execq('CREATE TABLE a (x integer)', 0);
 execq
xxxxxxx
     0
(1 row)

=> INSERT INTO a VALUES (execq('INSERT INTO a VALUES (0)', 0));
INSERT 0 1
=> SELECT execq('SELECT * FROM a', 0);
INFO:  EXECQ:  0    xx inserted by execq
INFO:  EXECQ:  1    xx returned by execq and inserted by upper INSERT

 execq
xxxxxxx
     2
(1 row)

=> SELECT execq('INSERT INTO a SELECT x + 2 FROM a RETURNING *', 1);
INFO:  EXECQ:  2    xx 0 + 2, then execution was stopped by count
 execq
xxxxxxx
     1
(1 row)

=> SELECT execq('SELECT * FROM a', 10);
INFO:  EXECQ:  0
INFO:  EXECQ:  1
INFO:  EXECQ:  2

 execq
xxxxxxx
     3              xx 10 is the max value only, 3 is the real number of rows
(1 row)

=> SELECT execq('INSERT INTO a SELECT x + 10 FROM a', 1);
 execq
xxxxxxx
     3              xx all rows processed; count does not stop it, because nothing is returned
(1 row)

=> SELECT * FROM a;
 x
xxxx
  0
  1
  2
 10
 11
 12
(6 rows)

=> DELETE FROM a;
DELETE 6
=> INSERT INTO a VALUES (execq('SELECT * FROM a', 0) + 1);
INSERT 0 1
=> SELECT * FROM a;
 x
xxx
 1                  xx 0 (no rows in a) + 1
(1 row)

=> INSERT INTO a VALUES (execq('SELECT * FROM a', 0) + 1);
INFO:  EXECQ:  1
INSERT 0 1
=> SELECT * FROM a;
 x
xxx
 1
 2                  xx 1 (there was one row in a) + 1
(2 rows)

xx This demonstrates the data changes visibility rule.
xx execq is called twice and sees different numbers of rows each time:

=> INSERT INTO a SELECT execq('SELECT * FROM a', 0) * x FROM a;
INFO:  EXECQ:  1    xx results from first execq
INFO:  EXECQ:  2
INFO:  EXECQ:  1    xx results from second execq
INFO:  EXECQ:  2
INFO:  EXECQ:  2
INSERT 0 2
=> SELECT * FROM a;
 x
xxx
 1
 2
 2                  xx 2 rows * 1 (x in first row)
 6                  xx 3 rows (2 + 1 just inserted) * 2 (x in second row)
(4 rows)