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$PostgreSQL: pgsql/doc/src/sgml/ref/create_rule.sgml,v 1.46 2006/09/02 17:06:52 tgl Exp $
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<refentry id="SQL-CREATERULE">
 <refmeta>
  <refentrytitle id="sql-createrule-title">CREATE RULE</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>CREATE RULE</refname>
  <refpurpose>define a new rewrite rule</refpurpose>
 </refnamediv>

 <indexterm zone="sql-createrule">
  <primary>CREATE RULE</primary>
 </indexterm>

 <refsynopsisdiv>
<synopsis>
CREATE [ OR REPLACE ] RULE <replaceable class="parameter">name</replaceable> AS ON <replaceable class="parameter">event</replaceable>
    TO <replaceable class="parameter">table</replaceable> [ WHERE <replaceable class="parameter">condition</replaceable> ]
    DO [ ALSO | INSTEAD ] { NOTHING | <replaceable class="parameter">command</replaceable> | ( <replaceable class="parameter">command</replaceable> ; <replaceable class="parameter">command</replaceable> ... ) }
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

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

  <para>
   The <productname>PostgreSQL</productname> rule system allows one to
   define an alternate 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 <literal>INSTEAD</literal>
   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 <xref linkend="rules">.
  </para>
  
  <para>
   Presently, <literal>ON SELECT</literal> rules must be unconditional
   <literal>INSTEAD</literal> rules and must have actions that consist
   of a single <command>SELECT</command> command.  Thus, an
   <literal>ON SELECT</literal> rule effectively turns the table into
   a view, whose visible contents are the rows returned by the rule's
   <command>SELECT</command> command rather than whatever had been
   stored in the table (if anything).  It is considered better style
   to write a <command>CREATE VIEW</command> command than to create a
   real table and define an <literal>ON SELECT</literal> rule for it.
  </para>

  <para>
   You can create the illusion of an updatable view by defining
   <literal>ON INSERT</literal>, <literal>ON UPDATE</literal>, and
   <literal>ON DELETE</literal> 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
   <command>INSERT RETURNING</> and so on, then be sure to put a suitable
   <literal>RETURNING</> clause into each of these rules.
  </para>

  <para>
   There is a catch if you try to use conditional rules for view
   updates: there <emphasis>must</> be an unconditional
   <literal>INSTEAD</literal> rule for each action you wish to allow
   on the view.  If the rule is conditional, or is not
   <literal>INSTEAD</literal>, 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 <literal>DO
   INSTEAD NOTHING</literal> rule to ensure that the system
   understands it will never be called on to update the dummy table.
   Then make the conditional rules non-<literal>INSTEAD</literal>; in
   the cases where they are applied, they add to the default
   <literal>INSTEAD NOTHING</literal> action.  (This method does not
   currently work to support <literal>RETURNING</> queries, however.)
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><replaceable class="parameter">name</replaceable></term>
    <listitem>
     <para>
      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.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">event</replaceable></term>
    <listitem>
     <para>
      The event is one of <literal>SELECT</literal>,
      <literal>INSERT</literal>, <literal>UPDATE</literal>, or
      <literal>DELETE</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">table</replaceable></term>
    <listitem>
     <para>
      The name (optionally schema-qualified) of the table or view the
      rule applies to.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">condition</replaceable></term>
    <listitem>
     <para>
      Any <acronym>SQL</acronym> conditional expression (returning
      <type>boolean</type>).  The condition expression may not refer
      to any tables except <literal>NEW</> and <literal>OLD</>, and
      may not contain aggregate functions.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><option>INSTEAD</option></term>
    <listitem>
     <para>
      <literal>INSTEAD</literal> indicates that the commands should be
      executed <emphasis>instead of</> the original command.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><option>ALSO</option></term>
    <listitem>
     <para>
      <literal>ALSO</literal> indicates that the commands should be
      executed <emphasis>in addition to</emphasis> the original
      command.
     </para>

     <para>
      If neither <literal>ALSO</literal> nor
      <literal>INSTEAD</literal> is specified, <literal>ALSO</literal>
      is the default.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">command</replaceable></term>
    <listitem>
     <para>
      The command or commands that make up the rule action.  Valid
      commands are <command>SELECT</command>,
      <command>INSERT</command>, <command>UPDATE</command>,
      <command>DELETE</command>, or <command>NOTIFY</command>.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>

  <para>
   Within <replaceable class="parameter">condition</replaceable> and
   <replaceable class="parameter">command</replaceable>, the special
   table names <literal>NEW</literal> and <literal>OLD</literal> may
   be used to refer to values in the referenced table.
   <literal>NEW</literal> is valid in <literal>ON INSERT</literal> and
   <literal>ON UPDATE</literal> rules to refer to the new row being
   inserted or updated.  <literal>OLD</literal> is valid in
   <literal>ON UPDATE</literal> and <literal>ON DELETE</literal> rules
   to refer to the existing row being updated or deleted.
  </para>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   You must have the privilege <literal>RULE</literal> on a table to
   be allowed to define a rule on it.
  </para>

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

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

<programlisting>
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;
</programlisting>
  </para>

  <para>
   Presently, if a rule action contains a <command>NOTIFY</command>
   command, the <command>NOTIFY</command> command will be executed
   unconditionally, that is, the <command>NOTIFY</command> will be
   issued even if there are not any rows that the rule should apply
   to.  For example, in
<programlisting>
CREATE RULE notify_me AS ON UPDATE TO mytable DO ALSO NOTIFY mytable;

UPDATE mytable SET name = 'foo' WHERE id = 42;
</programlisting>
   one <command>NOTIFY</command> event will be sent during the
   <command>UPDATE</command>, whether or not there are any rows that
   match the condition <literal>id = 42</literal>.  This is an
   implementation restriction that may be fixed in future releases.
  </para>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   <command>CREATE RULE</command> is a
   <productname>PostgreSQL</productname> language extension, as is the
   entire query rewrite system.
  </para>
 </refsect1>
</refentry>

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