Name
CREATE TRIGGER -- define a new triggerSynopsis
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 |
---|---|---|---|
BEFORE |
INSERT /UPDATE /DELETE
| Tables and foreign tables | Tables, views, and foreign tables |
TRUNCATE | — | Tables | |
AFTER |
INSERT /UPDATE /DELETE
| Tables and foreign tables | Tables, views, and foreign tables |
TRUNCATE | — | Tables | |
INSTEAD OF |
INSERT /UPDATE /DELETE
| Views | — |
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. 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 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
, orTRUNCATE
; this specifies the event that will fire the trigger. Multiple events can be specified usingOR
.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 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 specifiedFOR 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 thecondition
returnstrue
. InFOR EACH ROW
triggers, theWHEN
condition can refer to columns of the old and/or new row values by writingOLD.
orcolumn_name
NEW.
respectively. Of course,column_name
INSERT
triggers cannot refer toOLD
andDELETE
triggers cannot refer toNEW
.INSTEAD OF
triggers do not supportWHEN
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 to remove a trigger.
A column-specific trigger (one defined using the UPDATE OF
syntax) will fire when any of its columns are listed as targets in the column_name
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.
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.
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 37.4 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
andNEW 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.
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