1CREATE TRIGGER(7) SQL Commands CREATE TRIGGER(7)
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6 CREATE TRIGGER - define a new trigger
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10 CREATE TRIGGER name { BEFORE | AFTER } { event [ OR ... ] }
11 ON table [ FOR [ EACH ] { ROW | STATEMENT } ]
12 EXECUTE PROCEDURE funcname ( arguments )
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16 CREATE TRIGGER creates a new trigger. The trigger will be associated
17 with the specified table and will execute the specified function func‐
18 name when certain events occur.
19 The trigger can be specified to fire either before the operation is
21 attempted on a row (before constraints are checked and the INSERT,
22 UPDATE, or DELETE is attempted) or after the operation has completed
23 (after constraints are checked and the INSERT, UPDATE, or DELETE has
24 completed). If the trigger fires before the event, the trigger can skip
25 the operation for the current row, or change the row being inserted
26 (for INSERT and UPDATE operations only). If the trigger fires after the
27 event, all changes, including the last insertion, update, or deletion,
28 are ``visible'' to the trigger.
29 A trigger that is marked FOR EACH ROW is called once for every row that
31 the operation modifies. For example, a DELETE that affects 10 rows will
32 cause any ON DELETE triggers on the target relation to be called 10
33 separate times, once for each deleted row. In contrast, a trigger that
34 is marked FOR EACH STATEMENT only executes once for any given opera‐
35 tion, regardless of how many rows it modifies (in particular, an opera‐
36 tion that modifies zero rows will still result in the execution of any
37 applicable FOR EACH STATEMENT triggers).
38 In addition, triggers may be defined to fire for a TRUNCATE, though
40 only FOR EACH STATEMENT.
41 If multiple triggers of the same kind are defined for the same event,
43 they will be fired in alphabetical order by name.
44 SELECT does not modify any rows so you cannot create SELECT triggers.
46 Rules and views are more appropriate in such cases.
47 Refer to in the documentation for more information about triggers.
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51 name The name to give the new trigger. This must be distinct from the
52 name of any other trigger for the same table.
53 BEFORE
55 AFTER Determines whether the function is called before or after the
57 event.
58 event One of INSERT, UPDATE, DELETE, or TRUNCATE; this specifies the
60 event that will fire the trigger. Multiple events can be speci‐
61 fied using OR.
62 table The name (optionally schema-qualified) of the table the trigger
64 is for.
65 FOR EACH ROW
67 FOR EACH STATEMENT
69 This specifies whether the trigger procedure should be fired
70 once for every row affected by the trigger event, or just once
71 per SQL statement. If neither is specified, FOR EACH STATEMENT
72 is the default.
73 funcname
75 A user-supplied function that is declared as taking no arguments
76 and returning type trigger, which is executed when the trigger
77 fires.
78 arguments
80 An optional comma-separated list of arguments to be provided to
81 the function when the trigger is executed. The arguments are
82 literal string constants. Simple names and numeric constants can
83 be written here, too, but they will all be converted to strings.
84 Please check the description of the implementation language of
85 the trigger function about how the trigger arguments are acces‐
86 sible within the function; it might be different from normal
87 function arguments.
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90 To create a trigger on a table, the user must have the TRIGGER privi‐
91 lege on the table. The user must also have EXECUTE privilege on the
92 trigger function.
93 Use DROP TRIGGER [drop_trigger(7)] to remove a trigger.
95 In PostgreSQL versions before 7.3, it was necessary to declare trigger
97 functions as returning the placeholder type opaque, rather than trig‐
98 ger. To support loading of old dump files, CREATE TRIGGER will accept a
99 function declared as returning opaque, but it will issue a notice and
100 change the function's declared return type to trigger.
101
103 in the documentation contains a complete example.
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106 The CREATE TRIGGER statement in PostgreSQL implements a subset of the
107 SQL standard. The following functionality is currently missing:
108 · SQL allows triggers to fire on updates to specific columns (e.g.,
110 AFTER UPDATE OF col1, col2).
111 · SQL allows you to define aliases for the ``old'' and ``new'' rows or
113 tables for use in the definition of the triggered action (e.g., CRE‐
114 ATE TRIGGER ... ON tablename REFERENCING OLD ROW AS somename NEW ROW
115 AS othername ...). Since PostgreSQL allows trigger procedures to be
116 written in any number of user-defined languages, access to the data
117 is handled in a language-specific way.
118 · PostgreSQL only allows the execution of a user-defined function for
120 the triggered action. The standard allows the execution of a number
121 of other SQL commands, such as CREATE TABLE as the triggered action.
122 This limitation is not hard to work around by creating a user-defined
123 function that executes the desired commands.
124 SQL specifies that multiple triggers should be fired in time-of-cre‐
126 ation order. PostgreSQL uses name order, which was judged to be more
127 convenient.
128 SQL specifies that BEFORE DELETE triggers on cascaded deletes fire
130 after the cascaded DELETE completes. The PostgreSQL behavior is for
131 BEFORE DELETE to always fire before the delete action, even a cascading
132 one. This is considered more consistent. There is also unpredictable
133 behavior when BEFORE triggers modify rows that are later to be modified
134 by referential actions. This can lead to constraint violations or
135 stored data that does not honor the referential constraint.
136 The ability to specify multiple actions for a single trigger using OR
138 is a PostgreSQL extension of the SQL standard.
139 The ability to fire triggers for TRUNCATE is a PostgreSQL extension of
141 the SQL standard.
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144 CREATE FUNCTION [create_function(7)], ALTER TRIGGER [alter_trigger(7)],
145 DROP TRIGGER [drop_trigger(7)]
146SQL - Language Statements 2014-02-17 CREATE TRIGGER(7)