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create_rule(7) - CREATE RULE - define a new rewrite rule - man 7 create_rule

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CREATE RULE(7)                   SQL Commands                   CREATE RULE(7)



NAME
       CREATE RULE - define a new rewrite rule


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


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 alternate action to
       be performed on insertions, updates, or deletions in(1,8)  database  tables.
       Roughly speaking, a rule causes additional commands to be executed when
       a given command on a given table is executed. Alternatively, a rule can
       replace  a  given command by another, or cause a command not to be exe-
       cuted 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 exe-
       cution  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(1,8) the
       chapter called ``The Rule System'' in(1,8) 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 con-
       tents  are  the  rows returned by the rule's SELECT command rather than
       whatever had been stored in(1,8) the table (if(3,n) anything). It  is  considered
       better style to write(1,2) 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 suffi-
       cient for your purposes) to replace update(7,n) actions  on  the  view  with
       appropriate updates on other tables.

       There  is a catch if(3,n) 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(7,n)  action,
       because  it  thinks it might end up trying to perform the action on the
       dummy table of the view in(1,8) some cases. If you want to  handle  all  the
       useful  cases  in(1,8) conditional rules, you can; just add an unconditional
       DO INSTEAD NOTHING rule to ensure that the system understands  it  will
       never  be  called  on  to update(7,n) the dummy table.  Then make the condi-
       tional rules not INSTEAD; in(1,8) the cases where they are applied, they add
       to the default INSTEAD NOTHING action.

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(1,8) alphabetical name
              order.

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

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

       condition
              Any  SQL conditional expression (returning boolean).  The condi-
              tion expression may not refer to any tables except NEW and  OLD,
              and may not contain aggregate functions.

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

       Within condition and command, the special table names NEW and  OLD  may
       be used to refer to values in(1,8) the referenced table.  NEW is valid in(1,8) ON
       INSERT and ON UPDATE rules to refer to the new row  being  inserted  or
       updated.  OLD is valid in(1,8) ON UPDATE and ON DELETE rules to refer to the
       existing row being updated or deleted.


NOTES
       You must have the privilege RULE on a table to be allowed to  define  a
       rule on it.

       It is very important to take care to avoid circular rules. For example,
       though each of the following two rule definitions are accepted by Post-
       greSQL,  the  SELECT  command would cause PostgreSQL to report an error(8,n)
       because the query cycled too many times:

       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(3,n) a rule action contains a NOTIFY command, the NOTIFY  com-
       mand  will  be  executed  unconditionally,  that is, the NOTIFY will be
       issued even if(3,n) there are not any rows that the rule  should  apply  to.
       For example, in(1,8)

       CREATE RULE notify_me AS ON UPDATE TO mytable DO 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 with id = 42. This is an implementation  restriction  that
       may be fixed in(1,8) future releases.

COMPATIBILITY
       CREATE  RULE is a PostgreSQL language extension, as is the entire rules
       system.



SQL - Language Statements         2003-11-02                    CREATE RULE(7)

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