<Chapter>
<Title>Triggers</Title>

<Para>
   While the current version of <ProductName>Postgres</ProductName> has various client interfaces
such as Perl, Tcl, Python and C, it lacks an actual <FirstTerm>Procedural Language</FirstTerm>
(PL).  We hope to have a proper PL one day.  In the meantime it is possible
to call C functions as trigger actions.  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 &lt;trigger name&gt; &lt;BEFORE|AFTER&gt; &lt;INSERT|DELETE|UPDATE&gt;
       ON &lt;relation name&gt; FOR EACH &lt;ROW|STATEMENT&gt;
       EXECUTE PROCEDURE &lt;procedure name&gt; (&lt;function args&gt;);
</ProgramListing>
</Para>

<Para>
   The name of the trigger is used if you ever have to delete the trigger.
It is used as an argument to the DROP TRIGGER command.
</Para>

<Para>
   The next word determines whether the function is called before or after
the event.
</Para>

<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>

<Para>
   The relation name determines which table the event applies to.
</Para>

<Para>
   The FOR EACH statement determines whether the trigger is fired for each
affected row or before (or after) the entire statement has completed.
</Para>

<Para>
   The procedure name is the C function called.
</Para>

<Para>
   The args are 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, function 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>

<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:

   - return NULL to skip the operation for the current tuple (and so the
     tuple will not be inserted/updated/deleted);
   - 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>

<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>

<ProgramListing>
tg_event 
   describes event for which the function is called. You may use the
   following macros to examine tg_event:

   TRIGGER_FIRED_BEFORE(event) returns TRUE if trigger fired BEFORE;
   TRIGGER_FIRED_AFTER(event) returns TRUE if trigger fired AFTER;
   TRIGGER_FIRED_FOR_ROW(event) returns TRUE if trigger fired for
                                ROW-level event;
   TRIGGER_FIRED_FOR_STATEMENT(event) returns TRUE if trigger fired for
                                STATEMENT-level event;
   TRIGGER_FIRED_BY_INSERT(event) returns TRUE if trigger fired by INSERT;
   TRIGGER_FIRED_BY_DELETE(event) returns TRUE if trigger fired by DELETE;
   TRIGGER_FIRED_BY_UPDATE(event) returns TRUE if trigger fired by UPDATE.

tg_relation
   is 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).

tg_trigtuple
   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.

tg_newtuple
   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.

tg_trigger
   is pointer to structure Trigger defined in src/include/utils/rel.h:

typedef struct Trigger
{
	char		*tgname;
	Oid		tgfoid;
	func_ptr	tgfunc;
	int16		tgtype;
	int16		tgnargs;
	int16		tgattr[8];
	char		**tgargs;
} Trigger;

   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.
</ProgramListing>
</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:

<ProgramListing>
   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.
</ProgramListing>
</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 in src/test/regress/regress.c and
in contrib/spi.
</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(WARN, "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(WARN, "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(WARN, "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);
create function trigf () returns opaque as 
'...path_to_so' language 'c';

<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>
