1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
|
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>CREATE TRIGGER</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="sql-createtransform.html" title="CREATE TRANSFORM" /><link rel="next" href="sql-createtype.html" title="CREATE TYPE" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">CREATE TRIGGER</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="sql-createtransform.html" title="CREATE TRANSFORM">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><th width="60%" align="center">SQL Commands</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="sql-createtype.html" title="CREATE TYPE">Next</a></td></tr></table><hr /></div><div class="refentry" id="SQL-CREATETRIGGER"><div class="titlepage"></div><a id="id-1.9.3.93.1" class="indexterm"></a><a id="id-1.9.3.93.2" class="indexterm"></a><div class="refnamediv"><h2><span class="refentrytitle">CREATE TRIGGER</span></h2><p>CREATE TRIGGER — define a new trigger</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><pre class="synopsis">
CREATE [ OR REPLACE ] [ CONSTRAINT ] TRIGGER <em class="replaceable"><code>name</code></em> { BEFORE | AFTER | INSTEAD OF } { <em class="replaceable"><code>event</code></em> [ OR ... ] }
ON <em class="replaceable"><code>table_name</code></em>
[ FROM <em class="replaceable"><code>referenced_table_name</code></em> ]
[ NOT DEFERRABLE | [ DEFERRABLE ] [ INITIALLY IMMEDIATE | INITIALLY DEFERRED ] ]
[ REFERENCING { { OLD | NEW } TABLE [ AS ] <em class="replaceable"><code>transition_relation_name</code></em> } [ ... ] ]
[ FOR [ EACH ] { ROW | STATEMENT } ]
[ WHEN ( <em class="replaceable"><code>condition</code></em> ) ]
EXECUTE { FUNCTION | PROCEDURE } <em class="replaceable"><code>function_name</code></em> ( <em class="replaceable"><code>arguments</code></em> )
<span class="phrase">where <em class="replaceable"><code>event</code></em> can be one of:</span>
INSERT
UPDATE [ OF <em class="replaceable"><code>column_name</code></em> [, ... ] ]
DELETE
TRUNCATE
</pre></div><div class="refsect1" id="id-1.9.3.93.6"><h2>Description</h2><p>
<code class="command">CREATE TRIGGER</code> creates a new trigger.
<code class="command">CREATE OR REPLACE TRIGGER</code> will either create a
new trigger, or replace an existing trigger. The
trigger will be associated with the specified table, view, or foreign table
and will execute the specified
function <em class="replaceable"><code>function_name</code></em> when
certain operations are performed on that table.
</p><p>
To replace the current definition of an existing trigger, use
<code class="command">CREATE OR REPLACE TRIGGER</code>, specifying the existing
trigger's name and parent table. All other properties are replaced.
</p><p>
The trigger can be specified to fire before the
operation is attempted on a row (before constraints are checked and
the <code class="command">INSERT</code>, <code class="command">UPDATE</code>, or
<code class="command">DELETE</code> is attempted); or after the operation has
completed (after constraints are checked and the
<code class="command">INSERT</code>, <code class="command">UPDATE</code>, or
<code class="command">DELETE</code> has completed); or instead of the operation
(in the case of inserts, updates or deletes on a view).
If the trigger fires before or instead of the event, the trigger can skip
the operation for the current row, or change the row being inserted (for
<code class="command">INSERT</code> and <code class="command">UPDATE</code> operations
only). If the trigger fires after the event, all changes, including
the effects of other triggers, are <span class="quote">“<span class="quote">visible</span>”</span>
to the trigger.
</p><p>
A trigger that is marked <code class="literal">FOR EACH ROW</code> is called
once for every row that the operation modifies. For example, a
<code class="command">DELETE</code> that affects 10 rows will cause any
<code class="literal">ON DELETE</code> triggers on the target relation to be
called 10 separate times, once for each deleted row. In contrast, a
trigger that is marked <code class="literal">FOR EACH STATEMENT</code> only
executes once for any given operation, regardless of how many rows
it modifies (in particular, an operation that modifies zero rows
will still result in the execution of any applicable <code class="literal">FOR
EACH STATEMENT</code> triggers).
</p><p>
Triggers that are specified to fire <code class="literal">INSTEAD OF</code> the trigger
event must be marked <code class="literal">FOR EACH ROW</code>, and can only be defined
on views. <code class="literal">BEFORE</code> and <code class="literal">AFTER</code> triggers on a view
must be marked as <code class="literal">FOR EACH STATEMENT</code>.
</p><p>
In addition, triggers may be defined to fire for
<code class="command">TRUNCATE</code>, though only
<code class="literal">FOR EACH STATEMENT</code>.
</p><p>
The following table summarizes which types of triggers may be used on
tables, views, and foreign tables:
</p><div class="informaltable" id="SUPPORTED-TRIGGER-TYPES"><table class="informaltable" border="1"><colgroup><col /><col /><col /><col /></colgroup><thead><tr><th>When</th><th>Event</th><th>Row-level</th><th>Statement-level</th></tr></thead><tbody><tr><td rowspan="2" align="center"><code class="literal">BEFORE</code></td><td align="center"><code class="command">INSERT</code>/<code class="command">UPDATE</code>/<code class="command">DELETE</code></td><td align="center">Tables and foreign tables</td><td align="center">Tables, views, and foreign tables</td></tr><tr><td align="center"><code class="command">TRUNCATE</code></td><td align="center">—</td><td align="center">Tables</td></tr><tr><td rowspan="2" align="center"><code class="literal">AFTER</code></td><td align="center"><code class="command">INSERT</code>/<code class="command">UPDATE</code>/<code class="command">DELETE</code></td><td align="center">Tables and foreign tables</td><td align="center">Tables, views, and foreign tables</td></tr><tr><td align="center"><code class="command">TRUNCATE</code></td><td align="center">—</td><td align="center">Tables</td></tr><tr><td rowspan="2" align="center"><code class="literal">INSTEAD OF</code></td><td align="center"><code class="command">INSERT</code>/<code class="command">UPDATE</code>/<code class="command">DELETE</code></td><td align="center">Views</td><td align="center">—</td></tr><tr><td align="center"><code class="command">TRUNCATE</code></td><td align="center">—</td><td align="center">—</td></tr></tbody></table></div><p>
Also, a trigger definition can specify a Boolean <code class="literal">WHEN</code>
condition, which will be tested to see whether the trigger should
be fired. In row-level triggers the <code class="literal">WHEN</code> condition can
examine the old and/or new values of columns of the row. Statement-level
triggers can also have <code class="literal">WHEN</code> conditions, although the feature
is not so useful for them since the condition cannot refer to any values
in the table.
</p><p>
If multiple triggers of the same kind are defined for the same event,
they will be fired in alphabetical order by name.
</p><p>
When the <code class="literal">CONSTRAINT</code> option is specified, this command creates a
<em class="firstterm">constraint trigger</em>.<a id="id-1.9.3.93.6.12.3" class="indexterm"></a>
This is the same as a regular trigger
except that the timing of the trigger firing can be adjusted using
<a class="link" href="sql-set-constraints.html" title="SET CONSTRAINTS"><code class="command">SET CONSTRAINTS</code></a>.
Constraint triggers must be <code class="literal">AFTER ROW</code> triggers on plain
tables (not foreign tables). They
can be fired either at the end of the statement causing the triggering
event, or at the end of the containing transaction; in the latter case they
are said to be <em class="firstterm">deferred</em>. A pending deferred-trigger firing
can also be forced to happen immediately by using <code class="command">SET
CONSTRAINTS</code>. Constraint triggers are expected to raise an exception
when the constraints they implement are violated.
</p><p>
The <code class="literal">REFERENCING</code> option enables collection
of <em class="firstterm">transition relations</em>, which are row sets that include all
of the rows inserted, deleted, or modified by the current SQL statement.
This feature lets the trigger see a global view of what the statement did,
not just one row at a time. This option is only allowed for
an <code class="literal">AFTER</code> trigger that is not a constraint trigger; also, if
the trigger is an <code class="literal">UPDATE</code> trigger, it must not specify
a <em class="replaceable"><code>column_name</code></em> list.
<code class="literal">OLD TABLE</code> may only be specified once, and only for a trigger
that can fire on <code class="literal">UPDATE</code> or <code class="literal">DELETE</code>; it creates a
transition relation containing the <em class="firstterm">before-images</em> of all rows
updated or deleted by the statement.
Similarly, <code class="literal">NEW TABLE</code> may only be specified once, and only for
a trigger that can fire on <code class="literal">UPDATE</code> or <code class="literal">INSERT</code>;
it creates a transition relation containing the <em class="firstterm">after-images</em>
of all rows updated or inserted by the statement.
</p><p>
<code class="command">SELECT</code> does not modify any rows so you cannot
create <code class="command">SELECT</code> triggers. Rules and views may provide
workable solutions to problems that seem to need <code class="command">SELECT</code>
triggers.
</p><p>
Refer to <a class="xref" href="triggers.html" title="Chapter 39. Triggers">Chapter 39</a> for more information about triggers.
</p></div><div class="refsect1" id="id-1.9.3.93.7"><h2>Parameters</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="replaceable"><code>name</code></em></span></dt><dd><p>
The name to give the new trigger. This must be distinct from
the name of any other trigger for the same table.
The name cannot be schema-qualified — the trigger inherits the
schema of its table. For a constraint trigger, this is also the name to
use when modifying the trigger's behavior using
<code class="command">SET CONSTRAINTS</code>.
</p></dd><dt><span class="term"><code class="literal">BEFORE</code><br /></span><span class="term"><code class="literal">AFTER</code><br /></span><span class="term"><code class="literal">INSTEAD OF</code></span></dt><dd><p>
Determines whether the function is called before, after, or instead of
the event. A constraint trigger can only be specified as
<code class="literal">AFTER</code>.
</p></dd><dt><span class="term"><em class="replaceable"><code>event</code></em></span></dt><dd><p>
One of <code class="literal">INSERT</code>, <code class="literal">UPDATE</code>,
<code class="literal">DELETE</code>, or <code class="literal">TRUNCATE</code>;
this specifies the event that will fire the trigger. Multiple
events can be specified using <code class="literal">OR</code>, except when
transition relations are requested.
</p><p>
For <code class="literal">UPDATE</code> events, it is possible to
specify a list of columns using this syntax:
</p><pre class="synopsis">
UPDATE OF <em class="replaceable"><code>column_name1</code></em> [, <em class="replaceable"><code>column_name2</code></em> ... ]
</pre><p>
The trigger will only fire if at least one of the listed columns
is mentioned as a target of the <code class="command">UPDATE</code> command
or if one of the listed columns is a generated column that depends on a
column that is the target of the <code class="command">UPDATE</code>.
</p><p>
<code class="literal">INSTEAD OF UPDATE</code> events do not allow a list of columns.
A column list cannot be specified when requesting transition relations,
either.
</p></dd><dt><span class="term"><em class="replaceable"><code>table_name</code></em></span></dt><dd><p>
The name (optionally schema-qualified) of the table, view, or foreign
table the trigger is for.
</p></dd><dt><span class="term"><em class="replaceable"><code>referenced_table_name</code></em></span></dt><dd><p>
The (possibly schema-qualified) name of another table referenced by the
constraint. This option is used for foreign-key constraints and is not
recommended for general use. This can only be specified for
constraint triggers.
</p></dd><dt><span class="term"><code class="literal">DEFERRABLE</code><br /></span><span class="term"><code class="literal">NOT DEFERRABLE</code><br /></span><span class="term"><code class="literal">INITIALLY IMMEDIATE</code><br /></span><span class="term"><code class="literal">INITIALLY DEFERRED</code></span></dt><dd><p>
The default timing of the trigger.
See the <a class="xref" href="sql-createtable.html" title="CREATE TABLE"><span class="refentrytitle">CREATE TABLE</span></a> documentation for details of
these constraint options. This can only be specified for constraint
triggers.
</p></dd><dt><span class="term"><code class="literal">REFERENCING</code></span></dt><dd><p>
This keyword immediately precedes the declaration of one or two
relation names that provide access to the transition relations of the
triggering statement.
</p></dd><dt><span class="term"><code class="literal">OLD TABLE</code><br /></span><span class="term"><code class="literal">NEW TABLE</code></span></dt><dd><p>
This clause indicates whether the following relation name is for the
before-image transition relation or the after-image transition
relation.
</p></dd><dt><span class="term"><em class="replaceable"><code>transition_relation_name</code></em></span></dt><dd><p>
The (unqualified) name to be used within the trigger for this
transition relation.
</p></dd><dt><span class="term"><code class="literal">FOR EACH ROW</code><br /></span><span class="term"><code class="literal">FOR EACH STATEMENT</code></span></dt><dd><p>
This specifies whether the trigger function should be fired
once for every row affected by the trigger event, or just once
per SQL statement. If neither is specified, <code class="literal">FOR EACH
STATEMENT</code> is the default. Constraint triggers can only
be specified <code class="literal">FOR EACH ROW</code>.
</p></dd><dt><span class="term"><em class="replaceable"><code>condition</code></em></span></dt><dd><p>
A Boolean expression that determines whether the trigger function
will actually be executed. If <code class="literal">WHEN</code> is specified, the
function will only be called if the <em class="replaceable"><code>condition</code></em> returns <code class="literal">true</code>.
In <code class="literal">FOR EACH ROW</code> triggers, the <code class="literal">WHEN</code>
condition can refer to columns of the old and/or new row values
by writing <code class="literal">OLD.<em class="replaceable"><code>column_name</code></em></code> or
<code class="literal">NEW.<em class="replaceable"><code>column_name</code></em></code> respectively.
Of course, <code class="literal">INSERT</code> triggers cannot refer to <code class="literal">OLD</code>
and <code class="literal">DELETE</code> triggers cannot refer to <code class="literal">NEW</code>.
</p><p><code class="literal">INSTEAD OF</code> triggers do not support <code class="literal">WHEN</code>
conditions.
</p><p>
Currently, <code class="literal">WHEN</code> expressions cannot contain
subqueries.
</p><p>
Note that for constraint triggers, evaluation of the <code class="literal">WHEN</code>
condition is not deferred, but occurs immediately after the row update
operation is performed. If the condition does not evaluate to true then
the trigger is not queued for deferred execution.
</p></dd><dt><span class="term"><em class="replaceable"><code>function_name</code></em></span></dt><dd><p>
A user-supplied function that is declared as taking no arguments
and returning type <code class="literal">trigger</code>, which is executed when
the trigger fires.
</p><p>
In the syntax of <code class="literal">CREATE TRIGGER</code>, the keywords
<code class="literal">FUNCTION</code> and <code class="literal">PROCEDURE</code> are
equivalent, but the referenced function must in any case be a function,
not a procedure. The use of the keyword <code class="literal">PROCEDURE</code>
here is historical and deprecated.
</p></dd><dt><span class="term"><em class="replaceable"><code>arguments</code></em></span></dt><dd><p>
An optional comma-separated list of arguments to be provided to
the function when the trigger is executed. The arguments are
literal string constants. Simple names and numeric constants
can be written here, too, but they will all be converted to
strings. Please check the description of the implementation
language of the trigger function to find out how these arguments
can be accessed within the function; it might be different from
normal function arguments.
</p></dd></dl></div></div><div class="refsect1" id="SQL-CREATETRIGGER-NOTES"><h2>Notes</h2><p>
To create or replace a trigger on a table, the user must have the
<code class="literal">TRIGGER</code> privilege on the table. The user must
also have <code class="literal">EXECUTE</code> privilege on the trigger function.
</p><p>
Use <a class="link" href="sql-droptrigger.html" title="DROP TRIGGER"><code class="command">DROP TRIGGER</code></a> to remove a trigger.
</p><p>
Creating a row-level trigger on a partitioned table will cause an
identical <span class="quote">“<span class="quote">clone</span>”</span> trigger to be created on each of its
existing partitions; and any partitions created or attached later will have
an identical trigger, too. If there is a conflictingly-named trigger on a
child partition already, an error occurs unless <code class="command">CREATE OR REPLACE
TRIGGER</code> is used, in which case that trigger is replaced with a
clone trigger. When a partition is detached from its parent, its clone
triggers are removed.
</p><p>
A column-specific trigger (one defined using the <code class="literal">UPDATE OF
<em class="replaceable"><code>column_name</code></em></code> syntax) will fire when any
of its columns are listed as targets in the <code class="command">UPDATE</code>
command's <code class="literal">SET</code> list. It is possible for a column's value
to change even when the trigger is not fired, because changes made to the
row's contents by <code class="literal">BEFORE UPDATE</code> triggers are not considered.
Conversely, a command such as <code class="literal">UPDATE ... SET x = x ...</code>
will fire a trigger on column <code class="literal">x</code>, even though the column's
value did not change.
</p><p>
In a <code class="literal">BEFORE</code> trigger, the <code class="literal">WHEN</code> condition is
evaluated just before the function is or would be executed, so using
<code class="literal">WHEN</code> is not materially different from testing the same
condition at the beginning of the trigger function. Note in particular
that the <code class="literal">NEW</code> row seen by the condition is the current value,
as possibly modified by earlier triggers. Also, a <code class="literal">BEFORE</code>
trigger's <code class="literal">WHEN</code> condition is not allowed to examine the
system columns of the <code class="literal">NEW</code> row (such as <code class="literal">ctid</code>),
because those won't have been set yet.
</p><p>
In an <code class="literal">AFTER</code> trigger, the <code class="literal">WHEN</code> condition is
evaluated just after the row update occurs, and it determines whether an
event is queued to fire the trigger at the end of statement. So when an
<code class="literal">AFTER</code> trigger's <code class="literal">WHEN</code> condition does not return
true, it is not necessary to queue an event nor to re-fetch the row at end
of statement. This can result in significant speedups in statements that
modify many rows, if the trigger only needs to be fired for a few of the
rows.
</p><p>
In some cases it is possible for a single SQL command to fire more than
one kind of trigger. For instance an <code class="command">INSERT</code> with
an <code class="literal">ON CONFLICT DO UPDATE</code> clause may cause both insert and
update operations, so it will fire both kinds of triggers as needed.
The transition relations supplied to triggers are
specific to their event type; thus an <code class="command">INSERT</code> trigger
will see only the inserted rows, while an <code class="command">UPDATE</code>
trigger will see only the updated rows.
</p><p>
Row updates or deletions caused by foreign-key enforcement actions, such
as <code class="literal">ON UPDATE CASCADE</code> or <code class="literal">ON DELETE SET NULL</code>, are
treated as part of the SQL command that caused them (note that such
actions are never deferred). Relevant triggers on the affected table will
be fired, so that this provides another way in which an SQL command might
fire triggers not directly matching its type. In simple cases, triggers
that request transition relations will see all changes caused in their
table by a single original SQL command as a single transition relation.
However, there are cases in which the presence of an <code class="literal">AFTER ROW</code>
trigger that requests transition relations will cause the foreign-key
enforcement actions triggered by a single SQL command to be split into
multiple steps, each with its own transition relation(s). In such cases,
any statement-level triggers that are present will be fired once per
creation of a transition relation set, ensuring that the triggers see
each affected row in a transition relation once and only once.
</p><p>
Statement-level triggers on a view are fired only if the action on the
view is handled by a row-level <code class="literal">INSTEAD OF</code> trigger.
If the action is handled by an <code class="literal">INSTEAD</code> rule, then
whatever statements are emitted by the rule are executed in place of the
original statement naming the view, so that the triggers that will be
fired are those on tables named in the replacement statements.
Similarly, if the view is automatically updatable, then the action is
handled by automatically rewriting the statement into an action on the
view's base table, so that the base table's statement-level triggers are
the ones that are fired.
</p><p>
Modifying a partitioned table or a table with inheritance children fires
statement-level triggers attached to the explicitly named table, but not
statement-level triggers for its partitions or child tables. In contrast,
row-level triggers are fired on the rows in affected partitions or
child tables, even if they are not explicitly named in the query.
If a statement-level trigger has been defined with transition relations
named by a <code class="literal">REFERENCING</code> clause, then before and after
images of rows are visible from all affected partitions or child tables.
In the case of inheritance children, the row images include only columns
that are present in the table that the trigger is attached to.
</p><p>
Currently, row-level triggers with transition relations cannot be defined
on partitions or inheritance child tables. Also, triggers on partitioned
tables may not be <code class="literal">INSTEAD OF</code>.
</p><p>
Currently, the <code class="literal">OR REPLACE</code> option is not supported for
constraint triggers.
</p><p>
Replacing an existing trigger within a transaction that has already
performed updating actions on the trigger's table is not recommended.
Trigger firing decisions, or portions of firing decisions, that have
already been made will not be reconsidered, so the effects could be
surprising.
</p><p>
There are a few built-in trigger functions that can be used to
solve common problems without having to write your own trigger code;
see <a class="xref" href="functions-trigger.html" title="9.28. Trigger Functions">Section 9.28</a>.
</p></div><div class="refsect1" id="SQL-CREATETRIGGER-EXAMPLES"><h2>Examples</h2><p>
Execute the function <code class="function">check_account_update</code> whenever
a row of the table <code class="literal">accounts</code> is about to be updated:
</p><pre class="programlisting">
CREATE TRIGGER check_update
BEFORE UPDATE ON accounts
FOR EACH ROW
EXECUTE FUNCTION check_account_update();
</pre><p>
Modify that trigger definition to only execute the function if
column <code class="literal">balance</code> is specified as a target in
the <code class="command">UPDATE</code> command:
</p><pre class="programlisting">
CREATE OR REPLACE TRIGGER check_update
BEFORE UPDATE OF balance ON accounts
FOR EACH ROW
EXECUTE FUNCTION check_account_update();
</pre><p>
This form only executes the function if column <code class="literal">balance</code>
has in fact changed value:
</p><pre class="programlisting">
CREATE TRIGGER check_update
BEFORE UPDATE ON accounts
FOR EACH ROW
WHEN (OLD.balance IS DISTINCT FROM NEW.balance)
EXECUTE FUNCTION check_account_update();
</pre><p>
Call a function to log updates of <code class="literal">accounts</code>, but only if
something changed:
</p><pre class="programlisting">
CREATE TRIGGER log_update
AFTER UPDATE ON accounts
FOR EACH ROW
WHEN (OLD.* IS DISTINCT FROM NEW.*)
EXECUTE FUNCTION log_account_update();
</pre><p>
Execute the function <code class="function">view_insert_row</code> for each row to insert
rows into the tables underlying a view:
</p><pre class="programlisting">
CREATE TRIGGER view_insert
INSTEAD OF INSERT ON my_view
FOR EACH ROW
EXECUTE FUNCTION view_insert_row();
</pre><p>
Execute the function <code class="function">check_transfer_balances_to_zero</code> for each
statement to confirm that the <code class="literal">transfer</code> rows offset to a net of
zero:
</p><pre class="programlisting">
CREATE TRIGGER transfer_insert
AFTER INSERT ON transfer
REFERENCING NEW TABLE AS inserted
FOR EACH STATEMENT
EXECUTE FUNCTION check_transfer_balances_to_zero();
</pre><p>
Execute the function <code class="function">check_matching_pairs</code> for each row to
confirm that changes are made to matching pairs at the same time (by the
same statement):
</p><pre class="programlisting">
CREATE TRIGGER paired_items_update
AFTER UPDATE ON paired_items
REFERENCING NEW TABLE AS newtab OLD TABLE AS oldtab
FOR EACH ROW
EXECUTE FUNCTION check_matching_pairs();
</pre><p>
</p><p>
<a class="xref" href="trigger-example.html" title="39.4. A Complete Trigger Example">Section 39.4</a> contains a complete example of a trigger
function written in C.
</p></div><div class="refsect1" id="SQL-CREATETRIGGER-COMPATIBILITY"><h2>Compatibility</h2><p>
The <code class="command">CREATE TRIGGER</code> statement in
<span class="productname">PostgreSQL</span> implements a subset of the
<acronym class="acronym">SQL</acronym> standard. The following functionalities are currently
missing:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
While transition table names for <code class="literal">AFTER</code> triggers are
specified using the <code class="literal">REFERENCING</code> clause in the standard way,
the row variables used in <code class="literal">FOR EACH ROW</code> triggers may not be
specified in a <code class="literal">REFERENCING</code> clause. They are available in a
manner that is dependent on the language in which the trigger function
is written, but is fixed for any one language. Some languages
effectively behave as though there is a <code class="literal">REFERENCING</code> clause
containing <code class="literal">OLD ROW AS OLD NEW ROW AS NEW</code>.
</p></li><li class="listitem"><p>
The standard allows transition tables to be used with
column-specific <code class="literal">UPDATE</code> triggers, but then the set of rows
that should be visible in the transition tables depends on the
trigger's column list. This is not currently implemented by
<span class="productname">PostgreSQL</span>.
</p></li><li class="listitem"><p>
<span class="productname">PostgreSQL</span> only allows the execution
of a user-defined function for the triggered action. The standard
allows the execution of a number of other SQL commands, such as
<code class="command">CREATE TABLE</code>, as the triggered action. This
limitation is not hard to work around by creating a user-defined
function that executes the desired commands.
</p></li></ul></div><p>
</p><p>
SQL specifies that multiple triggers should be fired in
time-of-creation order. <span class="productname">PostgreSQL</span> uses
name order, which was judged to be more convenient.
</p><p>
SQL specifies that <code class="literal">BEFORE DELETE</code> triggers on cascaded
deletes fire <span class="emphasis"><em>after</em></span> the cascaded <code class="literal">DELETE</code> completes.
The <span class="productname">PostgreSQL</span> behavior is for <code class="literal">BEFORE
DELETE</code> to always fire before the delete action, even a cascading
one. This is considered more consistent. There is also nonstandard
behavior if <code class="literal">BEFORE</code> triggers modify rows or prevent
updates during an update that is caused by a referential action. This can
lead to constraint violations or stored data that does not honor the
referential constraint.
</p><p>
The ability to specify multiple actions for a single trigger using
<code class="literal">OR</code> is a <span class="productname">PostgreSQL</span> extension of
the SQL standard.
</p><p>
The ability to fire triggers for <code class="command">TRUNCATE</code> is a
<span class="productname">PostgreSQL</span> extension of the SQL standard, as is the
ability to define statement-level triggers on views.
</p><p>
<code class="command">CREATE CONSTRAINT TRIGGER</code> is a
<span class="productname">PostgreSQL</span> extension of the <acronym class="acronym">SQL</acronym>
standard.
So is the <code class="literal">OR REPLACE</code> option.
</p></div><div class="refsect1" id="id-1.9.3.93.11"><h2>See Also</h2><span class="simplelist"><a class="xref" href="sql-altertrigger.html" title="ALTER TRIGGER"><span class="refentrytitle">ALTER TRIGGER</span></a>, <a class="xref" href="sql-droptrigger.html" title="DROP TRIGGER"><span class="refentrytitle">DROP TRIGGER</span></a>, <a class="xref" href="sql-createfunction.html" title="CREATE FUNCTION"><span class="refentrytitle">CREATE FUNCTION</span></a>, <a class="xref" href="sql-set-constraints.html" title="SET CONSTRAINTS"><span class="refentrytitle">SET CONSTRAINTS</span></a></span></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="sql-createtransform.html" title="CREATE TRANSFORM">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-createtype.html" title="CREATE TYPE">Next</a></td></tr><tr><td width="40%" align="left" valign="top">CREATE TRANSFORM </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> CREATE TYPE</td></tr></table></div></body></html>
|
