CREATE TRIGGER(7) PostgreSQL 10.1 Documentation CREATE TRIGGER(7)NAMECREATE_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 ] ]
[ REFERENCING { { OLD | NEW } TABLE [ AS ] transition_relation_name } [ ... ] ]
[ 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 operations are performed
on that table.
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 │
├───────────┼──────────────────────┼────────────────┼─────────────────┤
│ │ 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
plain tables (not foreign 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.
The REFERENCING option enables collection of transition relations,
which are row sets that include all of the rows inserted, deleted, or
modified by the current SQL statement. This feature lets the trigger
see a global view of what the statement did, not just one row at a
time. This option is only allowed for an AFTER trigger that is not a
constraint trigger; also, if the trigger is an UPDATE trigger, it must
not specify a column_name list. OLD TABLE may only be specified once,
and only for a trigger that can fire on UPDATE or DELETE; it creates a
transition relation containing the before-images of all rows updated or
deleted by the statement. Similarly, NEW TABLE may only be specified
once, and only for a trigger that can fire on UPDATE or INSERT; it
creates a transition relation containing the after-images of all rows
updated or inserted by the statement.
SELECT does not modify any rows so you cannot create SELECT triggers.
Rules and views may provide workable solutions to problems that seem to
need SELECT triggers.
Refer to Chapter 38 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, except when transition relations are requested.
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 allow a list of columns. A column
list cannot be specified when requesting transition relations,
either.
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.
REFERENCING
This keyword immediately precedes the declaration of one or two
relation names that provide access to the transition relations of
the triggering statement.
OLD TABLE
NEW TABLE
This clause indicates whether the following relation name is for
the before-image transition relation or the after-image transition
relation.
transition_relation_name
The (unqualified) name to be used within the trigger for this
transition relation.
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.
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 some cases it is possible for a single SQL command to fire more than
one kind of trigger. For instance an INSERT with an ON CONFLICT DO
UPDATE clause may cause both insert and update operations, so it will
fire both kinds of triggers as needed. The transition relations
supplied to triggers are specific to their event type; thus an INSERT
trigger will see only the inserted rows, while an UPDATE trigger will
see only the updated rows.
Row updates or deletions caused by foreign-key enforcement actions,
such as ON UPDATE CASCADE or ON DELETE SET NULL, are treated as part of
the SQL command that caused them (note that such actions are never
deferred). Relevant triggers on the affected table will be fired, so
that this provides another way in which a SQL command might fire
triggers not directly matching its type. In simple cases, triggers that
request transition relations will see all changes caused in their table
by a single original SQL command as a single transition relation.
However, there are cases in which the presence of an AFTER ROW trigger
that requests transition relations will cause the foreign-key
enforcement actions triggered by a single SQL command to be split into
multiple steps, each with its own transition relation(s). In such
cases, any statement-level triggers that are present will be fired once
per creation of a transition relation set, ensuring that the triggers
see each affected row in a transition relation once and only once.
Modifying a partitioned table or a table with inheritance children
fires statement-level triggers directly attached to that table, but not
statement-level triggers for its partitions or child tables. In
contrast, row-level triggers are fired for all affected partitions or
child tables. If a statement-level trigger has been defined with
transition relations named by a REFERENCING clause, then before and
after images of rows are visible from all affected partitions or child
tables. In the case of inheritance children, the row images include
only columns that are present in the table that the trigger is attached
to. Currently, row-level triggers with transition relations cannot be
defined on partitions or inheritance child tables.
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();
Execute the function check_transfer_balances_to_zero for each statement
to confirm that the transfer rows offset to a net of zero:
CREATE TRIGGER transfer_insert
AFTER INSERT ON transfer
REFERENCING NEW TABLE AS inserted
FOR EACH STATEMENT
EXECUTE PROCEDURE check_transfer_balances_to_zero();
Execute the function check_matching_pairs for each row to confirm that
changes are made to matching pairs at the same time (by the same
statement):
CREATE TRIGGER paired_items_update
AFTER UPDATE ON paired_items
REFERENCING NEW TABLE AS newtab OLD TABLE AS oldtab
FOR EACH ROW
EXECUTE PROCEDURE check_matching_pairs();
Section 38.4 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:
· While transition table names for AFTER triggers are specified using
the REFERENCING clause in the standard way, the row variables used
in FOR EACH ROW triggers may not be specified in a REFERENCING
clause. They are available in a manner that is dependent on the
language in which the trigger function is written, but is fixed for
any one language. Some languages effectively behave as though there
is a REFERENCING clause containing OLD ROW AS OLD NEW ROW AS NEW.
· The standard allows transition tables to be used with
column-specific UPDATE triggers, but then the set of rows that
should be visible in the transition tables depends on the trigger's
column list. This is not currently implemented by PostgreSQL.
· 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))
PostgreSQL 10.1 2017 CREATE TRIGGER(7)
