NAME

       CREATE_RULE - define a new rewrite rule

SYNOPSIS

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

       where event can be one of:

           SELECT | INSERT | UPDATE | DELETE

DESCRIPTION

       CREATE RULE defines a new rule applying to a specified table or view.  CREATE OR REPLACE
       RULE will either create a new rule, or replace an existing rule of the same name for the
       same table.

       The PostgreSQL rule system allows one to define an alternative action to be performed on
       insertions, updates, or deletions in database tables. Roughly speaking, a rule causes
       additional commands to be executed when a given command on a given table is executed.
       Alternatively, an INSTEAD rule can replace a given command by another, or cause a command
       not to be executed at all. Rules are used to implement SQL views as well. It is important
       to realize that a rule is really a command transformation mechanism, or command macro. The
       transformation happens before the execution of the command starts. If you actually want an
       operation that fires independently for each physical row, you probably want to use a
       trigger, not a rule. More information about the rules system is in Chapter 40.

       Presently, ON SELECT rules can only be attached to views. (Attaching one to a table
       converts the table into a view.) Such a rule must be named "_RETURN", must be an
       unconditional INSTEAD rule, and must have an action that consists of a single SELECT
       command. This command defines the visible contents of the view. (The view itself is
       basically a dummy table with no storage.) It's best to regard such a rule as an
       implementation detail. While a view can be redefined via CREATE OR REPLACE RULE "_RETURN"
       AS ..., it's better style to use CREATE OR REPLACE VIEW.

       You can create the illusion of an updatable view by defining ON INSERT, ON UPDATE, and ON
       DELETE rules (or any subset of those that's sufficient for your purposes) to replace
       update actions on the view with appropriate updates on other tables. If you want to
       support INSERT RETURNING and so on, then be sure to put a suitable RETURNING clause into
       each of these rules.

       There is a catch if you try to use conditional rules for complex view updates: there must
       be an unconditional INSTEAD rule for each action you wish to allow on the view. If the
       rule is conditional, or is not INSTEAD, then the system will still reject attempts to
       perform the update action, because it thinks it might end up trying to perform the action
       on the dummy table of the view in some cases. If you want to handle all the useful cases
       in conditional rules, add an unconditional DO INSTEAD NOTHING rule to ensure that the
       system understands it will never be called on to update the dummy table. Then make the
       conditional rules non-INSTEAD; in the cases where they are applied, they add to the
       default INSTEAD NOTHING action. (This method does not currently work to support RETURNING
       queries, however.)

           Note
           A view that is simple enough to be automatically updatable (see CREATE VIEW
           (CREATE_VIEW(7))) does not require a user-created rule in order to be updatable. While
           you can create an explicit rule anyway, the automatic update transformation will
           generally outperform an explicit rule.

           Another alternative worth considering is to use INSTEAD OF triggers (see CREATE
           TRIGGER (CREATE_TRIGGER(7))) in place of rules.

PARAMETERS

       name
           The name of a rule to create. This must be distinct from the name of any other rule
           for the same table. Multiple rules on the same table and same event type are applied
           in alphabetical name order.

       event
           The event is one of SELECT, INSERT, UPDATE, or DELETE. Note that an INSERT containing
           an ON CONFLICT clause cannot be used on tables that have either INSERT or UPDATE
           rules. Consider using an updatable view instead.

       table_name
           The name (optionally schema-qualified) of the table or view the rule applies to.

       condition
           Any SQL conditional expression (returning boolean). The condition expression cannot
           refer to any tables except NEW and OLD, and cannot contain aggregate functions.

       INSTEAD
           INSTEAD indicates that the commands should be executed instead of the original
           command.

       ALSO
           ALSO indicates that the commands should be executed in addition to the original
           command.

           If neither ALSO nor INSTEAD is specified, ALSO is the default.

       command
           The command or commands that make up the rule action. Valid commands are SELECT,
           INSERT, UPDATE, DELETE, or NOTIFY.

       Within condition and command, the special table names NEW and OLD can be used to refer to
       values in the referenced table.  NEW is valid in ON INSERT and ON UPDATE rules to refer to
       the new row being inserted or updated.  OLD is valid in ON UPDATE and ON DELETE rules to
       refer to the existing row being updated or deleted.

NOTES

       You must be the owner of a table to create or change rules for it.

       In a rule for INSERT, UPDATE, or DELETE on a view, you can add a RETURNING clause that
       emits the view's columns. This clause will be used to compute the outputs if the rule is
       triggered by an INSERT RETURNING, UPDATE RETURNING, or DELETE RETURNING command
       respectively. When the rule is triggered by a command without RETURNING, the rule's
       RETURNING clause will be ignored. The current implementation allows only unconditional
       INSTEAD rules to contain RETURNING; furthermore there can be at most one RETURNING clause
       among all the rules for the same event. (This ensures that there is only one candidate
       RETURNING clause to be used to compute the results.)  RETURNING queries on the view will
       be rejected if there is no RETURNING clause in any available rule.

       It is very important to take care to avoid circular rules. For example, though each of the
       following two rule definitions are accepted by PostgreSQL, the SELECT command would cause
       PostgreSQL to report an error because of recursive expansion of a rule:

           CREATE RULE "_RETURN" AS
               ON SELECT TO t1
               DO INSTEAD
                   SELECT * FROM t2;

           CREATE RULE "_RETURN" AS
               ON SELECT TO t2
               DO INSTEAD
                   SELECT * FROM t1;

           SELECT * FROM t1;

       Presently, if a rule action contains a NOTIFY command, the NOTIFY command will be executed
       unconditionally, that is, the NOTIFY will be issued even if there are not any rows that
       the rule should apply to. For example, in:

           CREATE RULE notify_me AS ON UPDATE TO mytable DO ALSO NOTIFY mytable;

           UPDATE mytable SET name = 'foo' WHERE id = 42;

       one NOTIFY event will be sent during the UPDATE, whether or not there are any rows that
       match the condition id = 42. This is an implementation restriction that might be fixed in
       future releases.

COMPATIBILITY

       CREATE RULE is a PostgreSQL language extension, as is the entire query rewrite system.

SEE ALSO

       ALTER RULE (ALTER_RULE(7)), DROP RULE (DROP_RULE(7))