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CREATE TRIGGER(7)	PostgreSQL 9.6.19 Documentation	     CREATE TRIGGER(7)

       CREATE_TRIGGER -	define a new trigger

       CREATE [	CONSTRAINT ] TRIGGER name { BEFORE | AFTER | INSTEAD OF	} { event [ OR ... ] }
	   ON table_name
	   [ FROM referenced_table_name	]
	   [ FOR [ EACH	] { ROW	| STATEMENT } ]
	   [ WHEN ( condition )	]
	   EXECUTE PROCEDURE function_name ( arguments )

       where event can be one of:

	   UPDATE [ OF column_name [, ... ] ]

       CREATE TRIGGER creates a	new trigger. The trigger will be associated
       with the	specified table, view, or foreign table	and will execute the
       specified function function_name	when certain events occur.

       The trigger can be specified to fire before the operation is attempted
       on a row	(before	constraints are	checked	and the	INSERT,	UPDATE,	or
       DELETE is attempted); or	after the operation has	completed (after
       constraints are checked and the INSERT, UPDATE, or DELETE has
       completed); or instead of the operation (in the case of inserts,
       updates or deletes on a view). If the trigger fires before or instead
       of the event, the trigger can skip the operation	for the	current	row,
       or change the row being inserted	(for INSERT and	UPDATE operations
       only). If the trigger fires after the event, all	changes, including the
       effects of other	triggers, are "visible"	to the trigger.

       A trigger that is marked	FOR EACH ROW is	called once for	every row that
       the operation modifies. For example, a DELETE that affects 10 rows will
       cause any ON DELETE triggers on the target relation to be called	10
       separate	times, once for	each deleted row. In contrast, a trigger that
       is marked FOR EACH STATEMENT only executes once for any given
       operation, regardless of	how many rows it modifies (in particular, an
       operation that modifies zero rows will still result in the execution of
       any applicable FOR EACH STATEMENT triggers). Note that with an INSERT
       with an ON CONFLICT DO UPDATE clause, both INSERT and UPDATE statement
       level trigger will be fired.

       Triggers	that are specified to fire INSTEAD OF the trigger event	must
       be marked FOR EACH ROW, and can only be defined on views.  BEFORE and
       AFTER triggers on a view	must be	marked as FOR EACH STATEMENT.

       In addition, triggers may be defined to fire for	TRUNCATE, though only

       The following table summarizes which types of triggers may be used on
       tables, views, and foreign tables:

       |When	   | Event		  | Row-level	   | Statement-level |
       |	   | INSERT/UPDATE/DELETE | Tables and	   | Tables, views,  |
       |	   |			  | foreign tables | and foreign     |
       |  BEFORE   |			  |		   | tables	     |
       |	   +----------------------+----------------+-----------------+
       |	   |	   TRUNCATE	  |	  --	   |	 Tables	     |
       |	   | INSERT/UPDATE/DELETE | Tables and	   | Tables, views,  |
       |	   |			  | foreign tables | and foreign     |
       |  AFTER	   |			  |		   | tables	     |
       |	   +----------------------+----------------+-----------------+
       |	   |	   TRUNCATE	  |	  --	   |	 Tables	     |
       |	   | INSERT/UPDATE/DELETE |	Views	   |	   --	     |
       |INSTEAD	OF +----------------------+----------------+-----------------+
       |	   |	   TRUNCATE	  |	  --	   |	   --	     |

       Also, a trigger definition can specify a	Boolean	WHEN condition,	which
       will be tested to see whether the trigger should	be fired. In row-level
       triggers	the WHEN condition can examine the old and/or new values of
       columns of the row. Statement-level triggers can	also have WHEN
       conditions, although the	feature	is not so useful for them since	the
       condition cannot	refer to any values in the table.

       If multiple triggers of the same	kind are defined for the same event,
       they will be fired in alphabetical order	by name.

       When the	CONSTRAINT option is specified,	this command creates a
       constraint trigger. This	is the same as a regular trigger except	that
       the timing of the trigger firing	can be adjusted	using SET CONSTRAINTS
       (SET_CONSTRAINTS(7)). Constraint	triggers must be AFTER ROW triggers on
       tables. They can	be fired either	at the end of the statement causing
       the triggering event, or	at the end of the containing transaction; in
       the latter case they are	said to	be deferred. A pending
       deferred-trigger	firing can also	be forced to happen immediately	by
       using SET CONSTRAINTS. Constraint triggers are expected to raise	an
       exception when the constraints they implement are violated.

       SELECT does not modify any rows so you cannot create SELECT triggers.
       Rules and views are more	appropriate in such cases.

       Refer to	Chapter	37, Triggers, in the documentation for more
       information about triggers.

	   The name to give the	new trigger. This must be distinct from	the
	   name	of any other trigger for the same table. The name cannot be
	   schema-qualified -- the trigger inherits the	schema of its table.
	   For a constraint trigger, this is also the name to use when
	   modifying the trigger's behavior using SET CONSTRAINTS.

	   Determines whether the function is called before, after, or instead
	   of the event. A constraint trigger can only be specified as AFTER.

	   One of INSERT, UPDATE, DELETE, or TRUNCATE; this specifies the
	   event that will fire	the trigger. Multiple events can be specified
	   using OR.

	   For UPDATE events, it is possible to	specify	a list of columns
	   using this syntax:

	       UPDATE OF column_name1 [, column_name2 ... ]

	   The trigger will only fire if at least one of the listed columns is
	   mentioned as	a target of the	UPDATE command.

	   INSTEAD OF UPDATE events do not support lists of columns.

	   The name (optionally	schema-qualified) of the table,	view, or
	   foreign table the trigger is	for.

	   The (possibly schema-qualified) name	of another table referenced by
	   the constraint. This	option is used for foreign-key constraints and
	   is not recommended for general use. This can	only be	specified for
	   constraint triggers.

	   The default timing of the trigger. See the CREATE TABLE
	   (CREATE_TABLE(7)) documentation for details of these	constraint
	   options. This can only be specified for constraint triggers.

	   This	specifies whether the trigger procedure	should be fired	once
	   for every row affected by the trigger event,	or just	once per SQL
	   statement. If neither is specified, FOR EACH	STATEMENT is the
	   default. Constraint triggers	can only be specified FOR EACH ROW.

	   A Boolean expression	that determines	whether	the trigger function
	   will	actually be executed. If WHEN is specified, the	function will
	   only	be called if the condition returns true. In FOR	EACH ROW
	   triggers, the WHEN condition	can refer to columns of	the old	and/or
	   new row values by writing OLD.column_name or	NEW.column_name
	   respectively. Of course, INSERT triggers cannot refer to OLD	and
	   DELETE triggers cannot refer	to NEW.

	   INSTEAD OF triggers do not support WHEN conditions.

	   Currently, WHEN expressions cannot contain subqueries.

	   Note	that for constraint triggers, evaluation of the	WHEN condition
	   is not deferred, but	occurs immediately after the row update
	   operation is	performed. If the condition does not evaluate to true
	   then	the trigger is not queued for deferred execution.

	   A user-supplied function that is declared as	taking no arguments
	   and returning type trigger, which is	executed when the trigger

	   An optional comma-separated list of arguments to be provided	to the
	   function when the trigger is	executed. The arguments	are literal
	   string constants. Simple names and numeric constants	can be written
	   here, too, but they will all	be converted to	strings. Please	check
	   the description of the implementation language of the trigger
	   function to find out	how these arguments can	be accessed within the
	   function; it	might be different from	normal function	arguments.

       To create a trigger on a	table, the user	must have the TRIGGER
       privilege on the	table. The user	must also have EXECUTE privilege on
       the trigger function.

       Use DROP	TRIGGER	(DROP_TRIGGER(7)) to remove a trigger.

       A column-specific trigger (one defined using the	UPDATE OF column_name
       syntax) will fire when any of its columns are listed as targets in the
       UPDATE command's	SET list. It is	possible for a column's	value to
       change even when	the trigger is not fired, because changes made to the
       row's contents by BEFORE	UPDATE triggers	are not	considered.
       Conversely, a command such as UPDATE ...	SET x =	x ...  will fire a
       trigger on column x, even though	the column's value did not change.

       There are a few built-in	trigger	functions that can be used to solve
       common problems without having to write your own	trigger	code; see
       Section 9.27, "Trigger Functions", in the documentation.

       In a BEFORE trigger, the	WHEN condition is evaluated just before	the
       function	is or would be executed, so using WHEN is not materially
       different from testing the same condition at the	beginning of the
       trigger function. Note in particular that the NEW row seen by the
       condition is the	current	value, as possibly modified by earlier
       triggers. Also, a BEFORE	trigger's WHEN condition is not	allowed	to
       examine the system columns of the NEW row (such as oid),	because	those
       won't have been set yet.

       In an AFTER trigger, the	WHEN condition is evaluated just after the row
       update occurs, and it determines	whether	an event is queued to fire the
       trigger at the end of statement.	So when	an AFTER trigger's WHEN
       condition does not return true, it is not necessary to queue an event
       nor to re-fetch the row at end of statement. This can result in
       significant speedups in statements that modify many rows, if the
       trigger only needs to be	fired for a few	of the rows.

       Statement-level triggers	on a view are fired only if the	action on the
       view is handled by a row-level INSTEAD OF trigger. If the action	is
       handled by an INSTEAD rule, then	whatever statements are	emitted	by the
       rule are	executed in place of the original statement naming the view,
       so that the triggers that will be fired are those on tables named in
       the replacement statements. Similarly, if the view is automatically
       updatable, then the action is handled by	automatically rewriting	the
       statement into an action	on the view's base table, so that the base
       table's statement-level triggers	are the	ones that are fired.

       In PostgreSQL versions before 7.3, it was necessary to declare trigger
       functions as returning the placeholder type opaque, rather than
       trigger.	To support loading of old dump files, CREATE TRIGGER will
       accept a	function declared as returning opaque, but it will issue a
       notice and change the function's	declared return	type to	trigger.

       Execute the function check_account_update whenever a row	of the table
       accounts	is about to be updated:

	   CREATE TRIGGER check_update
	       BEFORE UPDATE ON	accounts
	       FOR EACH	ROW
	       EXECUTE PROCEDURE check_account_update();

       The same, but only execute the function if column balance is specified
       as a target in the UPDATE command:

	   CREATE TRIGGER check_update
	       BEFORE UPDATE OF	balance	ON accounts
	       FOR EACH	ROW
	       EXECUTE PROCEDURE check_account_update();

       This form only executes the function if column balance has in fact
       changed value:

	   CREATE TRIGGER check_update
	       BEFORE UPDATE ON	accounts
	       FOR EACH	ROW
	       WHEN (OLD.balance IS DISTINCT FROM NEW.balance)
	       EXECUTE PROCEDURE check_account_update();

       Call a function to log updates of accounts, but only if something

	   CREATE TRIGGER log_update
	       AFTER UPDATE ON accounts
	       FOR EACH	ROW
	       EXECUTE PROCEDURE log_account_update();

       Execute the function view_insert_row for	each row to insert rows	into
       the tables underlying a view:

	   CREATE TRIGGER view_insert
	       INSTEAD OF INSERT ON my_view
	       FOR EACH	ROW
	       EXECUTE PROCEDURE view_insert_row();

       Section 37.4, "A	Complete Trigger Example", in the documentation
       contains	a complete example of a	trigger	function written in C.

       The CREATE TRIGGER statement in PostgreSQL implements a subset of the
       SQL standard. The following functionalities are currently missing:

       o   SQL allows you to define aliases for	the "old" and "new" rows or
	   tables for use in the definition of the triggered action (e.g.,
	   ROW AS othername ...). Since	PostgreSQL allows trigger procedures
	   to be written in any	number of user-defined languages, access to
	   the data is handled in a language-specific way.

       o   PostgreSQL does not allow the old and new tables to be referenced
	   in statement-level triggers,	i.e., the tables that contain all the
	   old and/or new rows,	which are referred to by the OLD TABLE and NEW
	   TABLE clauses in the	SQL standard.

       o   PostgreSQL only allows the execution	of a user-defined function for
	   the triggered action. The standard allows the execution of a	number
	   of other SQL	commands, such as CREATE TABLE,	as the triggered
	   action. This	limitation is not hard to work around by creating a
	   user-defined	function that executes the desired commands.

       SQL specifies that multiple triggers should be fired in
       time-of-creation	order.	PostgreSQL uses	name order, which was judged
       to be more convenient.

       SQL specifies that BEFORE DELETE	triggers on cascaded deletes fire
       after the cascaded DELETE completes. The	PostgreSQL behavior is for
       BEFORE DELETE to	always fire before the delete action, even a cascading
       one. This is considered more consistent.	There is also nonstandard
       behavior	if BEFORE triggers modify rows or prevent updates during an
       update that is caused by	a referential action. This can lead to
       constraint violations or	stored data that does not honor	the
       referential constraint.

       The ability to specify multiple actions for a single trigger using OR
       is a PostgreSQL extension of the	SQL standard.

       The ability to fire triggers for	TRUNCATE is a PostgreSQL extension of
       the SQL standard, as is the ability to define statement-level triggers
       on views.

       CREATE CONSTRAINT TRIGGER is a PostgreSQL extension of the SQL


PostgreSQL 9.6.19		     2020		     CREATE TRIGGER(7)


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