1CREATE TRIGGER(7) PostgreSQL 16.1 Documentation CREATE TRIGGER(7)
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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 and │
74 │ │ │ │ foreign tables │
75 ├───────────┼──────────────────────┼────────────────┼─────────────────┤
76 │ │ INSERT/UPDATE/DELETE │ Tables and │ Tables, views, │
77 │ │ │ foreign tables │ and foreign │
78 │ AFTER │ │ │ tables │
79 │ ├──────────────────────┼────────────────┼─────────────────┤
80 │ │ TRUNCATE │ — │ Tables and │
81 │ │ │ │ foreign tables │
82 ├───────────┼──────────────────────┼────────────────┼─────────────────┤
83 │ │ INSERT/UPDATE/DELETE │ Views │ — │
84 │INSTEAD OF ├──────────────────────┼────────────────┼─────────────────┤
85 │ │ TRUNCATE │ — │ — │
86 └───────────┴──────────────────────┴────────────────┴─────────────────┘
87 Also, a trigger definition can specify a Boolean WHEN condition, which
89 will be tested to see whether the trigger should be fired. In row-level
90 triggers the WHEN condition can examine the old and/or new values of
91 columns of the row. Statement-level triggers can also have WHEN
92 conditions, although the feature is not so useful for them since the
93 condition cannot refer to any values in the table.
94 If multiple triggers of the same kind are defined for the same event,
96 they will be fired in alphabetical order by name.
97 When the CONSTRAINT option is specified, this command creates a
99 constraint trigger. This is the same as a regular trigger except that
100 the timing of the trigger firing can be adjusted using SET CONSTRAINTS.
101 Constraint triggers must be AFTER ROW triggers on plain tables (not
102 foreign tables). They can be fired either at the end of the statement
103 causing the triggering event, or at the end of the containing
104 transaction; in the latter case they are said to be deferred. A pending
105 deferred-trigger firing can also be forced to happen immediately by
106 using SET CONSTRAINTS. Constraint triggers are expected to raise an
107 exception when the constraints they implement are violated.
108 The REFERENCING option enables collection of transition relations,
110 which are row sets that include all of the rows inserted, deleted, or
111 modified by the current SQL statement. This feature lets the trigger
112 see a global view of what the statement did, not just one row at a
113 time. This option is only allowed for an AFTER trigger that is not a
114 constraint trigger; also, if the trigger is an UPDATE trigger, it must
115 not specify a column_name list. OLD TABLE may only be specified once,
116 and only for a trigger that can fire on UPDATE or DELETE; it creates a
117 transition relation containing the before-images of all rows updated or
118 deleted by the statement. Similarly, NEW TABLE may only be specified
119 once, and only for a trigger that can fire on UPDATE or INSERT; it
120 creates a transition relation containing the after-images of all rows
121 updated or inserted by the statement.
122 SELECT does not modify any rows so you cannot create SELECT triggers.
124 Rules and views may provide workable solutions to problems that seem to
125 need SELECT triggers.
126 Refer to Chapter 39 for more information about triggers.
128
130 name
131 The name to give the new trigger. This must be distinct from the
132 name of any other trigger for the same table. The name cannot be
133 schema-qualified — the trigger inherits the schema of its table.
134 For a constraint trigger, this is also the name to use when
135 modifying the trigger's behavior using SET CONSTRAINTS.
136 BEFORE
138 AFTER
139 INSTEAD OF
140 Determines whether the function is called before, after, or instead
141 of the event. A constraint trigger can only be specified as AFTER.
142 event
144 One of INSERT, UPDATE, DELETE, or TRUNCATE; this specifies the
145 event that will fire the trigger. Multiple events can be specified
146 using OR, except when transition relations are requested.
147 For UPDATE events, it is possible to specify a list of columns
149 using this syntax:
150 UPDATE OF column_name1 [, column_name2 ... ]
152 The trigger will only fire if at least one of the listed columns is
154 mentioned as a target of the UPDATE command or if one of the listed
155 columns is a generated column that depends on a column that is the
156 target of the UPDATE.
157 INSTEAD OF UPDATE events do not allow a list of columns. A column
159 list cannot be specified when requesting transition relations,
160 either.
161 table_name
163 The name (optionally schema-qualified) of the table, view, or
164 foreign table the trigger is for.
165 referenced_table_name
167 The (possibly schema-qualified) name of another table referenced by
168 the constraint. This option is used for foreign-key constraints and
169 is not recommended for general use. This can only be specified for
170 constraint triggers.
171 DEFERRABLE
173 NOT DEFERRABLE
174 INITIALLY IMMEDIATE
175 INITIALLY DEFERRED
176 The default timing of the trigger. See the CREATE TABLE
177 (CREATE_TABLE(7)) documentation for details of these constraint
178 options. This can only be specified for constraint triggers.
179 REFERENCING
181 This keyword immediately precedes the declaration of one or two
182 relation names that provide access to the transition relations of
183 the triggering statement.
184 OLD TABLE
186 NEW TABLE
187 This clause indicates whether the following relation name is for
188 the before-image transition relation or the after-image transition
189 relation.
190 transition_relation_name
192 The (unqualified) name to be used within the trigger for this
193 transition relation.
194 FOR EACH ROW
196 FOR EACH STATEMENT
197 This specifies whether the trigger function should be fired once
198 for every row affected by the trigger event, or just once per SQL
199 statement. If neither is specified, FOR EACH STATEMENT is the
200 default. Constraint triggers can only be specified FOR EACH ROW.
201 condition
203 A Boolean expression that determines whether the trigger function
204 will actually be executed. If WHEN is specified, the function will
205 only be called if the condition returns true. In FOR EACH ROW
206 triggers, the WHEN condition can refer to columns of the old and/or
207 new row values by writing OLD.column_name or NEW.column_name
208 respectively. Of course, INSERT triggers cannot refer to OLD and
209 DELETE triggers cannot refer to NEW.
210 INSTEAD OF triggers do not support WHEN conditions.
212 Currently, WHEN expressions cannot contain subqueries.
214 Note that for constraint triggers, evaluation of the WHEN condition
216 is not deferred, but occurs immediately after the row update
217 operation is performed. If the condition does not evaluate to true
218 then the trigger is not queued for deferred execution.
219 function_name
221 A user-supplied function that is declared as taking no arguments
222 and returning type trigger, which is executed when the trigger
223 fires.
224 In the syntax of CREATE TRIGGER, the keywords FUNCTION and
226 PROCEDURE are equivalent, but the referenced function must in any
227 case be a function, not a procedure. The use of the keyword
228 PROCEDURE here is historical and deprecated.
229 arguments
231 An optional comma-separated list of arguments to be provided to the
232 function when the trigger is executed. The arguments are literal
233 string constants. Simple names and numeric constants can be written
234 here, too, but they will all be converted to strings. Please check
235 the description of the implementation language of the trigger
236 function to find out how these arguments can be accessed within the
237 function; it might be different from normal function arguments.
238
240 To create or replace a trigger on a table, the user must have the
241 TRIGGER privilege on the table. The user must also have EXECUTE
242 privilege on the trigger function.
243 Use DROP TRIGGER to remove a trigger.
245 Creating a row-level trigger on a partitioned table will cause an
247 identical “clone” trigger to be created on each of its existing
248 partitions; and any partitions created or attached later will have an
249 identical trigger, too. If there is a conflictingly-named trigger on a
250 child partition already, an error occurs unless CREATE OR REPLACE
251 TRIGGER is used, in which case that trigger is replaced with a clone
252 trigger. When a partition is detached from its parent, its clone
253 triggers are removed.
254 A column-specific trigger (one defined using the UPDATE OF column_name
256 syntax) will fire when any of its columns are listed as targets in the
257 UPDATE command's SET list. It is possible for a column's value to
258 change even when the trigger is not fired, because changes made to the
259 row's contents by BEFORE UPDATE triggers are not considered.
260 Conversely, a command such as UPDATE ... SET x = x ... will fire a
261 trigger on column x, even though the column's value did not change.
262 In a BEFORE trigger, the WHEN condition is evaluated just before the
264 function is or would be executed, so using WHEN is not materially
265 different from testing the same condition at the beginning of the
266 trigger function. Note in particular that the NEW row seen by the
267 condition is the current value, as possibly modified by earlier
268 triggers. Also, a BEFORE trigger's WHEN condition is not allowed to
269 examine the system columns of the NEW row (such as ctid), because those
270 won't have been set yet.
271 In an AFTER trigger, the WHEN condition is evaluated just after the row
273 update occurs, and it determines whether an event is queued to fire the
274 trigger at the end of statement. So when an AFTER trigger's WHEN
275 condition does not return true, it is not necessary to queue an event
276 nor to re-fetch the row at end of statement. This can result in
277 significant speedups in statements that modify many rows, if the
278 trigger only needs to be fired for a few of the rows.
279 In some cases it is possible for a single SQL command to fire more than
281 one kind of trigger. For instance an INSERT with an ON CONFLICT DO
282 UPDATE clause may cause both insert and update operations, so it will
283 fire both kinds of triggers as needed. The transition relations
284 supplied to triggers are specific to their event type; thus an INSERT
285 trigger will see only the inserted rows, while an UPDATE trigger will
286 see only the updated rows.
287 Row updates or deletions caused by foreign-key enforcement actions,
289 such as ON UPDATE CASCADE or ON DELETE SET NULL, are treated as part of
290 the SQL command that caused them (note that such actions are never
291 deferred). Relevant triggers on the affected table will be fired, so
292 that this provides another way in which an SQL command might fire
293 triggers not directly matching its type. In simple cases, triggers that
294 request transition relations will see all changes caused in their table
295 by a single original SQL command as a single transition relation.
296 However, there are cases in which the presence of an AFTER ROW trigger
297 that requests transition relations will cause the foreign-key
298 enforcement actions triggered by a single SQL command to be split into
299 multiple steps, each with its own transition relation(s). In such
300 cases, any statement-level triggers that are present will be fired once
301 per creation of a transition relation set, ensuring that the triggers
302 see each affected row in a transition relation once and only once.
303 Statement-level triggers on a view are fired only if the action on the
305 view is handled by a row-level INSTEAD OF trigger. If the action is
306 handled by an INSTEAD rule, then whatever statements are emitted by the
307 rule are executed in place of the original statement naming the view,
308 so that the triggers that will be fired are those on tables named in
309 the replacement statements. Similarly, if the view is automatically
310 updatable, then the action is handled by automatically rewriting the
311 statement into an action on the view's base table, so that the base
312 table's statement-level triggers are the ones that are fired.
313 Modifying a partitioned table or a table with inheritance children
315 fires statement-level triggers attached to the explicitly named table,
316 but not statement-level triggers for its partitions or child tables. In
317 contrast, row-level triggers are fired on the rows in affected
318 partitions or child tables, even if they are not explicitly named in
319 the query. If a statement-level trigger has been defined with
320 transition relations named by a REFERENCING clause, then before and
321 after images of rows are visible from all affected partitions or child
322 tables. In the case of inheritance children, the row images include
323 only columns that are present in the table that the trigger is attached
324 to.
325 Currently, row-level triggers with transition relations cannot be
327 defined on partitions or inheritance child tables. Also, triggers on
328 partitioned tables may not be INSTEAD OF.
329 Currently, the OR REPLACE option is not supported for constraint
331 triggers.
332 Replacing an existing trigger within a transaction that has already
334 performed updating actions on the trigger's table is not recommended.
335 Trigger firing decisions, or portions of firing decisions, that have
336 already been made will not be reconsidered, so the effects could be
337 surprising.
338 There are a few built-in trigger functions that can be used to solve
340 common problems without having to write your own trigger code; see
341 Section 9.28.
342
344 Execute the function check_account_update whenever a row of the table
345 accounts is about to be updated:
346 CREATE TRIGGER check_update
348 BEFORE UPDATE ON accounts
349 FOR EACH ROW
350 EXECUTE FUNCTION check_account_update();
351 Modify that trigger definition to only execute the function if column
353 balance is specified as a target in the UPDATE command:
354 CREATE OR REPLACE TRIGGER check_update
356 BEFORE UPDATE OF balance ON accounts
357 FOR EACH ROW
358 EXECUTE FUNCTION check_account_update();
359 This form only executes the function if column balance has in fact
361 changed value:
362 CREATE TRIGGER check_update
364 BEFORE UPDATE ON accounts
365 FOR EACH ROW
366 WHEN (OLD.balance IS DISTINCT FROM NEW.balance)
367 EXECUTE FUNCTION check_account_update();
368 Call a function to log updates of accounts, but only if something
370 changed:
371 CREATE TRIGGER log_update
373 AFTER UPDATE ON accounts
374 FOR EACH ROW
375 WHEN (OLD.* IS DISTINCT FROM NEW.*)
376 EXECUTE FUNCTION log_account_update();
377 Execute the function view_insert_row for each row to insert rows into
379 the tables underlying a view:
380 CREATE TRIGGER view_insert
382 INSTEAD OF INSERT ON my_view
383 FOR EACH ROW
384 EXECUTE FUNCTION view_insert_row();
385 Execute the function check_transfer_balances_to_zero for each statement
387 to confirm that the transfer rows offset to a net of zero:
388 CREATE TRIGGER transfer_insert
390 AFTER INSERT ON transfer
391 REFERENCING NEW TABLE AS inserted
392 FOR EACH STATEMENT
393 EXECUTE FUNCTION check_transfer_balances_to_zero();
394 Execute the function check_matching_pairs for each row to confirm that
396 changes are made to matching pairs at the same time (by the same
397 statement):
398 CREATE TRIGGER paired_items_update
400 AFTER UPDATE ON paired_items
401 REFERENCING NEW TABLE AS newtab OLD TABLE AS oldtab
402 FOR EACH ROW
403 EXECUTE FUNCTION check_matching_pairs();
404 Section 39.4 contains a complete example of a trigger function written
406 in C.
407
409 The CREATE TRIGGER statement in PostgreSQL implements a subset of the
410 SQL standard. The following functionalities are currently missing:
411 • While transition table names for AFTER triggers are specified using
413 the REFERENCING clause in the standard way, the row variables used
414 in FOR EACH ROW triggers may not be specified in a REFERENCING
415 clause. They are available in a manner that is dependent on the
416 language in which the trigger function is written, but is fixed for
417 any one language. Some languages effectively behave as though there
418 is a REFERENCING clause containing OLD ROW AS OLD NEW ROW AS NEW.
419 • The standard allows transition tables to be used with
421 column-specific UPDATE triggers, but then the set of rows that
422 should be visible in the transition tables depends on the trigger's
423 column list. This is not currently implemented by PostgreSQL.
424 • PostgreSQL only allows the execution of a user-defined function for
426 the triggered action. The standard allows the execution of a number
427 of other SQL commands, such as CREATE TABLE, as the triggered
428 action. This limitation is not hard to work around by creating a
429 user-defined function that executes the desired commands.
430 SQL specifies that multiple triggers should be fired in
432 time-of-creation order. PostgreSQL uses name order, which was judged
433 to be more convenient.
434 SQL specifies that BEFORE DELETE triggers on cascaded deletes fire
436 after the cascaded DELETE completes. The PostgreSQL behavior is for
437 BEFORE DELETE to always fire before the delete action, even a cascading
438 one. This is considered more consistent. There is also nonstandard
439 behavior if BEFORE triggers modify rows or prevent updates during an
440 update that is caused by a referential action. This can lead to
441 constraint violations or stored data that does not honor the
442 referential constraint.
443 The ability to specify multiple actions for a single trigger using OR
445 is a PostgreSQL extension of the SQL standard.
446 The ability to fire triggers for TRUNCATE is a PostgreSQL extension of
448 the SQL standard, as is the ability to define statement-level triggers
449 on views.
450 CREATE CONSTRAINT TRIGGER is a PostgreSQL extension of the SQL
452 standard. So is the OR REPLACE option.
453
455 ALTER TRIGGER (ALTER_TRIGGER(7)), DROP TRIGGER (DROP_TRIGGER(7)),
456 CREATE FUNCTION (CREATE_FUNCTION(7)), SET CONSTRAINTS
457 (SET_CONSTRAINTS(7))
458PostgreSQL 16.1 2023 CREATE TRIGGER(7)