1CREATE TRIGGER(7) PostgreSQL 15.4 Documentation CREATE TRIGGER(7)
2
6 CREATE_TRIGGER - define a new trigger
7
9 CREATE [ OR REPLACE ] [ CONSTRAINT ] TRIGGER name { BEFORE | AFTER | INSTEAD OF } { event [ OR ... ] }
10 ON table_name
11 [ FROM referenced_table_name ]
12 [ NOT DEFERRABLE | [ DEFERRABLE ] [ INITIALLY IMMEDIATE | INITIALLY DEFERRED ] ]
13 [ REFERENCING { { OLD | NEW } TABLE [ AS ] transition_relation_name } [ ... ] ]
14 [ FOR [ EACH ] { ROW | STATEMENT } ]
15 [ WHEN ( condition ) ]
16 EXECUTE { FUNCTION | PROCEDURE } function_name ( arguments )
17 where event can be one of:
19 INSERT
21 UPDATE [ OF column_name [, ... ] ]
22 DELETE
23 TRUNCATE
24
26 CREATE TRIGGER creates a new trigger. CREATE OR REPLACE TRIGGER will
27 either create a new trigger, or replace an existing trigger. The
28 trigger will be associated with the specified table, view, or foreign
29 table and will execute the specified function function_name when
30 certain operations are performed on that table.
31 To replace the current definition of an existing trigger, use CREATE OR
33 REPLACE TRIGGER, specifying the existing trigger's name and parent
34 table. All other properties are replaced.
35 The trigger can be specified to fire before the operation is attempted
37 on a row (before constraints are checked and the INSERT, UPDATE, or
38 DELETE is attempted); or after the operation has completed (after
39 constraints are checked and the INSERT, UPDATE, or DELETE has
40 completed); or instead of the operation (in the case of inserts,
41 updates or deletes on a view). If the trigger fires before or instead
42 of the event, the trigger can skip the operation for the current row,
43 or change the row being inserted (for INSERT and UPDATE operations
44 only). If the trigger fires after the event, all changes, including the
45 effects of other triggers, are “visible” to the trigger.
46 A trigger that is marked FOR EACH ROW is called once for every row that
48 the operation modifies. For example, a DELETE that affects 10 rows will
49 cause any ON DELETE triggers on the target relation to be called 10
50 separate times, once for each deleted row. In contrast, a trigger that
51 is marked FOR EACH STATEMENT only executes once for any given
52 operation, regardless of how many rows it modifies (in particular, an
53 operation that modifies zero rows will still result in the execution of
54 any applicable FOR EACH STATEMENT triggers).
55 Triggers that are specified to fire INSTEAD OF the trigger event must
57 be marked FOR EACH ROW, and can only be defined on views. BEFORE and
58 AFTER triggers on a view must be marked as FOR EACH STATEMENT.
59 In addition, triggers may be defined to fire for TRUNCATE, though only
61 FOR EACH STATEMENT.
62 The following table summarizes which types of triggers may be used on
64 tables, views, and foreign tables:
65 ┌───────────┬──────────────────────┬────────────────┬─────────────────┐
67 │When │ Event │ Row-level │ Statement-level │
68 ├───────────┼──────────────────────┼────────────────┼─────────────────┤
69 │ │ INSERT/UPDATE/DELETE │ Tables and │ Tables, views, │
70 │ │ │ foreign tables │ and foreign │
71 │ BEFORE │ │ │ tables │
72 │ ├──────────────────────┼────────────────┼─────────────────┤
73 │ │ TRUNCATE │ — │ Tables │
74 ├───────────┼──────────────────────┼────────────────┼─────────────────┤
75 │ │ INSERT/UPDATE/DELETE │ Tables and │ Tables, views, │
76 │ │ │ foreign tables │ and foreign │
77 │ AFTER │ │ │ tables │
78 │ ├──────────────────────┼────────────────┼─────────────────┤
79 │ │ TRUNCATE │ — │ Tables │
80 ├───────────┼──────────────────────┼────────────────┼─────────────────┤
81 │ │ INSERT/UPDATE/DELETE │ Views │ — │
82 │INSTEAD OF ├──────────────────────┼────────────────┼─────────────────┤
83 │ │ TRUNCATE │ — │ — │
84 └───────────┴──────────────────────┴────────────────┴─────────────────┘
85 Also, a trigger definition can specify a Boolean WHEN condition, which
87 will be tested to see whether the trigger should be fired. In row-level
88 triggers the WHEN condition can examine the old and/or new values of
89 columns of the row. Statement-level triggers can also have WHEN
90 conditions, although the feature is not so useful for them since the
91 condition cannot refer to any values in the table.
92 If multiple triggers of the same kind are defined for the same event,
94 they will be fired in alphabetical order by name.
95 When the CONSTRAINT option is specified, this command creates a
97 constraint trigger. This is the same as a regular trigger except that
98 the timing of the trigger firing can be adjusted using SET CONSTRAINTS.
99 Constraint triggers must be AFTER ROW triggers on plain tables (not
100 foreign tables). They can be fired either at the end of the statement
101 causing the triggering event, or at the end of the containing
102 transaction; in the latter case they are said to be deferred. A pending
103 deferred-trigger firing can also be forced to happen immediately by
104 using SET CONSTRAINTS. Constraint triggers are expected to raise an
105 exception when the constraints they implement are violated.
106 The REFERENCING option enables collection of transition relations,
108 which are row sets that include all of the rows inserted, deleted, or
109 modified by the current SQL statement. This feature lets the trigger
110 see a global view of what the statement did, not just one row at a
111 time. This option is only allowed for an AFTER trigger that is not a
112 constraint trigger; also, if the trigger is an UPDATE trigger, it must
113 not specify a column_name list. OLD TABLE may only be specified once,
114 and only for a trigger that can fire on UPDATE or DELETE; it creates a
115 transition relation containing the before-images of all rows updated or
116 deleted by the statement. Similarly, NEW TABLE may only be specified
117 once, and only for a trigger that can fire on UPDATE or INSERT; it
118 creates a transition relation containing the after-images of all rows
119 updated or inserted by the statement.
120 SELECT does not modify any rows so you cannot create SELECT triggers.
122 Rules and views may provide workable solutions to problems that seem to
123 need SELECT triggers.
124 Refer to Chapter 39 for more information about triggers.
126
128 name
129 The name to give the new trigger. This must be distinct from the
130 name of any other trigger for the same table. The name cannot be
131 schema-qualified — the trigger inherits the schema of its table.
132 For a constraint trigger, this is also the name to use when
133 modifying the trigger's behavior using SET CONSTRAINTS.
134 BEFORE
136 AFTER
137 INSTEAD OF
138 Determines whether the function is called before, after, or instead
139 of the event. A constraint trigger can only be specified as AFTER.
140 event
142 One of INSERT, UPDATE, DELETE, or TRUNCATE; this specifies the
143 event that will fire the trigger. Multiple events can be specified
144 using OR, except when transition relations are requested.
145 For UPDATE events, it is possible to specify a list of columns
147 using this syntax:
148 UPDATE OF column_name1 [, column_name2 ... ]
150 The trigger will only fire if at least one of the listed columns is
152 mentioned as a target of the UPDATE command or if one of the listed
153 columns is a generated column that depends on a column that is the
154 target of the UPDATE.
155 INSTEAD OF UPDATE events do not allow a list of columns. A column
157 list cannot be specified when requesting transition relations,
158 either.
159 table_name
161 The name (optionally schema-qualified) of the table, view, or
162 foreign table the trigger is for.
163 referenced_table_name
165 The (possibly schema-qualified) name of another table referenced by
166 the constraint. This option is used for foreign-key constraints and
167 is not recommended for general use. This can only be specified for
168 constraint triggers.
169 DEFERRABLE
171 NOT DEFERRABLE
172 INITIALLY IMMEDIATE
173 INITIALLY DEFERRED
174 The default timing of the trigger. See the CREATE TABLE
175 (CREATE_TABLE(7)) documentation for details of these constraint
176 options. This can only be specified for constraint triggers.
177 REFERENCING
179 This keyword immediately precedes the declaration of one or two
180 relation names that provide access to the transition relations of
181 the triggering statement.
182 OLD TABLE
184 NEW TABLE
185 This clause indicates whether the following relation name is for
186 the before-image transition relation or the after-image transition
187 relation.
188 transition_relation_name
190 The (unqualified) name to be used within the trigger for this
191 transition relation.
192 FOR EACH ROW
194 FOR EACH STATEMENT
195 This specifies whether the trigger function should be fired once
196 for every row affected by the trigger event, or just once per SQL
197 statement. If neither is specified, FOR EACH STATEMENT is the
198 default. Constraint triggers can only be specified FOR EACH ROW.
199 condition
201 A Boolean expression that determines whether the trigger function
202 will actually be executed. If WHEN is specified, the function will
203 only be called if the condition returns true. In FOR EACH ROW
204 triggers, the WHEN condition can refer to columns of the old and/or
205 new row values by writing OLD.column_name or NEW.column_name
206 respectively. Of course, INSERT triggers cannot refer to OLD and
207 DELETE triggers cannot refer to NEW.
208 INSTEAD OF triggers do not support WHEN conditions.
210 Currently, WHEN expressions cannot contain subqueries.
212 Note that for constraint triggers, evaluation of the WHEN condition
214 is not deferred, but occurs immediately after the row update
215 operation is performed. If the condition does not evaluate to true
216 then the trigger is not queued for deferred execution.
217 function_name
219 A user-supplied function that is declared as taking no arguments
220 and returning type trigger, which is executed when the trigger
221 fires.
222 In the syntax of CREATE TRIGGER, the keywords FUNCTION and
224 PROCEDURE are equivalent, but the referenced function must in any
225 case be a function, not a procedure. The use of the keyword
226 PROCEDURE here is historical and deprecated.
227 arguments
229 An optional comma-separated list of arguments to be provided to the
230 function when the trigger is executed. The arguments are literal
231 string constants. Simple names and numeric constants can be written
232 here, too, but they will all be converted to strings. Please check
233 the description of the implementation language of the trigger
234 function to find out how these arguments can be accessed within the
235 function; it might be different from normal function arguments.
236
238 To create or replace a trigger on a table, the user must have the
239 TRIGGER privilege on the table. The user must also have EXECUTE
240 privilege on the trigger function.
241 Use DROP TRIGGER to remove a trigger.
243 Creating a row-level trigger on a partitioned table will cause an
245 identical “clone” trigger to be created on each of its existing
246 partitions; and any partitions created or attached later will have an
247 identical trigger, too. If there is a conflictingly-named trigger on a
248 child partition already, an error occurs unless CREATE OR REPLACE
249 TRIGGER is used, in which case that trigger is replaced with a clone
250 trigger. When a partition is detached from its parent, its clone
251 triggers are removed.
252 A column-specific trigger (one defined using the UPDATE OF column_name
254 syntax) will fire when any of its columns are listed as targets in the
255 UPDATE command's SET list. It is possible for a column's value to
256 change even when the trigger is not fired, because changes made to the
257 row's contents by BEFORE UPDATE triggers are not considered.
258 Conversely, a command such as UPDATE ... SET x = x ... will fire a
259 trigger on column x, even though the column's value did not change.
260 In a BEFORE trigger, the WHEN condition is evaluated just before the
262 function is or would be executed, so using WHEN is not materially
263 different from testing the same condition at the beginning of the
264 trigger function. Note in particular that the NEW row seen by the
265 condition is the current value, as possibly modified by earlier
266 triggers. Also, a BEFORE trigger's WHEN condition is not allowed to
267 examine the system columns of the NEW row (such as ctid), because those
268 won't have been set yet.
269 In an AFTER trigger, the WHEN condition is evaluated just after the row
271 update occurs, and it determines whether an event is queued to fire the
272 trigger at the end of statement. So when an AFTER trigger's WHEN
273 condition does not return true, it is not necessary to queue an event
274 nor to re-fetch the row at end of statement. This can result in
275 significant speedups in statements that modify many rows, if the
276 trigger only needs to be fired for a few of the rows.
277 In some cases it is possible for a single SQL command to fire more than
279 one kind of trigger. For instance an INSERT with an ON CONFLICT DO
280 UPDATE clause may cause both insert and update operations, so it will
281 fire both kinds of triggers as needed. The transition relations
282 supplied to triggers are specific to their event type; thus an INSERT
283 trigger will see only the inserted rows, while an UPDATE trigger will
284 see only the updated rows.
285 Row updates or deletions caused by foreign-key enforcement actions,
287 such as ON UPDATE CASCADE or ON DELETE SET NULL, are treated as part of
288 the SQL command that caused them (note that such actions are never
289 deferred). Relevant triggers on the affected table will be fired, so
290 that this provides another way in which an SQL command might fire
291 triggers not directly matching its type. In simple cases, triggers that
292 request transition relations will see all changes caused in their table
293 by a single original SQL command as a single transition relation.
294 However, there are cases in which the presence of an AFTER ROW trigger
295 that requests transition relations will cause the foreign-key
296 enforcement actions triggered by a single SQL command to be split into
297 multiple steps, each with its own transition relation(s). In such
298 cases, any statement-level triggers that are present will be fired once
299 per creation of a transition relation set, ensuring that the triggers
300 see each affected row in a transition relation once and only once.
301 Statement-level triggers on a view are fired only if the action on the
303 view is handled by a row-level INSTEAD OF trigger. If the action is
304 handled by an INSTEAD rule, then whatever statements are emitted by the
305 rule are executed in place of the original statement naming the view,
306 so that the triggers that will be fired are those on tables named in
307 the replacement statements. Similarly, if the view is automatically
308 updatable, then the action is handled by automatically rewriting the
309 statement into an action on the view's base table, so that the base
310 table's statement-level triggers are the ones that are fired.
311 Modifying a partitioned table or a table with inheritance children
313 fires statement-level triggers attached to the explicitly named table,
314 but not statement-level triggers for its partitions or child tables. In
315 contrast, row-level triggers are fired on the rows in affected
316 partitions or child tables, even if they are not explicitly named in
317 the query. If a statement-level trigger has been defined with
318 transition relations named by a REFERENCING clause, then before and
319 after images of rows are visible from all affected partitions or child
320 tables. In the case of inheritance children, the row images include
321 only columns that are present in the table that the trigger is attached
322 to.
323 Currently, row-level triggers with transition relations cannot be
325 defined on partitions or inheritance child tables. Also, triggers on
326 partitioned tables may not be INSTEAD OF.
327 Currently, the OR REPLACE option is not supported for constraint
329 triggers.
330 Replacing an existing trigger within a transaction that has already
332 performed updating actions on the trigger's table is not recommended.
333 Trigger firing decisions, or portions of firing decisions, that have
334 already been made will not be reconsidered, so the effects could be
335 surprising.
336 There are a few built-in trigger functions that can be used to solve
338 common problems without having to write your own trigger code; see
339 Section 9.28.
340
342 Execute the function check_account_update whenever a row of the table
343 accounts is about to be updated:
344 CREATE TRIGGER check_update
346 BEFORE UPDATE ON accounts
347 FOR EACH ROW
348 EXECUTE FUNCTION check_account_update();
349 Modify that trigger definition to only execute the function if column
351 balance is specified as a target in the UPDATE command:
352 CREATE OR REPLACE TRIGGER check_update
354 BEFORE UPDATE OF balance ON accounts
355 FOR EACH ROW
356 EXECUTE FUNCTION check_account_update();
357 This form only executes the function if column balance has in fact
359 changed value:
360 CREATE TRIGGER check_update
362 BEFORE UPDATE ON accounts
363 FOR EACH ROW
364 WHEN (OLD.balance IS DISTINCT FROM NEW.balance)
365 EXECUTE FUNCTION check_account_update();
366 Call a function to log updates of accounts, but only if something
368 changed:
369 CREATE TRIGGER log_update
371 AFTER UPDATE ON accounts
372 FOR EACH ROW
373 WHEN (OLD.* IS DISTINCT FROM NEW.*)
374 EXECUTE FUNCTION log_account_update();
375 Execute the function view_insert_row for each row to insert rows into
377 the tables underlying a view:
378 CREATE TRIGGER view_insert
380 INSTEAD OF INSERT ON my_view
381 FOR EACH ROW
382 EXECUTE FUNCTION view_insert_row();
383 Execute the function check_transfer_balances_to_zero for each statement
385 to confirm that the transfer rows offset to a net of zero:
386 CREATE TRIGGER transfer_insert
388 AFTER INSERT ON transfer
389 REFERENCING NEW TABLE AS inserted
390 FOR EACH STATEMENT
391 EXECUTE FUNCTION check_transfer_balances_to_zero();
392 Execute the function check_matching_pairs for each row to confirm that
394 changes are made to matching pairs at the same time (by the same
395 statement):
396 CREATE TRIGGER paired_items_update
398 AFTER UPDATE ON paired_items
399 REFERENCING NEW TABLE AS newtab OLD TABLE AS oldtab
400 FOR EACH ROW
401 EXECUTE FUNCTION check_matching_pairs();
402 Section 39.4 contains a complete example of a trigger function written
404 in C.
405
407 The CREATE TRIGGER statement in PostgreSQL implements a subset of the
408 SQL standard. The following functionalities are currently missing:
409 • While transition table names for AFTER triggers are specified using
411 the REFERENCING clause in the standard way, the row variables used
412 in FOR EACH ROW triggers may not be specified in a REFERENCING
413 clause. They are available in a manner that is dependent on the
414 language in which the trigger function is written, but is fixed for
415 any one language. Some languages effectively behave as though there
416 is a REFERENCING clause containing OLD ROW AS OLD NEW ROW AS NEW.
417 • The standard allows transition tables to be used with
419 column-specific UPDATE triggers, but then the set of rows that
420 should be visible in the transition tables depends on the trigger's
421 column list. This is not currently implemented by PostgreSQL.
422 • PostgreSQL only allows the execution of a user-defined function for
424 the triggered action. The standard allows the execution of a number
425 of other SQL commands, such as CREATE TABLE, as the triggered
426 action. This limitation is not hard to work around by creating a
427 user-defined function that executes the desired commands.
428 SQL specifies that multiple triggers should be fired in
430 time-of-creation order. PostgreSQL uses name order, which was judged
431 to be more convenient.
432 SQL specifies that BEFORE DELETE triggers on cascaded deletes fire
434 after the cascaded DELETE completes. The PostgreSQL behavior is for
435 BEFORE DELETE to always fire before the delete action, even a cascading
436 one. This is considered more consistent. There is also nonstandard
437 behavior if BEFORE triggers modify rows or prevent updates during an
438 update that is caused by a referential action. This can lead to
439 constraint violations or stored data that does not honor the
440 referential constraint.
441 The ability to specify multiple actions for a single trigger using OR
443 is a PostgreSQL extension of the SQL standard.
444 The ability to fire triggers for TRUNCATE is a PostgreSQL extension of
446 the SQL standard, as is the ability to define statement-level triggers
447 on views.
448 CREATE CONSTRAINT TRIGGER is a PostgreSQL extension of the SQL
450 standard. So is the OR REPLACE option.
451
453 ALTER TRIGGER (ALTER_TRIGGER(7)), DROP TRIGGER (DROP_TRIGGER(7)),
454 CREATE FUNCTION (CREATE_FUNCTION(7)), SET CONSTRAINTS
455 (SET_CONSTRAINTS(7))
456PostgreSQL 15.4 2023 CREATE TRIGGER(7)