trigger.sgml

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          <term><literal>TRIGGER_FIRED_FOR_STATEMENT(tg_event)</literal></term>
          <listitem>
           <para>
            Returns true if the trigger fired for a statement-level event.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>TRIGGER_FIRED_BY_INSERT(tg_event)</literal></term>
          <listitem>
           <para>
            Returns true if the trigger was fired by an <command>INSERT</command> command.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>TRIGGER_FIRED_BY_UPDATE(tg_event)</literal></term>
          <listitem>
           <para>
            Returns true if the trigger was fired by an <command>UPDATE</command> command.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>TRIGGER_FIRED_BY_DELETE(tg_event)</literal></term>
          <listitem>
           <para>
            Returns true if the trigger was fired by a <command>DELETE</command> command.
           </para>
          </listitem>
         </varlistentry>
        </variablelist>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><structfield>tg_relation</></term>
      <listitem>
       <para>
        A pointer to a structure describing the relation that the trigger fired for.
        Look at <filename>utils/rel.h</> for details about
        this structure.  The most interesting things are
        <literal>tg_relation-&gt;rd_att</> (descriptor of the relation
        tuples) and <literal>tg_relation-&gt;rd_rel-&gt;relname</>
        (relation name; the type is not <type>char*</> but
        <type>NameData</>; use
        <literal>SPI_getrelname(tg_relation)</> to get a <type>char*</> if you
        need a copy of the name).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><structfield>tg_trigtuple</></term>
      <listitem>
       <para>
        A pointer to the row for which the trigger was fired. This is
        the row being inserted, updated, or deleted.  If this trigger
        was fired for an <command>INSERT</command> or
        <command>DELETE</command> then this is what you should return
        from the function if you don't want to replace the row with
        a different one (in the case of <command>INSERT</command>) or
        skip the operation.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><structfield>tg_newtuple</></term>
      <listitem>
       <para>
        A pointer to the new version of the row, if the trigger was
        fired for an <command>UPDATE</command>, and <symbol>NULL</> if
        it is for an <command>INSERT</command> or a
        <command>DELETE</command>. This is what you have to return
        from the function if the event is an <command>UPDATE</command>
        and you don't want to replace this row by a different one or
        skip the operation.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><structfield>tg_trigger</></term>
      <listitem>
       <para>
        A pointer to a structure of type <structname>Trigger</>,
        defined in <filename>utils/rel.h</>:

<programlisting>
typedef struct Trigger
{
    Oid         tgoid;
    char       *tgname;
    Oid         tgfoid;
    int16       tgtype;
    bool        tgenabled;
    bool        tgisconstraint;
    Oid         tgconstrrelid;
    bool        tgdeferrable;
    bool        tginitdeferred;
    int16       tgnargs;
    int16       tgnattr;
    int16      *tgattr;
    char      **tgargs;
} Trigger;
</programlisting>

       where <structfield>tgname</> is the trigger's name,
       <structfield>tgnargs</> is number of arguments in
       <structfield>tgargs</>, and <structfield>tgargs</> is an array of
       pointers to the arguments specified in the <command>CREATE
       TRIGGER</command> statement. The other members are for internal use
       only.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><structfield>tg_trigtuplebuf</></term>
      <listitem>
       <para>
        The buffer containing <structfield>tg_trigtuple</structfield>, or <symbol>InvalidBuffer</symbol> if there
        is no such tuple or it is not stored in a disk buffer.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><structfield>tg_newtuplebuf</></term>
      <listitem>
       <para>
        The buffer containing <structfield>tg_newtuple</structfield>, or <symbol>InvalidBuffer</symbol> if there
        is no such tuple or it is not stored in a disk buffer.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
   </para>

   <para>
    A trigger function must return either a
    <structname>HeapTuple</> pointer or a <symbol>NULL</> pointer
    (<emphasis>not</> an SQL null value, that is, do not set <parameter>isNull</parameter> true).
    Be careful to return either
    <structfield>tg_trigtuple</> or <structfield>tg_newtuple</>,
    as appropriate, if you don't want to modify the row being operated on.
   </para>
  </sect1>

  <sect1 id="trigger-example">
   <title>A Complete Example</title>

   <para>
    Here is a very simple example of a trigger function written in C.
    (Examples of triggers written in procedural languages may be found
    in the documentation of the procedural languages.)
   </para>

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

   <para>
    First, the table definition:
<programlisting>
CREATE TABLE ttest (
    x integer
);
</programlisting>
   </para>

   <para>
    This is the source code of the trigger function:
<programlisting>
#include "postgres.h"
#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-&gt;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-&gt;tg_event))
        rettuple = trigdata-&gt;tg_newtuple;
    else
        rettuple = trigdata-&gt;tg_trigtuple;

    /* check for null values */
    if (!TRIGGER_FIRED_BY_DELETE(trigdata-&gt;tg_event)
        && TRIGGER_FIRED_BEFORE(trigdata-&gt;tg_event))
        checknull = true;

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

    tupdesc = trigdata-&gt;tg_relation-&gt;rd_att;

    /* connect to SPI manager */
    if ((ret = SPI_connect()) &lt; 0)
        elog(INFO, "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 &lt; 0)
        elog(NOTICE, "trigf (fired %s): SPI_exec returned %d", when, ret);

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

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

    SPI_finish();

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

    return PointerGetDatum(rettuple);
}
</programlisting>
   </para>

   <para>
    After you have compiled the source code, declare the function and
    the triggers:
<programlisting>
CREATE FUNCTION trigf() RETURNS trigger
    AS '<replaceable>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();
</programlisting>
   </para>

   <para>
    Now you can test the operation of the trigger:
<screen>
=&gt; 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

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

=&gt; 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=&gt; SELECT * FROM ttest;
 x
---
 1
(1 row)

=&gt; 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
=&gt; SELECT * FROM ttest;
 x
---
 1
 2
(2 rows)

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

=&gt; 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
=&gt; SELECT * FROM ttest;
 x
---
(0 rows)
</screen>

   </para>

   <para>
    There are more complex examples in
    <filename>src/test/regress/regress.c</filename> and
    in <filename>contrib/spi</filename>.
   </para>
  </sect1>
 </chapter>

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