1CREATE TRIGGER(7) PostgreSQL 11.3 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 39 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.
149 INSTEAD OF UPDATE events do not allow a list of columns. A column
151 list cannot be specified when requesting transition relations,
152 either.
153 table_name
155 The name (optionally schema-qualified) of the table, view, or
156 foreign table the trigger is for.
157 referenced_table_name
159 The (possibly schema-qualified) name of another table referenced by
160 the constraint. This option is used for foreign-key constraints and
161 is not recommended for general use. This can only be specified for
162 constraint triggers.
163 DEFERRABLE
165 NOT DEFERRABLE
166 INITIALLY IMMEDIATE
167 INITIALLY DEFERRED
168 The default timing of the trigger. See the CREATE TABLE
169 (CREATE_TABLE(7)) documentation for details of these constraint
170 options. This can only be specified for constraint triggers.
171 REFERENCING
173 This keyword immediately precedes the declaration of one or two
174 relation names that provide access to the transition relations of
175 the triggering statement.
176 OLD TABLE
178 NEW TABLE
179 This clause indicates whether the following relation name is for
180 the before-image transition relation or the after-image transition
181 relation.
182 transition_relation_name
184 The (unqualified) name to be used within the trigger for this
185 transition relation.
186 FOR EACH ROW
188 FOR EACH STATEMENT
189 This specifies whether the trigger function should be fired once
190 for every row affected by the trigger event, or just once per SQL
191 statement. If neither is specified, FOR EACH STATEMENT is the
192 default. Constraint triggers can only be specified FOR EACH ROW.
193 condition
195 A Boolean expression that determines whether the trigger function
196 will actually be executed. If WHEN is specified, the function will
197 only be called if the condition returns true. In FOR EACH ROW
198 triggers, the WHEN condition can refer to columns of the old and/or
199 new row values by writing OLD.column_name or NEW.column_name
200 respectively. Of course, INSERT triggers cannot refer to OLD and
201 DELETE triggers cannot refer to NEW.
202 INSTEAD OF triggers do not support WHEN conditions.
204 Currently, WHEN expressions cannot contain subqueries.
206 Note that for constraint triggers, evaluation of the WHEN condition
208 is not deferred, but occurs immediately after the row update
209 operation is performed. If the condition does not evaluate to true
210 then the trigger is not queued for deferred execution.
211 function_name
213 A user-supplied function that is declared as taking no arguments
214 and returning type trigger, which is executed when the trigger
215 fires.
216 In the syntax of CREATE TRIGGER, the keywords FUNCTION and
218 PROCEDURE are equivalent, but the referenced function must in any
219 case be a function, not a procedure. The use of the keyword
220 PROCEDURE here is historical and deprecated.
221 arguments
223 An optional comma-separated list of arguments to be provided to the
224 function when the trigger is executed. The arguments are literal
225 string constants. Simple names and numeric constants can be written
226 here, too, but they will all be converted to strings. Please check
227 the description of the implementation language of the trigger
228 function to find out how these arguments can be accessed within the
229 function; it might be different from normal function arguments.
230
232 To create a trigger on a table, the user must have the TRIGGER
233 privilege on the table. The user must also have EXECUTE privilege on
234 the trigger function.
235 Use DROP TRIGGER (DROP_TRIGGER(7)) to remove a trigger.
237 A column-specific trigger (one defined using the UPDATE OF column_name
239 syntax) will fire when any of its columns are listed as targets in the
240 UPDATE command's SET list. It is possible for a column's value to
241 change even when the trigger is not fired, because changes made to the
242 row's contents by BEFORE UPDATE triggers are not considered.
243 Conversely, a command such as UPDATE ... SET x = x ... will fire a
244 trigger on column x, even though the column's value did not change.
245 In a BEFORE trigger, the WHEN condition is evaluated just before the
247 function is or would be executed, so using WHEN is not materially
248 different from testing the same condition at the beginning of the
249 trigger function. Note in particular that the NEW row seen by the
250 condition is the current value, as possibly modified by earlier
251 triggers. Also, a BEFORE trigger's WHEN condition is not allowed to
252 examine the system columns of the NEW row (such as oid), because those
253 won't have been set yet.
254 In an AFTER trigger, the WHEN condition is evaluated just after the row
256 update occurs, and it determines whether an event is queued to fire the
257 trigger at the end of statement. So when an AFTER trigger's WHEN
258 condition does not return true, it is not necessary to queue an event
259 nor to re-fetch the row at end of statement. This can result in
260 significant speedups in statements that modify many rows, if the
261 trigger only needs to be fired for a few of the rows.
262 In some cases it is possible for a single SQL command to fire more than
264 one kind of trigger. For instance an INSERT with an ON CONFLICT DO
265 UPDATE clause may cause both insert and update operations, so it will
266 fire both kinds of triggers as needed. The transition relations
267 supplied to triggers are specific to their event type; thus an INSERT
268 trigger will see only the inserted rows, while an UPDATE trigger will
269 see only the updated rows.
270 Row updates or deletions caused by foreign-key enforcement actions,
272 such as ON UPDATE CASCADE or ON DELETE SET NULL, are treated as part of
273 the SQL command that caused them (note that such actions are never
274 deferred). Relevant triggers on the affected table will be fired, so
275 that this provides another way in which a SQL command might fire
276 triggers not directly matching its type. In simple cases, triggers that
277 request transition relations will see all changes caused in their table
278 by a single original SQL command as a single transition relation.
279 However, there are cases in which the presence of an AFTER ROW trigger
280 that requests transition relations will cause the foreign-key
281 enforcement actions triggered by a single SQL command to be split into
282 multiple steps, each with its own transition relation(s). In such
283 cases, any statement-level triggers that are present will be fired once
284 per creation of a transition relation set, ensuring that the triggers
285 see each affected row in a transition relation once and only once.
286 Statement-level triggers on a view are fired only if the action on the
288 view is handled by a row-level INSTEAD OF trigger. If the action is
289 handled by an INSTEAD rule, then whatever statements are emitted by the
290 rule are executed in place of the original statement naming the view,
291 so that the triggers that will be fired are those on tables named in
292 the replacement statements. Similarly, if the view is automatically
293 updatable, then the action is handled by automatically rewriting the
294 statement into an action on the view's base table, so that the base
295 table's statement-level triggers are the ones that are fired.
296 Creating a row-level trigger on a partitioned table will cause
298 identical triggers to be created in all its existing partitions; and
299 any partitions created or attached later will contain an identical
300 trigger, too. Triggers on partitioned tables may only be AFTER.
301 Modifying a partitioned table or a table with inheritance children
303 fires statement-level triggers attached to the explicitly named table,
304 but not statement-level triggers for its partitions or child tables. In
305 contrast, row-level triggers are fired on the rows in affected
306 partitions or child tables, even if they are not explicitly named in
307 the query. If a statement-level trigger has been defined with
308 transition relations named by a REFERENCING clause, then before and
309 after images of rows are visible from all affected partitions or child
310 tables. In the case of inheritance children, the row images include
311 only columns that are present in the table that the trigger is attached
312 to. Currently, row-level triggers with transition relations cannot be
313 defined on partitions or inheritance child tables.
314 In PostgreSQL versions before 7.3, it was necessary to declare trigger
316 functions as returning the placeholder type opaque, rather than
317 trigger. To support loading of old dump files, CREATE TRIGGER will
318 accept a function declared as returning opaque, but it will issue a
319 notice and change the function's declared return type to trigger.
320
322 Execute the function check_account_update whenever a row of the table
323 accounts is about to be updated:
324 CREATE TRIGGER check_update
326 BEFORE UPDATE ON accounts
327 FOR EACH ROW
328 EXECUTE FUNCTION check_account_update();
329 The same, but only execute the function if column balance is specified
331 as a target in the UPDATE command:
332 CREATE TRIGGER check_update
334 BEFORE UPDATE OF balance ON accounts
335 FOR EACH ROW
336 EXECUTE FUNCTION check_account_update();
337 This form only executes the function if column balance has in fact
339 changed value:
340 CREATE TRIGGER check_update
342 BEFORE UPDATE ON accounts
343 FOR EACH ROW
344 WHEN (OLD.balance IS DISTINCT FROM NEW.balance)
345 EXECUTE FUNCTION check_account_update();
346 Call a function to log updates of accounts, but only if something
348 changed:
349 CREATE TRIGGER log_update
351 AFTER UPDATE ON accounts
352 FOR EACH ROW
353 WHEN (OLD.* IS DISTINCT FROM NEW.*)
354 EXECUTE FUNCTION log_account_update();
355 Execute the function view_insert_row for each row to insert rows into
357 the tables underlying a view:
358 CREATE TRIGGER view_insert
360 INSTEAD OF INSERT ON my_view
361 FOR EACH ROW
362 EXECUTE FUNCTION view_insert_row();
363 Execute the function check_transfer_balances_to_zero for each statement
365 to confirm that the transfer rows offset to a net of zero:
366 CREATE TRIGGER transfer_insert
368 AFTER INSERT ON transfer
369 REFERENCING NEW TABLE AS inserted
370 FOR EACH STATEMENT
371 EXECUTE FUNCTION check_transfer_balances_to_zero();
372 Execute the function check_matching_pairs for each row to confirm that
374 changes are made to matching pairs at the same time (by the same
375 statement):
376 CREATE TRIGGER paired_items_update
378 AFTER UPDATE ON paired_items
379 REFERENCING NEW TABLE AS newtab OLD TABLE AS oldtab
380 FOR EACH ROW
381 EXECUTE FUNCTION check_matching_pairs();
382 Section 39.4 contains a complete example of a trigger function written
384 in C.
385
387 The CREATE TRIGGER statement in PostgreSQL implements a subset of the
388 SQL standard. The following functionalities are currently missing:
389 · While transition table names for AFTER triggers are specified using
391 the REFERENCING clause in the standard way, the row variables used
392 in FOR EACH ROW triggers may not be specified in a REFERENCING
393 clause. They are available in a manner that is dependent on the
394 language in which the trigger function is written, but is fixed for
395 any one language. Some languages effectively behave as though there
396 is a REFERENCING clause containing OLD ROW AS OLD NEW ROW AS NEW.
397 · The standard allows transition tables to be used with
399 column-specific UPDATE triggers, but then the set of rows that
400 should be visible in the transition tables depends on the trigger's
401 column list. This is not currently implemented by PostgreSQL.
402 · PostgreSQL only allows the execution of a user-defined function for
404 the triggered action. The standard allows the execution of a number
405 of other SQL commands, such as CREATE TABLE, as the triggered
406 action. This limitation is not hard to work around by creating a
407 user-defined function that executes the desired commands.
408 SQL specifies that multiple triggers should be fired in
410 time-of-creation order. PostgreSQL uses name order, which was judged
411 to be more convenient.
412 SQL specifies that BEFORE DELETE triggers on cascaded deletes fire
414 after the cascaded DELETE completes. The PostgreSQL behavior is for
415 BEFORE DELETE to always fire before the delete action, even a cascading
416 one. This is considered more consistent. There is also nonstandard
417 behavior if BEFORE triggers modify rows or prevent updates during an
418 update that is caused by a referential action. This can lead to
419 constraint violations or stored data that does not honor the
420 referential constraint.
421 The ability to specify multiple actions for a single trigger using OR
423 is a PostgreSQL extension of the SQL standard.
424 The ability to fire triggers for TRUNCATE is a PostgreSQL extension of
426 the SQL standard, as is the ability to define statement-level triggers
427 on views.
428 CREATE CONSTRAINT TRIGGER is a PostgreSQL extension of the SQL
430 standard.
431
433 ALTER TRIGGER (ALTER_TRIGGER(7)), DROP TRIGGER (DROP_TRIGGER(7)),
434 CREATE FUNCTION (CREATE_FUNCTION(7)), SET CONSTRAINTS
435 (SET_CONSTRAINTS(7))
436PostgreSQL 11.3 2019 CREATE TRIGGER(7)