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>Chapter 36. Triggers</TD
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><A
NAME="TRIGGER-EXAMPLE"
>36.4. A Complete Trigger Example</A
></H1
><P
> 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.)
</P
><P
> The function <CODE
CLASS="FUNCTION"
>trigf</CODE
> reports the number of rows in the
table <TT
CLASS="STRUCTNAME"
>ttest</TT
> and skips the actual operation if the
command attempts to insert a null value into the column
<TT
CLASS="STRUCTFIELD"
>x</TT
>. (So the trigger acts as a not-null constraint but
doesn't abort the transaction.)
</P
><P
> First, the table definition:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE ttest (
x integer
);</PRE
><P>
</P
><P
> This is the source code of the trigger function:
</P><PRE
CLASS="PROGRAMLISTING"
>#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);
}</PRE
><P>
</P
><P
> After you have compiled the source code (see <A
HREF="xfunc-c.html#DFUNC"
>Section 35.9.6</A
>), declare the function and the triggers:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION trigf() RETURNS trigger
AS '<TT
CLASS="REPLACEABLE"
><I
>filename</I
></TT
>'
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();</PRE
><P>
</P
><P
> Now you can test the operation of the trigger:
</P><PRE
CLASS="SCREEN"
>=> 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)</PRE
><P>
</P
><P
> There are more complex examples in
<TT
CLASS="FILENAME"
>src/test/regress/regress.c</TT
> and
in <A
HREF="contrib-spi.html"
>spi</A
>.
</P
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