PostgreSQL 7.1.3 Documentation
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# 21.4. Examples

There are more complex examples in `src/test/regress/regress.c` and in `contrib/spi`.

Here is a very simple example of trigger usage. Function trigf reports the number of tuples in the triggered relation ttest and skips the operation if the query attempts to insert NULL into x (i.e - it acts as a NOT NULL constraint but doesn't abort the transaction).

    #include "executor/spi.h"	/* this is what you need to work with SPI */
    #include "commands/trigger.h"	/* -"- and triggers */

    extern Datum trigf(PG_FUNCTION_ARGS);

    PG_FUNCTION_INFO_V1(trigf);

    Datum
    trigf(PG_FUNCTION_ARGS)
    {
    	TriggerData *trigdata = (TriggerData *) fcinfo->context;
    	TupleDesc	tupdesc;
    	HeapTuple	rettuple;
    	char		*when;
    	bool		checknull = false;
    	bool		isnull;
    	int		ret, i;

    	/* Make sure trigdata is pointing at what I expect */
    	if (!CALLED_AS_TRIGGER(fcinfo))
    		elog(ERROR, "trigf: not fired by trigger manager");

    	/* tuple to return to Executor */
    	if (TRIGGER_FIRED_BY_UPDATE(trigdata->tg_event))
    		rettuple = trigdata->tg_newtuple;
    	else
    		rettuple = trigdata->tg_trigtuple;

    	/* check for NULLs ? */
    	if (!TRIGGER_FIRED_BY_DELETE(trigdata->tg_event) &&
    		TRIGGER_FIRED_BEFORE(trigdata->tg_event))
    		checknull = true;

    	if (TRIGGER_FIRED_BEFORE(trigdata->tg_event))
    		when = "before";
    	else
    		when = "after ";

    	tupdesc = trigdata->tg_relation->rd_att;

    	/* Connect to SPI manager */
    	if ((ret = SPI_connect()) < 0)
    		elog(NOTICE, "trigf (fired %s): SPI_connect returned %d", when, ret);

    	/* Get number of tuples in relation */
    	ret = SPI_exec("select count(*) from ttest", 0);

    	if (ret < 0)
    		elog(NOTICE, "trigf (fired %s): SPI_exec returned %d", when, ret);

    	i = SPI_getbinval(SPI_tuptable->vals[0], SPI_tuptable->tupdesc, 1, &isnull);

    	elog (NOTICE, "trigf (fired %s): there are %d tuples in ttest", when, i);

    	SPI_finish();

    	if (checknull)
    	{
    		i = SPI_getbinval(rettuple, tupdesc, 1, &isnull);
    		if (isnull)
    			rettuple = NULL;
    	}

    	return PointerGetDatum(rettuple);
    }

Now, compile and create the trigger function:

    create function trigf () returns opaque as
    '...path_to_so' language 'C';

    create table ttest (x int4);

    vac=> create trigger tbefore before insert or update or delete on ttest
    for each row execute procedure trigf();
    CREATE
    vac=> create trigger tafter after insert or update or delete on ttest
    for each row execute procedure trigf();
    CREATE
    vac=> insert into ttest values (null);
    NOTICE:trigf (fired before): there are 0 tuples in ttest
    INSERT 0 0

    -- Insertion skipped and AFTER trigger is not fired

    vac=> select * from ttest;
    x
    -
    (0 rows)

    vac=> insert into ttest values (1);
    NOTICE:trigf (fired before): there are 0 tuples in ttest
    NOTICE:trigf (fired after ): there are 1 tuples in ttest
                                           ^^^^^^^^
                                 remember what we said about visibility.
    INSERT 167793 1
    vac=> select * from ttest;
    x
    -
    1
    (1 row)

    vac=> insert into ttest select x * 2 from ttest;
    NOTICE:trigf (fired before): there are 1 tuples in ttest
    NOTICE:trigf (fired after ): there are 2 tuples in ttest
                                           ^^^^^^^^
                                 remember what we said about visibility.
    INSERT 167794 1
    vac=> select * from ttest;
    x
    -
    1
    2
    (2 rows)

    vac=> update ttest set x = null where x = 2;
    NOTICE:trigf (fired before): there are 2 tuples in ttest
    UPDATE 0
    vac=> update ttest set x = 4 where x = 2;
    NOTICE:trigf (fired before): there are 2 tuples in ttest
    NOTICE:trigf (fired after ): there are 2 tuples in ttest
    UPDATE 1
    vac=> select * from ttest;
    x
    -
    1
    4
    (2 rows)

    vac=> delete from ttest;
    NOTICE:trigf (fired before): there are 2 tuples in ttest
    NOTICE:trigf (fired after ): there are 1 tuples in ttest
    NOTICE:trigf (fired before): there are 1 tuples in ttest
    NOTICE:trigf (fired after ): there are 0 tuples in ttest
                                           ^^^^^^^^
                                 remember what we said about visibility.
    DELETE 2
    vac=> select * from ttest;
    x
    -
    (0 rows)

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