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
