1CREATE TRIGGER(7) PostgreSQL 13.3 Documentation CREATE TRIGGER(7)
2
6 CREATE_TRIGGER - define a new trigger
7
9 CREATE [ 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. The trigger will be associated
27 with the specified table, view, or foreign table and will execute the
28 specified function function_name when certain operations are performed
29 on that table.
30 The trigger can be specified to fire before the operation is attempted
32 on a row (before constraints are checked and the INSERT, UPDATE, or
33 DELETE is attempted); or after the operation has completed (after
34 constraints are checked and the INSERT, UPDATE, or DELETE has
35 completed); or instead of the operation (in the case of inserts,
36 updates or deletes on a view). If the trigger fires before or instead
37 of the event, the trigger can skip the operation for the current row,
38 or change the row being inserted (for INSERT and UPDATE operations
39 only). If the trigger fires after the event, all changes, including the
40 effects of other triggers, are “visible” to the trigger.
41 A trigger that is marked FOR EACH ROW is called once for every row that
43 the operation modifies. For example, a DELETE that affects 10 rows will
44 cause any ON DELETE triggers on the target relation to be called 10
45 separate times, once for each deleted row. In contrast, a trigger that
46 is marked FOR EACH STATEMENT only executes once for any given
47 operation, regardless of how many rows it modifies (in particular, an
48 operation that modifies zero rows will still result in the execution of
49 any applicable FOR EACH STATEMENT triggers).
50 Triggers that are specified to fire INSTEAD OF the trigger event must
52 be marked FOR EACH ROW, and can only be defined on views. BEFORE and
53 AFTER triggers on a view must be marked as FOR EACH STATEMENT.
54 In addition, triggers may be defined to fire for TRUNCATE, though only
56 FOR EACH STATEMENT.
57 The following table summarizes which types of triggers may be used on
59 tables, views, and foreign tables:
60 ┌───────────┬──────────────────────┬────────────────┬─────────────────┐
62 │When │ Event │ Row-level │ Statement-level │
63 ├───────────┼──────────────────────┼────────────────┼─────────────────┤
64 │ │ INSERT/UPDATE/DELETE │ Tables and │ Tables, views, │
65 │ │ │ foreign tables │ and foreign │
66 │ BEFORE │ │ │ tables │
67 │ ├──────────────────────┼────────────────┼─────────────────┤
68 │ │ TRUNCATE │ — │ Tables │
69 ├───────────┼──────────────────────┼────────────────┼─────────────────┤
70 │ │ INSERT/UPDATE/DELETE │ Tables and │ Tables, views, │
71 │ │ │ foreign tables │ and foreign │
72 │ AFTER │ │ │ tables │
73 │ ├──────────────────────┼────────────────┼─────────────────┤
74 │ │ TRUNCATE │ — │ Tables │
75 ├───────────┼──────────────────────┼────────────────┼─────────────────┤
76 │ │ INSERT/UPDATE/DELETE │ Views │ — │
77 │INSTEAD OF ├──────────────────────┼────────────────┼─────────────────┤
78 │ │ TRUNCATE │ — │ — │
79 └───────────┴──────────────────────┴────────────────┴─────────────────┘
80 Also, a trigger definition can specify a Boolean WHEN condition, which
82 will be tested to see whether the trigger should be fired. In row-level
83 triggers the WHEN condition can examine the old and/or new values of
84 columns of the row. Statement-level triggers can also have WHEN
85 conditions, although the feature is not so useful for them since the
86 condition cannot refer to any values in the table.
87 If multiple triggers of the same kind are defined for the same event,
89 they will be fired in alphabetical order by name.
90 When the CONSTRAINT option is specified, this command creates a
92 constraint trigger. This is the same as a regular trigger except that
93 the timing of the trigger firing can be adjusted using SET CONSTRAINTS
94 (SET_CONSTRAINTS(7)). Constraint triggers must be AFTER ROW triggers on
95 plain tables (not foreign tables). They can be fired either at the end
96 of the statement causing the triggering event, or at the end of the
97 containing transaction; in the latter case they are said to be
98 deferred. A pending deferred-trigger firing can also be forced to
99 happen immediately by using SET CONSTRAINTS. Constraint triggers are
100 expected to raise an exception when the constraints they implement are
101 violated.
102 The REFERENCING option enables collection of transition relations,
104 which are row sets that include all of the rows inserted, deleted, or
105 modified by the current SQL statement. This feature lets the trigger
106 see a global view of what the statement did, not just one row at a
107 time. This option is only allowed for an AFTER trigger that is not a
108 constraint trigger; also, if the trigger is an UPDATE trigger, it must
109 not specify a column_name list. OLD TABLE may only be specified once,
110 and only for a trigger that can fire on UPDATE or DELETE; it creates a
111 transition relation containing the before-images of all rows updated or
112 deleted by the statement. Similarly, NEW TABLE may only be specified
113 once, and only for a trigger that can fire on UPDATE or INSERT; it
114 creates a transition relation containing the after-images of all rows
115 updated or inserted by the statement.
116 SELECT does not modify any rows so you cannot create SELECT triggers.
118 Rules and views may provide workable solutions to problems that seem to
119 need SELECT triggers.
120 Refer to Chapter 38 for more information about triggers.
122
124 name
125 The name to give the new trigger. This must be distinct from the
126 name of any other trigger for the same table. The name cannot be
127 schema-qualified — the trigger inherits the schema of its table.
128 For a constraint trigger, this is also the name to use when
129 modifying the trigger's behavior using SET CONSTRAINTS.
130 BEFORE
132 AFTER
133 INSTEAD OF
134 Determines whether the function is called before, after, or instead
135 of the event. A constraint trigger can only be specified as AFTER.
136 event
138 One of INSERT, UPDATE, DELETE, or TRUNCATE; this specifies the
139 event that will fire the trigger. Multiple events can be specified
140 using OR, except when transition relations are requested.
141 For UPDATE events, it is possible to specify a list of columns
143 using this syntax:
144 UPDATE OF column_name1 [, column_name2 ... ]
146 The trigger will only fire if at least one of the listed columns is
148 mentioned as a target of the UPDATE command or if one of the listed
149 columns is a generated column that depends on a column that is the
150 target of the UPDATE.
151 INSTEAD OF UPDATE events do not allow a list of columns. A column
153 list cannot be specified when requesting transition relations,
154 either.
155 table_name
157 The name (optionally schema-qualified) of the table, view, or
158 foreign table the trigger is for.
159 referenced_table_name
161 The (possibly schema-qualified) name of another table referenced by
162 the constraint. This option is used for foreign-key constraints and
163 is not recommended for general use. This can only be specified for
164 constraint triggers.
165 DEFERRABLE
167 NOT DEFERRABLE
168 INITIALLY IMMEDIATE
169 INITIALLY DEFERRED
170 The default timing of the trigger. See the CREATE TABLE
171 (CREATE_TABLE(7)) documentation for details of these constraint
172 options. This can only be specified for constraint triggers.
173 REFERENCING
175 This keyword immediately precedes the declaration of one or two
176 relation names that provide access to the transition relations of
177 the triggering statement.
178 OLD TABLE
180 NEW TABLE
181 This clause indicates whether the following relation name is for
182 the before-image transition relation or the after-image transition
183 relation.
184 transition_relation_name
186 The (unqualified) name to be used within the trigger for this
187 transition relation.
188 FOR EACH ROW
190 FOR EACH STATEMENT
191 This specifies whether the trigger function should be fired once
192 for every row affected by the trigger event, or just once per SQL
193 statement. If neither is specified, FOR EACH STATEMENT is the
194 default. Constraint triggers can only be specified FOR EACH ROW.
195 condition
197 A Boolean expression that determines whether the trigger function
198 will actually be executed. If WHEN is specified, the function will
199 only be called if the condition returns true. In FOR EACH ROW
200 triggers, the WHEN condition can refer to columns of the old and/or
201 new row values by writing OLD.column_name or NEW.column_name
202 respectively. Of course, INSERT triggers cannot refer to OLD and
203 DELETE triggers cannot refer to NEW.
204 INSTEAD OF triggers do not support WHEN conditions.
206 Currently, WHEN expressions cannot contain subqueries.
208 Note that for constraint triggers, evaluation of the WHEN condition
210 is not deferred, but occurs immediately after the row update
211 operation is performed. If the condition does not evaluate to true
212 then the trigger is not queued for deferred execution.
213 function_name
215 A user-supplied function that is declared as taking no arguments
216 and returning type trigger, which is executed when the trigger
217 fires.
218 In the syntax of CREATE TRIGGER, the keywords FUNCTION and
220 PROCEDURE are equivalent, but the referenced function must in any
221 case be a function, not a procedure. The use of the keyword
222 PROCEDURE here is historical and deprecated.
223 arguments
225 An optional comma-separated list of arguments to be provided to the
226 function when the trigger is executed. The arguments are literal
227 string constants. Simple names and numeric constants can be written
228 here, too, but they will all be converted to strings. Please check
229 the description of the implementation language of the trigger
230 function to find out how these arguments can be accessed within the
231 function; it might be different from normal function arguments.
232
234 To create a trigger on a table, the user must have the TRIGGER
235 privilege on the table. The user must also have EXECUTE privilege on
236 the trigger function.
237 Use DROP TRIGGER (DROP_TRIGGER(7)) to remove a trigger.
239 A column-specific trigger (one defined using the UPDATE OF column_name
241 syntax) will fire when any of its columns are listed as targets in the
242 UPDATE command's SET list. It is possible for a column's value to
243 change even when the trigger is not fired, because changes made to the
244 row's contents by BEFORE UPDATE triggers are not considered.
245 Conversely, a command such as UPDATE ... SET x = x ... will fire a
246 trigger on column x, even though the column's value did not change.
247 There are a few built-in trigger functions that can be used to solve
249 common problems without having to write your own trigger code; see
250 Section 9.28.
251 In a BEFORE trigger, the WHEN condition is evaluated just before the
253 function is or would be executed, so using WHEN is not materially
254 different from testing the same condition at the beginning of the
255 trigger function. Note in particular that the NEW row seen by the
256 condition is the current value, as possibly modified by earlier
257 triggers. Also, a BEFORE trigger's WHEN condition is not allowed to
258 examine the system columns of the NEW row (such as ctid), because those
259 won't have been set yet.
260 In an AFTER trigger, the WHEN condition is evaluated just after the row
262 update occurs, and it determines whether an event is queued to fire the
263 trigger at the end of statement. So when an AFTER trigger's WHEN
264 condition does not return true, it is not necessary to queue an event
265 nor to re-fetch the row at end of statement. This can result in
266 significant speedups in statements that modify many rows, if the
267 trigger only needs to be fired for a few of the rows.
268 In some cases it is possible for a single SQL command to fire more than
270 one kind of trigger. For instance an INSERT with an ON CONFLICT DO
271 UPDATE clause may cause both insert and update operations, so it will
272 fire both kinds of triggers as needed. The transition relations
273 supplied to triggers are specific to their event type; thus an INSERT
274 trigger will see only the inserted rows, while an UPDATE trigger will
275 see only the updated rows.
276 Row updates or deletions caused by foreign-key enforcement actions,
278 such as ON UPDATE CASCADE or ON DELETE SET NULL, are treated as part of
279 the SQL command that caused them (note that such actions are never
280 deferred). Relevant triggers on the affected table will be fired, so
281 that this provides another way in which a SQL command might fire
282 triggers not directly matching its type. In simple cases, triggers that
283 request transition relations will see all changes caused in their table
284 by a single original SQL command as a single transition relation.
285 However, there are cases in which the presence of an AFTER ROW trigger
286 that requests transition relations will cause the foreign-key
287 enforcement actions triggered by a single SQL command to be split into
288 multiple steps, each with its own transition relation(s). In such
289 cases, any statement-level triggers that are present will be fired once
290 per creation of a transition relation set, ensuring that the triggers
291 see each affected row in a transition relation once and only once.
292 Statement-level triggers on a view are fired only if the action on the
294 view is handled by a row-level INSTEAD OF trigger. If the action is
295 handled by an INSTEAD rule, then whatever statements are emitted by the
296 rule are executed in place of the original statement naming the view,
297 so that the triggers that will be fired are those on tables named in
298 the replacement statements. Similarly, if the view is automatically
299 updatable, then the action is handled by automatically rewriting the
300 statement into an action on the view's base table, so that the base
301 table's statement-level triggers are the ones that are fired.
302 Creating a row-level trigger on a partitioned table will cause
304 identical triggers to be created in all its existing partitions; and
305 any partitions created or attached later will contain an identical
306 trigger, too. If the partition is detached from its parent, the trigger
307 is removed. Triggers on partitioned tables may not be INSTEAD OF.
308 Modifying a partitioned table or a table with inheritance children
310 fires statement-level triggers attached to the explicitly named table,
311 but not statement-level triggers for its partitions or child tables. In
312 contrast, row-level triggers are fired on the rows in affected
313 partitions or child tables, even if they are not explicitly named in
314 the query. If a statement-level trigger has been defined with
315 transition relations named by a REFERENCING clause, then before and
316 after images of rows are visible from all affected partitions or child
317 tables. In the case of inheritance children, the row images include
318 only columns that are present in the table that the trigger is attached
319 to. Currently, row-level triggers with transition relations cannot be
320 defined on partitions or inheritance child tables.
321
323 Execute the function check_account_update whenever a row of the table
324 accounts is about to be updated:
325 CREATE TRIGGER check_update
327 BEFORE UPDATE ON accounts
328 FOR EACH ROW
329 EXECUTE FUNCTION check_account_update();
330 The same, but only execute the function if column balance is specified
332 as a target in the UPDATE command:
333 CREATE TRIGGER check_update
335 BEFORE UPDATE OF balance ON accounts
336 FOR EACH ROW
337 EXECUTE FUNCTION check_account_update();
338 This form only executes the function if column balance has in fact
340 changed value:
341 CREATE TRIGGER check_update
343 BEFORE UPDATE ON accounts
344 FOR EACH ROW
345 WHEN (OLD.balance IS DISTINCT FROM NEW.balance)
346 EXECUTE FUNCTION check_account_update();
347 Call a function to log updates of accounts, but only if something
349 changed:
350 CREATE TRIGGER log_update
352 AFTER UPDATE ON accounts
353 FOR EACH ROW
354 WHEN (OLD.* IS DISTINCT FROM NEW.*)
355 EXECUTE FUNCTION log_account_update();
356 Execute the function view_insert_row for each row to insert rows into
358 the tables underlying a view:
359 CREATE TRIGGER view_insert
361 INSTEAD OF INSERT ON my_view
362 FOR EACH ROW
363 EXECUTE FUNCTION view_insert_row();
364 Execute the function check_transfer_balances_to_zero for each statement
366 to confirm that the transfer rows offset to a net of zero:
367 CREATE TRIGGER transfer_insert
369 AFTER INSERT ON transfer
370 REFERENCING NEW TABLE AS inserted
371 FOR EACH STATEMENT
372 EXECUTE FUNCTION check_transfer_balances_to_zero();
373 Execute the function check_matching_pairs for each row to confirm that
375 changes are made to matching pairs at the same time (by the same
376 statement):
377 CREATE TRIGGER paired_items_update
379 AFTER UPDATE ON paired_items
380 REFERENCING NEW TABLE AS newtab OLD TABLE AS oldtab
381 FOR EACH ROW
382 EXECUTE FUNCTION check_matching_pairs();
383 Section 38.4 contains a complete example of a trigger function written
385 in C.
386
388 The CREATE TRIGGER statement in PostgreSQL implements a subset of the
389 SQL standard. The following functionalities are currently missing:
390 • While transition table names for AFTER triggers are specified using
392 the REFERENCING clause in the standard way, the row variables used
393 in FOR EACH ROW triggers may not be specified in a REFERENCING
394 clause. They are available in a manner that is dependent on the
395 language in which the trigger function is written, but is fixed for
396 any one language. Some languages effectively behave as though there
397 is a REFERENCING clause containing OLD ROW AS OLD NEW ROW AS NEW.
398 • The standard allows transition tables to be used with
400 column-specific UPDATE triggers, but then the set of rows that
401 should be visible in the transition tables depends on the trigger's
402 column list. This is not currently implemented by PostgreSQL.
403 • PostgreSQL only allows the execution of a user-defined function for
405 the triggered action. The standard allows the execution of a number
406 of other SQL commands, such as CREATE TABLE, as the triggered
407 action. This limitation is not hard to work around by creating a
408 user-defined function that executes the desired commands.
409 SQL specifies that multiple triggers should be fired in
411 time-of-creation order. PostgreSQL uses name order, which was judged
412 to be more convenient.
413 SQL specifies that BEFORE DELETE triggers on cascaded deletes fire
415 after the cascaded DELETE completes. The PostgreSQL behavior is for
416 BEFORE DELETE to always fire before the delete action, even a cascading
417 one. This is considered more consistent. There is also nonstandard
418 behavior if BEFORE triggers modify rows or prevent updates during an
419 update that is caused by a referential action. This can lead to
420 constraint violations or stored data that does not honor the
421 referential constraint.
422 The ability to specify multiple actions for a single trigger using OR
424 is a PostgreSQL extension of the SQL standard.
425 The ability to fire triggers for TRUNCATE is a PostgreSQL extension of
427 the SQL standard, as is the ability to define statement-level triggers
428 on views.
429 CREATE CONSTRAINT TRIGGER is a PostgreSQL extension of the SQL
431 standard.
432
434 ALTER TRIGGER (ALTER_TRIGGER(7)), DROP TRIGGER (DROP_TRIGGER(7)),
435 CREATE FUNCTION (CREATE_FUNCTION(7)), SET CONSTRAINTS
436 (SET_CONSTRAINTS(7))
437PostgreSQL 13.3 2021 CREATE TRIGGER(7)