 <chapter id="triggers">
  <title>Triggers</title>

  <para>
   <productname>Postgres</productname> has various server-side function
   interfaces. Server-side functions can be written in SQL, PLPGSQL,
   TCL, or C. Trigger functions can be written in any of these
   languages except SQL. Note that STATEMENT-level trigger events are not
   supported in the current version. You can currently specify BEFORE or
   AFTER on INSERT, DELETE or UPDATE of a tuple as a trigger event.
  </para>

  <sect1>
   <title>Trigger Creation</title>

   <para>
    If a trigger event occurs, the trigger manager (called by the Executor)
    initializes the global structure TriggerData *CurrentTriggerData (described
    below) and calls the trigger function to handle the event.
   </para>

   <para>
    The trigger function must be created before the trigger is created as a
    function taking no arguments and returns opaque.
   </para>

   <para>
    The syntax for creating triggers is as follows:

    <programlisting>
CREATE TRIGGER <replaceable>trigger</replaceable> [ BEFORE | AFTER ] [ INSERT | DELETE | UPDATE [ OR ... ] ]
    ON <replaceable>relation</replaceable> FOR EACH [ ROW | STATEMENT ]
    EXECUTE PROCEDURE <replaceable>procedure</replaceable>
     (<replaceable>args</replaceable>);
    </programlisting>

    where the arguments are:

    <variablelist>
     <varlistentry>
      <term>
       <replaceable>trigger</replaceable>
      </term>
      <listitem>
       <para>
	The name of the trigger is
	used if you ever have to delete the trigger.
	It is used as an argument to the <command>DROP TRIGGER</command> command.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>BEFORE</term>
      <term>AFTER</term>
      <listitem>
       <para>
	Determines whether the function is called before or after
	the event.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>INSERT</term>
      <term>DELETE</term>
      <term>UPDATE</term>
      <listitem>
       <para>
	The next element of the command determines on what event(s) will trigger
	the function.  Multiple events can be specified separated by OR.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable>relation</replaceable></term>
      <listitem>
       <para>
	The relation name determines which table the event applies to.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ROW</term>
      <term>STATEMENT</term>
      <listitem>
       <para>
	The FOR EACH clause determines whether the trigger is fired for each
	affected row or before (or after) the entire statement has completed.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable>procedure</replaceable></term>
      <listitem>
       <para>
	The procedure name is the function called.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable>args</replaceable></term>
      <listitem>
       <para>
	The arguments passed to the function in the CurrentTriggerData structure.
	The purpose of passing arguments to the function is to allow different
	triggers with similar requirements to call the same function.
       </para>

       <para>
	Also, <replaceable>procedure</replaceable>
	may be used for triggering different relations (these
	functions are named as "general trigger functions").
       </para>

       <para>
	As example of using both features above, there could be a general
	function that takes as its arguments two field names and puts the current
	user in one and the current timestamp in the other. This allows triggers to
	be written on INSERT events to automatically track creation of records in a
	transaction table for example. It could also be used as a "last updated"
	function if used in an UPDATE event.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

   <para>
    Trigger functions return HeapTuple to the calling Executor.  This
    is ignored for triggers fired after an INSERT, DELETE or UPDATE operation
    but it allows BEFORE triggers to:

    <itemizedlist>
     <listitem>
      <para>
       Return NULL to skip the operation for the current tuple (and so the
       tuple will not be inserted/updated/deleted).
      </para>
     </listitem>

     <listitem>
      <para>
       Return a pointer to another tuple (INSERT and UPDATE only) which will
       be inserted (as the new version of the updated tuple if UPDATE) instead
       of original tuple.
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    Note that there is no initialization performed by the CREATE TRIGGER
    handler.  This will be changed in the future.  Also, if more than one trigger
    is defined for the same event on the same relation, the order of trigger
    firing is unpredictable. This may be changed in the future.
   </para>

   <para>
    If a trigger function executes SQL-queries (using SPI) then these queries
    may fire triggers again. This is known as cascading triggers.  There is no
    explicit limitation on the number of cascade levels.
   </para>

   <para>
    If a trigger is fired by INSERT and inserts a new tuple in the same
    relation then this trigger will be fired again.  Currently, there is nothing
    provided for synchronization (etc) of these cases but this may change.  At
    the moment, there is function funny_dup17() in the regress tests which uses
    some techniques to stop recursion (cascading) on itself...
   </para>
  </sect1>

  <sect1>
   <title>Interaction with the Trigger Manager</title>

   <para>
    As mentioned above, when function is called by the trigger manager,
    structure TriggerData *CurrentTriggerData is NOT NULL and initialized.  So
    it is better to check CurrentTriggerData against being NULL at the start
    and set it to NULL just after fetching the information to prevent calls to
    a trigger function not from the trigger manager.
   </para>

   <para>
    struct TriggerData is defined in src/include/commands/trigger.h:

    <programlisting>
typedef struct TriggerData
{
    TriggerEvent  tg_event;
    Relation      tg_relation;
    HeapTuple     tg_trigtuple;
    HeapTuple     tg_newtuple;
    Trigger      *tg_trigger;
} TriggerData;
    </programlisting>

    where the members are defined as follows:

    <variablelist>
     <varlistentry>
      <term>tg_event</term>
      <listitem>
       <para>
	describes the event for which the function is called. You may use the
	following macros to examine <literal>tg_event</literal>:

	<variablelist>
	 <varlistentry>
	  <term>TRIGGER_FIRED_BEFORE(tg_event)</term>
	  <listitem>
	   <para>
	    returns TRUE if trigger fired BEFORE.
	   </para>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term>TRIGGER_FIRED_AFTER(tg_event)</term>
	  <listitem>
	   <para>
	    Returns TRUE if trigger fired AFTER.
	   </para>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term>TRIGGER_FIRED_FOR_ROW(event)</term>
	  <listitem>
	   <para>
	    Returns TRUE if trigger fired for
	    a ROW-level event.
	   </para>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term>TRIGGER_FIRED_FOR_STATEMENT(event)</term>
	  <listitem>
	   <para>
	    Returns TRUE if trigger fired for
	    STATEMENT-level event.
	   </para>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term>TRIGGER_FIRED_BY_INSERT(event)</term>
	  <listitem>
	   <para>
	    Returns TRUE if trigger fired by INSERT.
	   </para>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term>TRIGGER_FIRED_BY_DELETE(event)</term>
	  <listitem>
	   <para>
	    Returns TRUE if trigger fired by DELETE.
	   </para>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term>TRIGGER_FIRED_BY_UPDATE(event)</term>
	  <listitem>
	   <para>
	    Returns TRUE if trigger fired by UPDATE.
	   </para>
	  </listitem>
	 </varlistentry>
	</variablelist>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>tg_relation</term>
      <listitem>
       <para>
	is a pointer to structure describing the triggered relation. Look at
	src/include/utils/rel.h for details about this structure.  The most
	interest things are tg_relation->rd_att (descriptor of the relation
	tuples) and tg_relation->rd_rel->relname (relation's name. This is not
	char*, but NameData.  Use SPI_getrelname(tg_relation) to get char* if
	you need a copy of name).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>tg_trigtuple</term>
      <listitem>
       <para>
	is a pointer to the tuple for which the trigger is fired. This is the tuple
	being inserted (if INSERT), deleted (if DELETE) or updated (if UPDATE).
	If INSERT/DELETE then this is what you are to return to Executor if 
	you don't want to replace tuple with another one (INSERT) or skip the
	operation.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>tg_newtuple</term>
      <listitem>
       <para>
	is a pointer to the new version of tuple if UPDATE and NULL if this is
	for an INSERT or a DELETE. This is what you are to return to Executor if
	UPDATE and you don't want to replace this tuple with another one or skip
	the operation.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>tg_trigger</term>
      <listitem>
       <para>
	is pointer to structure Trigger defined in src/include/utils/rel.h:

	<programlisting>
typedef struct Trigger
{
    Oid         tgoid;
    char       *tgname;
    Oid         tgfoid;
    FmgrInfo    tgfunc;
    int16       tgtype;
    bool        tgenabled;
    bool        tgisconstraint;
    bool        tgdeferrable;
    bool        tginitdeferred;
    int16       tgnargs;
    int16       tgattr[FUNC_MAX_ARGS];
    char      **tgargs;
} Trigger;
	</programlisting>

	where
	tgname is the trigger's name, tgnargs is number of arguments in tgargs,
	tgargs is an array of pointers to the arguments specified in the CREATE
	TRIGGER statement. Other members are for internal use only.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </sect1>

  <sect1>
   <title>Visibility of Data Changes</title>

   <para>
    <productname>Postgres</productname> data changes visibility rule: during a query execution, data
    changes made by the query itself (via SQL-function, SPI-function, triggers)
    are invisible to the query scan.  For example, in query

    <programlisting>
INSERT INTO a SELECT * FROM a;
    </programlisting>

    tuples inserted are invisible for SELECT scan.  In effect, this
    duplicates the database table within itself (subject to unique index
    rules, of course) without recursing.
   </para>

   <para>
    But keep in mind this notice about visibility in the SPI documentation:

    <blockquote>
     <para>
Changes made by query Q are visible by queries which are started after
query Q, no matter whether they are started inside Q (during the
execution of Q) or after Q is done.
     </para>
    </blockquote>
   </para>

   <para>
    This is true for triggers as well so, though a tuple being inserted
    (tg_trigtuple) is not visible to queries in a BEFORE trigger, this tuple
    (just inserted) is visible to queries in an AFTER trigger, and to queries
    in BEFORE/AFTER triggers fired after this!
   </para>
  </sect1>

  <sect1>
   <title>Examples</title>

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

   <para>
    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).

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

HeapTuple		trigf(void);

HeapTuple
trigf()
{
	TupleDesc	tupdesc;
	HeapTuple	rettuple;
	char		*when;
	bool		checknull = false;
	bool		isnull;
	int		ret, i;

	if (!CurrentTriggerData)
		elog(NOTICE, "trigf: triggers are not initialized");
	
	/* tuple to return to Executor */
	if (TRIGGER_FIRED_BY_UPDATE(CurrentTriggerData->tg_event))
		rettuple = CurrentTriggerData->tg_newtuple;
	else
		rettuple = CurrentTriggerData->tg_trigtuple;
	
	/* check for NULLs ? */
	if (!TRIGGER_FIRED_BY_DELETE(CurrentTriggerData->tg_event) &&
		TRIGGER_FIRED_BEFORE(CurrentTriggerData->tg_event))
		checknull = true;
	
	if (TRIGGER_FIRED_BEFORE(CurrentTriggerData->tg_event))
		when = "before";
	else
		when = "after ";
	
	tupdesc = CurrentTriggerData->tg_relation->rd_att;
	CurrentTriggerData = NULL;
	
	/* 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, &amp;isnull);
	
	elog (NOTICE, "trigf (fired %s): there are %d tuples in ttest", when, i);
	
	SPI_finish();
	
	if (checknull)
	{
		i = SPI_getbinval(rettuple, tupdesc, 1, &amp;isnull);
		if (isnull)
			rettuple = NULL;
	}

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

   <para>
    Now, compile and 
    create table ttest (x int4):

    <programlisting>
create function trigf () returns opaque as 
'...path_to_so' language 'c';
    </programlisting>

    <programlisting>
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)
    </programlisting>

   </para>
  </sect1>
 </chapter>

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