PostgreSQL 9.4.26 Documentation
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# 36.4. A Complete Trigger Example

Here is a very simple example of a trigger function written in C. (Examples of triggers written in procedural languages can be found in the documentation of the procedural languages.)

The function `trigf` reports the number of rows in the table `ttest` and skips the actual operation if the command attempts to insert a null value into the column `x`. (So the trigger acts as a not-null constraint but doesn't abort the transaction.)

First, the table definition:

    CREATE TABLE ttest (
        x integer
    );

This is the source code of the trigger function:

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

    #ifdef PG_MODULE_MAGIC
    PG_MODULE_MAGIC;
    #endif

    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 it's called as a trigger at all */
        if (!CALLED_AS_TRIGGER(fcinfo))
            elog(ERROR, "trigf: not called 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 null values */
        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(ERROR, "trigf (fired %s): SPI_connect returned %d", when, ret);

        /* get number of rows in table */
        ret = SPI_exec("SELECT count(*) FROM ttest", 0);

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

        /* count(*) returns int8, so be careful to convert */
        i = DatumGetInt64(SPI_getbinval(SPI_tuptable->vals[0],
                                        SPI_tuptable->tupdesc,
                                        1,
                                        &isnull));

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

        SPI_finish();

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

        return PointerGetDatum(rettuple);
    }

After you have compiled the source code (see Section 35.9.6), declare the function and the triggers:

    CREATE FUNCTION trigf() RETURNS trigger
        AS 'filename'
        LANGUAGE C;

    CREATE TRIGGER tbefore BEFORE INSERT OR UPDATE OR DELETE ON ttest
        FOR EACH ROW EXECUTE PROCEDURE trigf();

    CREATE TRIGGER tafter AFTER INSERT OR UPDATE OR DELETE ON ttest
        FOR EACH ROW EXECUTE PROCEDURE trigf();

Now you can test the operation of the trigger:

    => INSERT INTO ttest VALUES (NULL);
    INFO:  trigf (fired before): there are 0 rows in ttest
    INSERT 0 0

    -- Insertion skipped and AFTER trigger is not fired

    => SELECT * FROM ttest;
     x
    ---
    (0 rows)

    => INSERT INTO ttest VALUES (1);
    INFO:  trigf (fired before): there are 0 rows in ttest
    INFO:  trigf (fired after ): there are 1 rows in ttest
                                           ^^^^^^^^
                                 remember what we said about visibility.
    INSERT 167793 1
    vac=> SELECT * FROM ttest;
     x
    ---
     1
    (1 row)

    => INSERT INTO ttest SELECT x * 2 FROM ttest;
    INFO:  trigf (fired before): there are 1 rows in ttest
    INFO:  trigf (fired after ): there are 2 rows in ttest
                                           ^^^^^^
                                 remember what we said about visibility.
    INSERT 167794 1
    => SELECT * FROM ttest;
     x
    ---
     1
     2
    (2 rows)

    => UPDATE ttest SET x = NULL WHERE x = 2;
    INFO:  trigf (fired before): there are 2 rows in ttest
    UPDATE 0
    => UPDATE ttest SET x = 4 WHERE x = 2;
    INFO:  trigf (fired before): there are 2 rows in ttest
    INFO:  trigf (fired after ): there are 2 rows in ttest
    UPDATE 1
    vac=> SELECT * FROM ttest;
     x
    ---
     1
     4
    (2 rows)

    => DELETE FROM ttest;
    INFO:  trigf (fired before): there are 2 rows in ttest
    INFO:  trigf (fired before): there are 1 rows in ttest
    INFO:  trigf (fired after ): there are 0 rows in ttest
    INFO:  trigf (fired after ): there are 0 rows in ttest
                                           ^^^^^^
                                 remember what we said about visibility.
    DELETE 2
    => SELECT * FROM ttest;
     x
    ---
    (0 rows)

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

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