1CREATE RULE(7) PostgreSQL 12.6 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 40.
34 Presently, ON SELECT rules must be unconditional INSTEAD rules and must
36 have actions that consist of a single SELECT command. Thus, an ON
37 SELECT rule effectively turns the table into a view, whose visible
38 contents are the rows returned by the rule's SELECT command rather than
39 whatever had been stored in the table (if anything). It is considered
40 better style to write a CREATE VIEW command than to create a real table
41 and define an ON SELECT rule for it.
42 You can create the illusion of an updatable view by defining ON INSERT,
44 ON UPDATE, and ON DELETE rules (or any subset of those that's
45 sufficient for your purposes) to replace update actions on the view
46 with appropriate updates on other tables. If you want to support INSERT
47 RETURNING and so on, then be sure to put a suitable RETURNING clause
48 into each of these rules.
49 There is a catch if you try to use conditional rules for complex view
51 updates: there must be an unconditional INSTEAD rule for each action
52 you wish to allow on the view. If the rule is conditional, or is not
53 INSTEAD, then the system will still reject attempts to perform the
54 update action, because it thinks it might end up trying to perform the
55 action on the dummy table of the view in some cases. If you want to
56 handle all the useful cases in conditional rules, add an unconditional
57 DO INSTEAD NOTHING rule to ensure that the system understands it will
58 never be called on to update the dummy table. Then make the conditional
59 rules non-INSTEAD; in the cases where they are applied, they add to the
60 default INSTEAD NOTHING action. (This method does not currently work to
61 support RETURNING queries, however.)
62 Note
64 A view that is simple enough to be automatically updatable (see
65 CREATE VIEW (CREATE_VIEW(7))) does not require a user-created rule
66 in order to be updatable. While you can create an explicit rule
67 anyway, the automatic update transformation will generally
68 outperform an explicit rule.
69 Another alternative worth considering is to use INSTEAD OF triggers
71 (see CREATE TRIGGER (CREATE_TRIGGER(7))) in place of rules.
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74 name
75 The name of a rule to create. This must be distinct from the name
76 of any other rule for the same table. Multiple rules on the same
77 table and same event type are applied in alphabetical name order.
78 event
80 The event is one of SELECT, INSERT, UPDATE, or DELETE. Note that an
81 INSERT containing an ON CONFLICT clause cannot be used on tables
82 that have either INSERT or UPDATE rules. Consider using an
83 updatable view instead.
84 table_name
86 The name (optionally schema-qualified) of the table or view the
87 rule applies to.
88 condition
90 Any SQL conditional expression (returning boolean). The condition
91 expression cannot refer to any tables except NEW and OLD, and
92 cannot contain aggregate functions.
93 INSTEAD
95 INSTEAD indicates that the commands should be executed instead of
96 the original command.
97 ALSO
99 ALSO indicates that the commands should be executed in addition to
100 the original command.
101 If neither ALSO nor INSTEAD is specified, ALSO is the default.
103 command
105 The command or commands that make up the rule action. Valid
106 commands are SELECT, INSERT, UPDATE, DELETE, or NOTIFY.
107 Within condition and command, the special table names NEW and OLD can
109 be used to refer to values in the referenced table. NEW is valid in ON
110 INSERT and ON UPDATE rules to refer to the new row being inserted or
111 updated. OLD is valid in ON UPDATE and ON DELETE rules to refer to the
112 existing row being updated or deleted.
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115 You must be the owner of a table to create or change rules for it.
116 In a rule for INSERT, UPDATE, or DELETE on a view, you can add a
118 RETURNING clause that emits the view's columns. This clause will be
119 used to compute the outputs if the rule is triggered by an INSERT
120 RETURNING, UPDATE RETURNING, or DELETE RETURNING command respectively.
121 When the rule is triggered by a command without RETURNING, the rule's
122 RETURNING clause will be ignored. The current implementation allows
123 only unconditional INSTEAD rules to contain RETURNING; furthermore
124 there can be at most one RETURNING clause among all the rules for the
125 same event. (This ensures that there is only one candidate RETURNING
126 clause to be used to compute the results.) RETURNING queries on the
127 view will be rejected if there is no RETURNING clause in any available
128 rule.
129 It is very important to take care to avoid circular rules. For example,
131 though each of the following two rule definitions are accepted by
132 PostgreSQL, the SELECT command would cause PostgreSQL to report an
133 error because of recursive expansion of a rule:
134 CREATE RULE "_RETURN" AS
136 ON SELECT TO t1
137 DO INSTEAD
138 SELECT * FROM t2;
139 CREATE RULE "_RETURN" AS
141 ON SELECT TO t2
142 DO INSTEAD
143 SELECT * FROM t1;
144 SELECT * FROM t1;
146 Presently, if a rule action contains a NOTIFY command, the NOTIFY
148 command will be executed unconditionally, that is, the NOTIFY will be
149 issued even if there are not any rows that the rule should apply to.
150 For example, in:
151 CREATE RULE notify_me AS ON UPDATE TO mytable DO ALSO NOTIFY mytable;
153 UPDATE mytable SET name = 'foo' WHERE id = 42;
155 one NOTIFY event will be sent during the UPDATE, whether or not there
157 are any rows that match the condition id = 42. This is an
158 implementation restriction that might be fixed in future releases.
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161 CREATE RULE is a PostgreSQL language extension, as is the entire query
162 rewrite system.
163
165 ALTER RULE (ALTER_RULE(7)), DROP RULE (DROP_RULE(7))
166PostgreSQL 12.6 2021 CREATE RULE(7)