1CREATE RULE(7) PostgreSQL 16.1 Documentation CREATE RULE(7)
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6 CREATE_RULE - define a new rewrite rule
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9 CREATE [ OR REPLACE ] RULE name AS ON event
10 TO table_name [ WHERE condition ]
11 DO [ ALSO | INSTEAD ] { NOTHING | command | ( command ; command ... ) }
12 where event can be one of:
14 SELECT | INSERT | UPDATE | DELETE
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18 CREATE RULE defines a new rule applying to a specified table or view.
19 CREATE OR REPLACE RULE will either create a new rule, or replace an
20 existing rule of the same name for the same table.
21 The PostgreSQL rule system allows one to define an alternative action
23 to be performed on insertions, updates, or deletions in database
24 tables. Roughly speaking, a rule causes additional commands to be
25 executed when a given command on a given table is executed.
26 Alternatively, an INSTEAD rule can replace a given command by another,
27 or cause a command not to be executed at all. Rules are used to
28 implement SQL views as well. It is important to realize that a rule is
29 really a command transformation mechanism, or command macro. The
30 transformation happens before the execution of the command starts. If
31 you actually want an operation that fires independently for each
32 physical row, you probably want to use a trigger, not a rule. More
33 information about the rules system is in Chapter 41.
34 Presently, ON SELECT rules can only be attached to views. Such a rule
36 must be named "_RETURN", must be an unconditional INSTEAD rule, and
37 must have an action that consists of a single SELECT command. This
38 command defines the visible contents of the view. (The view itself is
39 basically a dummy table with no storage.) It's best to regard such a
40 rule as an implementation detail. While a view can be redefined via
41 CREATE OR REPLACE RULE "_RETURN" AS ..., it's better style to use
42 CREATE OR REPLACE VIEW.
43 You can create the illusion of an updatable view by defining ON INSERT,
45 ON UPDATE, and ON DELETE rules (or any subset of those that's
46 sufficient for your purposes) to replace update actions on the view
47 with appropriate updates on other tables. If you want to support INSERT
48 RETURNING and so on, then be sure to put a suitable RETURNING clause
49 into each of these rules.
50 There is a catch if you try to use conditional rules for complex view
52 updates: there must be an unconditional INSTEAD rule for each action
53 you wish to allow on the view. If the rule is conditional, or is not
54 INSTEAD, then the system will still reject attempts to perform the
55 update action, because it thinks it might end up trying to perform the
56 action on the dummy table of the view in some cases. If you want to
57 handle all the useful cases in conditional rules, add an unconditional
58 DO INSTEAD NOTHING rule to ensure that the system understands it will
59 never be called on to update the dummy table. Then make the conditional
60 rules non-INSTEAD; in the cases where they are applied, they add to the
61 default INSTEAD NOTHING action. (This method does not currently work to
62 support RETURNING queries, however.)
63 Note
65 A view that is simple enough to be automatically updatable (see
66 CREATE VIEW (CREATE_VIEW(7))) does not require a user-created rule
67 in order to be updatable. While you can create an explicit rule
68 anyway, the automatic update transformation will generally
69 outperform an explicit rule.
70 Another alternative worth considering is to use INSTEAD OF triggers
72 (see CREATE TRIGGER (CREATE_TRIGGER(7))) in place of rules.
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75 name
76 The name of a rule to create. This must be distinct from the name
77 of any other rule for the same table. Multiple rules on the same
78 table and same event type are applied in alphabetical name order.
79 event
81 The event is one of SELECT, INSERT, UPDATE, or DELETE. Note that an
82 INSERT containing an ON CONFLICT clause cannot be used on tables
83 that have either INSERT or UPDATE rules. Consider using an
84 updatable view instead.
85 table_name
87 The name (optionally schema-qualified) of the table or view the
88 rule applies to.
89 condition
91 Any SQL conditional expression (returning boolean). The condition
92 expression cannot refer to any tables except NEW and OLD, and
93 cannot contain aggregate functions.
94 INSTEAD
96 INSTEAD indicates that the commands should be executed instead of
97 the original command.
98 ALSO
100 ALSO indicates that the commands should be executed in addition to
101 the original command.
102 If neither ALSO nor INSTEAD is specified, ALSO is the default.
104 command
106 The command or commands that make up the rule action. Valid
107 commands are SELECT, INSERT, UPDATE, DELETE, or NOTIFY.
108 Within condition and command, the special table names NEW and OLD can
110 be used to refer to values in the referenced table. NEW is valid in ON
111 INSERT and ON UPDATE rules to refer to the new row being inserted or
112 updated. OLD is valid in ON UPDATE and ON DELETE rules to refer to the
113 existing row being updated or deleted.
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116 You must be the owner of a table to create or change rules for it.
117 In a rule for INSERT, UPDATE, or DELETE on a view, you can add a
119 RETURNING clause that emits the view's columns. This clause will be
120 used to compute the outputs if the rule is triggered by an INSERT
121 RETURNING, UPDATE RETURNING, or DELETE RETURNING command respectively.
122 When the rule is triggered by a command without RETURNING, the rule's
123 RETURNING clause will be ignored. The current implementation allows
124 only unconditional INSTEAD rules to contain RETURNING; furthermore
125 there can be at most one RETURNING clause among all the rules for the
126 same event. (This ensures that there is only one candidate RETURNING
127 clause to be used to compute the results.) RETURNING queries on the
128 view will be rejected if there is no RETURNING clause in any available
129 rule.
130 It is very important to take care to avoid circular rules. For example,
132 though each of the following two rule definitions are accepted by
133 PostgreSQL, the SELECT command would cause PostgreSQL to report an
134 error because of recursive expansion of a rule:
135 CREATE RULE "_RETURN" AS
137 ON SELECT TO t1
138 DO INSTEAD
139 SELECT * FROM t2;
140 CREATE RULE "_RETURN" AS
142 ON SELECT TO t2
143 DO INSTEAD
144 SELECT * FROM t1;
145 SELECT * FROM t1;
147 Presently, if a rule action contains a NOTIFY command, the NOTIFY
149 command will be executed unconditionally, that is, the NOTIFY will be
150 issued even if there are not any rows that the rule should apply to.
151 For example, in:
152 CREATE RULE notify_me AS ON UPDATE TO mytable DO ALSO NOTIFY mytable;
154 UPDATE mytable SET name = 'foo' WHERE id = 42;
156 one NOTIFY event will be sent during the UPDATE, whether or not there
158 are any rows that match the condition id = 42. This is an
159 implementation restriction that might be fixed in future releases.
160
162 CREATE RULE is a PostgreSQL language extension, as is the entire query
163 rewrite system.
164
166 ALTER RULE (ALTER_RULE(7)), DROP RULE (DROP_RULE(7))
167PostgreSQL 16.1 2023 CREATE RULE(7)