1CREATE RULE(7)          PostgreSQL 9.2.24 Documentation         CREATE RULE(7)
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NAME

6       CREATE_RULE - define a new rewrite rule
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SYNOPSIS

9       CREATE [ OR REPLACE ] RULE name AS ON event
10           TO table_name [ WHERE condition ]
11           DO [ ALSO | INSTEAD ] { NOTHING | command | ( command ; command ... ) }
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DESCRIPTION

14       CREATE RULE defines a new rule applying to a specified table or view.
15       CREATE OR REPLACE RULE will either create a new rule, or replace an
16       existing rule of the same name for the same table.
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18       The PostgreSQL rule system allows one to define an alternative action
19       to be performed on insertions, updates, or deletions in database
20       tables. Roughly speaking, a rule causes additional commands to be
21       executed when a given command on a given table is executed.
22       Alternatively, an INSTEAD rule can replace a given command by another,
23       or cause a command not to be executed at all. Rules are used to
24       implement table views as well. It is important to realize that a rule
25       is really a command transformation mechanism, or command macro. The
26       transformation happens before the execution of the commands starts. If
27       you actually want an operation that fires independently for each
28       physical row, you probably want to use a trigger, not a rule. More
29       information about the rules system is in Chapter 37, The Rule System,
30       in the documentation.
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32       Presently, ON SELECT rules must be unconditional INSTEAD rules and must
33       have actions that consist of a single SELECT command. Thus, an ON
34       SELECT rule effectively turns the table into a view, whose visible
35       contents are the rows returned by the rule's SELECT command rather than
36       whatever had been stored in the table (if anything). It is considered
37       better style to write a CREATE VIEW command than to create a real table
38       and define an ON SELECT rule for it.
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40       You can create the illusion of an updatable view by defining ON INSERT,
41       ON UPDATE, and ON DELETE rules (or any subset of those that's
42       sufficient for your purposes) to replace update actions on the view
43       with appropriate updates on other tables. If you want to support INSERT
44       RETURNING and so on, then be sure to put a suitable RETURNING clause
45       into each of these rules. Alternatively, an updatable view can be
46       implemented using INSTEAD OF triggers (see CREATE TRIGGER
47       (CREATE_TRIGGER(7))).
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49       There is a catch if you try to use conditional rules for view updates:
50       there must be an unconditional INSTEAD rule for each action you wish to
51       allow on the view. If the rule is conditional, or is not INSTEAD, then
52       the system will still reject attempts to perform the update action,
53       because it thinks it might end up trying to perform the action on the
54       dummy table of the view in some cases. If you want to handle all the
55       useful cases in conditional rules, add an unconditional DO INSTEAD
56       NOTHING rule to ensure that the system understands it will never be
57       called on to update the dummy table. Then make the conditional rules
58       non-INSTEAD; in the cases where they are applied, they add to the
59       default INSTEAD NOTHING action. (This method does not currently work to
60       support RETURNING queries, however.)
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PARAMETERS

63       name
64           The name of a rule to create. This must be distinct from the name
65           of any other rule for the same table. Multiple rules on the same
66           table and same event type are applied in alphabetical name order.
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68       event
69           The event is one of SELECT, INSERT, UPDATE, or DELETE.
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71       table_name
72           The name (optionally schema-qualified) of the table or view the
73           rule applies to.
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75       condition
76           Any SQL conditional expression (returning boolean). The condition
77           expression cannot refer to any tables except NEW and OLD, and
78           cannot contain aggregate functions.
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80       INSTEAD
81           INSTEAD indicates that the commands should be executed instead of
82           the original command.
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84       ALSO
85           ALSO indicates that the commands should be executed in addition to
86           the original command.
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88           If neither ALSO nor INSTEAD is specified, ALSO is the default.
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90       command
91           The command or commands that make up the rule action. Valid
92           commands are SELECT, INSERT, UPDATE, DELETE, or NOTIFY.
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94       Within condition and command, the special table names NEW and OLD can
95       be used to refer to values in the referenced table.  NEW is valid in ON
96       INSERT and ON UPDATE rules to refer to the new row being inserted or
97       updated.  OLD is valid in ON UPDATE and ON DELETE rules to refer to the
98       existing row being updated or deleted.
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NOTES

101       You must be the owner of a table to create or change rules for it.
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103       In a rule for INSERT, UPDATE, or DELETE on a view, you can add a
104       RETURNING clause that emits the view's columns. This clause will be
105       used to compute the outputs if the rule is triggered by an INSERT
106       RETURNING, UPDATE RETURNING, or DELETE RETURNING command respectively.
107       When the rule is triggered by a command without RETURNING, the rule's
108       RETURNING clause will be ignored. The current implementation allows
109       only unconditional INSTEAD rules to contain RETURNING; furthermore
110       there can be at most one RETURNING clause among all the rules for the
111       same event. (This ensures that there is only one candidate RETURNING
112       clause to be used to compute the results.)  RETURNING queries on the
113       view will be rejected if there is no RETURNING clause in any available
114       rule.
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116       It is very important to take care to avoid circular rules. For example,
117       though each of the following two rule definitions are accepted by
118       PostgreSQL, the SELECT command would cause PostgreSQL to report an
119       error because of recursive expansion of a rule:
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121           CREATE RULE "_RETURN" AS
122               ON SELECT TO t1
123               DO INSTEAD
124                   SELECT * FROM t2;
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126           CREATE RULE "_RETURN" AS
127               ON SELECT TO t2
128               DO INSTEAD
129                   SELECT * FROM t1;
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131           SELECT * FROM t1;
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133       Presently, if a rule action contains a NOTIFY command, the NOTIFY
134       command will be executed unconditionally, that is, the NOTIFY will be
135       issued even if there are not any rows that the rule should apply to.
136       For example, in:
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138           CREATE RULE notify_me AS ON UPDATE TO mytable DO ALSO NOTIFY mytable;
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140           UPDATE mytable SET name = 'foo' WHERE id = 42;
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142       one NOTIFY event will be sent during the UPDATE, whether or not there
143       are any rows that match the condition id = 42. This is an
144       implementation restriction that might be fixed in future releases.
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COMPATIBILITY

147       CREATE RULE is a PostgreSQL language extension, as is the entire query
148       rewrite system.
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152PostgreSQL 9.2.24                 2017-11-06                    CREATE RULE(7)
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