1CREATE TRIGGER(7) PostgreSQL 10.7 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 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.
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 procedure 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 arguments
218 An optional comma-separated list of arguments to be provided to the
219 function when the trigger is executed. The arguments are literal
220 string constants. Simple names and numeric constants can be written
221 here, too, but they will all be converted to strings. Please check
222 the description of the implementation language of the trigger
223 function to find out how these arguments can be accessed within the
224 function; it might be different from normal function arguments.
225
227 To create a trigger on a table, the user must have the TRIGGER
228 privilege on the table. The user must also have EXECUTE privilege on
229 the trigger function.
230 Use DROP TRIGGER (DROP_TRIGGER(7)) to remove a trigger.
232 A column-specific trigger (one defined using the UPDATE OF column_name
234 syntax) will fire when any of its columns are listed as targets in the
235 UPDATE command's SET list. It is possible for a column's value to
236 change even when the trigger is not fired, because changes made to the
237 row's contents by BEFORE UPDATE triggers are not considered.
238 Conversely, a command such as UPDATE ... SET x = x ... will fire a
239 trigger on column x, even though the column's value did not change.
240 In a BEFORE trigger, the WHEN condition is evaluated just before the
242 function is or would be executed, so using WHEN is not materially
243 different from testing the same condition at the beginning of the
244 trigger function. Note in particular that the NEW row seen by the
245 condition is the current value, as possibly modified by earlier
246 triggers. Also, a BEFORE trigger's WHEN condition is not allowed to
247 examine the system columns of the NEW row (such as oid), because those
248 won't have been set yet.
249 In an AFTER trigger, the WHEN condition is evaluated just after the row
251 update occurs, and it determines whether an event is queued to fire the
252 trigger at the end of statement. So when an AFTER trigger's WHEN
253 condition does not return true, it is not necessary to queue an event
254 nor to re-fetch the row at end of statement. This can result in
255 significant speedups in statements that modify many rows, if the
256 trigger only needs to be fired for a few of the rows.
257 In some cases it is possible for a single SQL command to fire more than
259 one kind of trigger. For instance an INSERT with an ON CONFLICT DO
260 UPDATE clause may cause both insert and update operations, so it will
261 fire both kinds of triggers as needed. The transition relations
262 supplied to triggers are specific to their event type; thus an INSERT
263 trigger will see only the inserted rows, while an UPDATE trigger will
264 see only the updated rows.
265 Row updates or deletions caused by foreign-key enforcement actions,
267 such as ON UPDATE CASCADE or ON DELETE SET NULL, are treated as part of
268 the SQL command that caused them (note that such actions are never
269 deferred). Relevant triggers on the affected table will be fired, so
270 that this provides another way in which a SQL command might fire
271 triggers not directly matching its type. In simple cases, triggers that
272 request transition relations will see all changes caused in their table
273 by a single original SQL command as a single transition relation.
274 However, there are cases in which the presence of an AFTER ROW trigger
275 that requests transition relations will cause the foreign-key
276 enforcement actions triggered by a single SQL command to be split into
277 multiple steps, each with its own transition relation(s). In such
278 cases, any statement-level triggers that are present will be fired once
279 per creation of a transition relation set, ensuring that the triggers
280 see each affected row in a transition relation once and only once.
281 Statement-level triggers on a view are fired only if the action on the
283 view is handled by a row-level INSTEAD OF trigger. If the action is
284 handled by an INSTEAD rule, then whatever statements are emitted by the
285 rule are executed in place of the original statement naming the view,
286 so that the triggers that will be fired are those on tables named in
287 the replacement statements. Similarly, if the view is automatically
288 updatable, then the action is handled by automatically rewriting the
289 statement into an action on the view's base table, so that the base
290 table's statement-level triggers are the ones that are fired.
291 Modifying a partitioned table or a table with inheritance children
293 fires statement-level triggers attached to the explicitly named table,
294 but not statement-level triggers for its partitions or child tables. In
295 contrast, row-level triggers are fired on the rows in affected
296 partitions or child tables, even if they are not explicitly named in
297 the query. If a statement-level trigger has been defined with
298 transition relations named by a REFERENCING clause, then before and
299 after images of rows are visible from all affected partitions or child
300 tables. In the case of inheritance children, the row images include
301 only columns that are present in the table that the trigger is attached
302 to. Currently, row-level triggers with transition relations cannot be
303 defined on partitions or inheritance child tables.
304 In PostgreSQL versions before 7.3, it was necessary to declare trigger
306 functions as returning the placeholder type opaque, rather than
307 trigger. To support loading of old dump files, CREATE TRIGGER will
308 accept a function declared as returning opaque, but it will issue a
309 notice and change the function's declared return type to trigger.
310
312 Execute the function check_account_update whenever a row of the table
313 accounts is about to be updated:
314 CREATE TRIGGER check_update
316 BEFORE UPDATE ON accounts
317 FOR EACH ROW
318 EXECUTE PROCEDURE check_account_update();
319 The same, but only execute the function if column balance is specified
321 as a target in the UPDATE command:
322 CREATE TRIGGER check_update
324 BEFORE UPDATE OF balance ON accounts
325 FOR EACH ROW
326 EXECUTE PROCEDURE check_account_update();
327 This form only executes the function if column balance has in fact
329 changed value:
330 CREATE TRIGGER check_update
332 BEFORE UPDATE ON accounts
333 FOR EACH ROW
334 WHEN (OLD.balance IS DISTINCT FROM NEW.balance)
335 EXECUTE PROCEDURE check_account_update();
336 Call a function to log updates of accounts, but only if something
338 changed:
339 CREATE TRIGGER log_update
341 AFTER UPDATE ON accounts
342 FOR EACH ROW
343 WHEN (OLD.* IS DISTINCT FROM NEW.*)
344 EXECUTE PROCEDURE log_account_update();
345 Execute the function view_insert_row for each row to insert rows into
347 the tables underlying a view:
348 CREATE TRIGGER view_insert
350 INSTEAD OF INSERT ON my_view
351 FOR EACH ROW
352 EXECUTE PROCEDURE view_insert_row();
353 Execute the function check_transfer_balances_to_zero for each statement
355 to confirm that the transfer rows offset to a net of zero:
356 CREATE TRIGGER transfer_insert
358 AFTER INSERT ON transfer
359 REFERENCING NEW TABLE AS inserted
360 FOR EACH STATEMENT
361 EXECUTE PROCEDURE check_transfer_balances_to_zero();
362 Execute the function check_matching_pairs for each row to confirm that
364 changes are made to matching pairs at the same time (by the same
365 statement):
366 CREATE TRIGGER paired_items_update
368 AFTER UPDATE ON paired_items
369 REFERENCING NEW TABLE AS newtab OLD TABLE AS oldtab
370 FOR EACH ROW
371 EXECUTE PROCEDURE check_matching_pairs();
372 Section 38.4 contains a complete example of a trigger function written
374 in C.
375
377 The CREATE TRIGGER statement in PostgreSQL implements a subset of the
378 SQL standard. The following functionalities are currently missing:
379 · While transition table names for AFTER triggers are specified using
381 the REFERENCING clause in the standard way, the row variables used
382 in FOR EACH ROW triggers may not be specified in a REFERENCING
383 clause. They are available in a manner that is dependent on the
384 language in which the trigger function is written, but is fixed for
385 any one language. Some languages effectively behave as though there
386 is a REFERENCING clause containing OLD ROW AS OLD NEW ROW AS NEW.
387 · The standard allows transition tables to be used with
389 column-specific UPDATE triggers, but then the set of rows that
390 should be visible in the transition tables depends on the trigger's
391 column list. This is not currently implemented by PostgreSQL.
392 · PostgreSQL only allows the execution of a user-defined function for
394 the triggered action. The standard allows the execution of a number
395 of other SQL commands, such as CREATE TABLE, as the triggered
396 action. This limitation is not hard to work around by creating a
397 user-defined function that executes the desired commands.
398 SQL specifies that multiple triggers should be fired in
400 time-of-creation order. PostgreSQL uses name order, which was judged
401 to be more convenient.
402 SQL specifies that BEFORE DELETE triggers on cascaded deletes fire
404 after the cascaded DELETE completes. The PostgreSQL behavior is for
405 BEFORE DELETE to always fire before the delete action, even a cascading
406 one. This is considered more consistent. There is also nonstandard
407 behavior if BEFORE triggers modify rows or prevent updates during an
408 update that is caused by a referential action. This can lead to
409 constraint violations or stored data that does not honor the
410 referential constraint.
411 The ability to specify multiple actions for a single trigger using OR
413 is a PostgreSQL extension of the SQL standard.
414 The ability to fire triggers for TRUNCATE is a PostgreSQL extension of
416 the SQL standard, as is the ability to define statement-level triggers
417 on views.
418 CREATE CONSTRAINT TRIGGER is a PostgreSQL extension of the SQL
420 standard.
421
423 ALTER TRIGGER (ALTER_TRIGGER(7)), DROP TRIGGER (DROP_TRIGGER(7)),
424 CREATE FUNCTION (CREATE_FUNCTION(7)), SET CONSTRAINTS
425 (SET_CONSTRAINTS(7))
426PostgreSQL 10.7 2019 CREATE TRIGGER(7)