Provided by: postgresql-client-9.1_9.1.3-2_amd64 bug


       CREATE_RULE - define a new rewrite rule


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


       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 table 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 commands 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
       37, The Rule System, in the documentation.

       Presently, ON SELECT rules must be unconditional INSTEAD rules and must have actions that
       consist of a single SELECT command. Thus, an ON SELECT rule effectively turns the table
       into a view, whose visible contents are the rows returned by the rule's SELECT command
       rather than whatever had been stored in the table (if anything). It is considered better
       style to write a CREATE VIEW command than to create a real table and define an ON SELECT
       rule for it.

       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. Alternatively, an updatable view can be implemented using INSTEAD OF
       triggers (see CREATE TRIGGER (CREATE_TRIGGER(7))).

       There is a catch if you try to use conditional rules for 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.)


           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.

           The event is one of SELECT, INSERT, UPDATE, or DELETE.

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

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

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

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

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

           The command or commands that make up the rule action. Valid commands are SELECT,

       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.


       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
       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:

               ON SELECT TO t1
               DO INSTEAD
                   SELECT * FROM t2;

               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.


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