xenial (7) CREATE_TRIGGER.7.gz
NAME
CREATE_TRIGGER - define a new trigger
SYNOPSIS
CREATE [ CONSTRAINT ] TRIGGER name { BEFORE | AFTER | INSTEAD OF } { event [ OR ... ] }
ON table_name
[ FROM referenced_table_name ]
[ NOT DEFERRABLE | [ DEFERRABLE ] [ INITIALLY IMMEDIATE | INITIALLY DEFERRED ] ]
[ FOR [ EACH ] { ROW | STATEMENT } ]
[ WHEN ( condition ) ]
EXECUTE PROCEDURE function_name ( arguments )
where event can be one of:
INSERT
UPDATE [ OF column_name [, ... ] ]
DELETE
TRUNCATE
DESCRIPTION
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 FOR EACH STATEMENT.
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 foreign │ Tables, views, and │
│ BEFORE │ │ tables │ foreign tables │
│ ├──────────────────────┼────────────────────┼────────────────────┤
│ │ TRUNCATE │ — │ Tables │
├───────────┼──────────────────────┼────────────────────┼────────────────────┤
│ │ INSERT/UPDATE/DELETE │ Tables and foreign │ Tables, views, and │
│ AFTER │ │ tables │ foreign 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 36, Triggers, in the documentation for more information about triggers.
PARAMETERS
name
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.
BEFORE
AFTER
INSTEAD OF
Determines whether the function is called before, after, or instead of the event. A constraint
trigger can only be specified as AFTER.
event
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.
table_name
The name (optionally schema-qualified) of the table, view, or foreign table the trigger is for.
referenced_table_name
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.
DEFERRABLE
NOT DEFERRABLE
INITIALLY IMMEDIATE
INITIALLY DEFERRED
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.
FOR EACH ROW
FOR EACH STATEMENT
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.
condition
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.
function_name
A user-supplied function that is declared as taking no arguments and returning type trigger, which is
executed when the trigger fires.
arguments
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.
NOTES
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.
EXAMPLES
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 changed:
CREATE TRIGGER log_update
AFTER UPDATE ON accounts
FOR EACH ROW
WHEN (OLD.* IS DISTINCT FROM NEW.*)
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 36.4, “A Complete Trigger Example”, in the documentation contains a complete example of a trigger
function written in C.
COMPATIBILITY
The CREATE TRIGGER statement in PostgreSQL implements a subset of the SQL standard. The following
functionalities are currently missing:
• 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., CREATE TRIGGER ... ON tablename REFERENCING OLD ROW AS somename NEW 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.
• 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.
• 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 standard.
SEE ALSO
ALTER TRIGGER (ALTER_TRIGGER(7)), DROP TRIGGER (DROP_TRIGGER(7)), CREATE FUNCTION (CREATE_FUNCTION(7)), SET CONSTRAINTS (SET_CONSTRAINTS(7))