CREATE TRIGGER — define a new trigger
CREATE [ CONSTRAINT ] TRIGGERname
{ BEFORE | AFTER | INSTEAD OF } {event
[ OR ... ] } ONtable_name
[ FROMreferenced_table_name
] [ NOT DEFERRABLE | [ DEFERRABLE ] { INITIALLY IMMEDIATE | INITIALLY DEFERRED } ] [ FOR [ EACH ] { ROW | STATEMENT } ] [ WHEN (condition
) ] EXECUTE PROCEDUREfunction_name
(arguments
) whereevent
can be one of: INSERT UPDATE [ OFcolumn_name
[, ... ] ] DELETE TRUNCATE
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).
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(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 for more information about triggers.
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 OFcolumn_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(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.
or
column_name
NEW.
respectively.
Of course, column_name
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.
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(7) 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 UPDATE
command's column_name
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.
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();
the section called “A Complete Trigger Example” 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:
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.