[pg-rex/syncrep.git] / doc / src / sgml / ref / create_rule.sgml

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$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_rule.sgml,v 1.34 2002/04/19 16:36:08 tgl Exp $
PostgreSQL documentation
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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>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>2001-01-05</date>
  </refsynopsisdivinfo>
  <synopsis>
CREATE 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 [ INSTEAD ] <replaceable class="parameter">action</replaceable>

where <replaceable class="PARAMETER">action</replaceable> can be:

NOTHING
|
<replaceable class="parameter">query</replaceable>
|
( <replaceable class="parameter">query</replaceable> ; <replaceable class="parameter">query</replaceable> ... )
  </synopsis>

  <refsect2 id="R2-SQL-CREATERULE-1">
   <refsect2info>
    <date>2001-01-05</date>
   </refsect2info>
   <title>
    Inputs
   </title>
   <para>

    <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.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><replaceable class="parameter">event</replaceable></term>
      <listitem>
       <para>
        Event is one of <literal>SELECT</literal>,
        <literal>UPDATE</literal>, <literal>DELETE</literal>
        or <literal>INSERT</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 SQL conditional expression (returning <type>boolean</type>).  The condition expression may not
        refer to any tables except <literal>new</literal> and
        <literal>old</literal>.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><replaceable class="parameter">query</replaceable></term>
      <listitem>
       <para>
        The query or queries making up the
        <replaceable class="PARAMETER">action</replaceable>
        can be any SQL <literal>SELECT</literal>, <literal>INSERT</literal>,
        <literal>UPDATE</literal>, <literal>DELETE</literal>, or
        <literal>NOTIFY</literal> statement.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

   <para>
    Within the <replaceable class="parameter">condition</replaceable>
    and <replaceable class="PARAMETER">action</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 ON INSERT and ON UPDATE rules
    to refer to the new row being inserted or updated.
    <literal>old</literal> is valid in ON UPDATE and ON DELETE
    rules to refer to the existing row being updated or deleted.
   </para>
  </refsect2>

  <refsect2 id="R2-SQL-CREATERULE-2">
   <refsect2info>
    <date>1998-09-11</date>
   </refsect2info>
   <title>
    Outputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term><computeroutput>
CREATE
       </computeroutput></term>
      <listitem>
       <para>
        Message returned if the rule is successfully created.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>
 </refsynopsisdiv>

 <refsect1 id="R1-SQL-CREATERULE-1">
  <refsect1info>
   <date>1998-09-11</date>
  </refsect1info>
  <title>
   Description
  </title>

  <para>
   The <productname>PostgreSQL</productname> 
   <firstterm>rule system</firstterm> allows one to define an
   alternate action to be performed on inserts, updates, or deletions
   from database tables. Rules are used to
   implement table views as well.
  </para>

  <para>
   The semantics of a rule is that at the time an individual instance (row)
   is
   accessed, inserted, updated, or deleted, there is an old instance (for
   selects, updates and deletes) and a new instance (for inserts and
   updates).  All the rules for the given event type and the given target
   table are examined successively (in order by name).  If the
   <replaceable class="parameter">condition</replaceable> specified in the
   WHERE clause (if any) is true, the 
   <replaceable class="parameter">action</replaceable> part of the rule is
   executed.  The <replaceable class="parameter">action</replaceable> is
   done instead of the original query if INSTEAD is specified; otherwise
   it is done after the original query in the case of ON INSERT, or before
   the original query in the case of ON UPDATE or ON DELETE.
   Within both the <replaceable class="parameter">condition</replaceable>
   and <replaceable class="parameter">action</replaceable>, values from
   fields in the old instance and/or the new instance are substituted for
   <literal>old.</literal><replaceable class="parameter">attribute-name</replaceable>
   and <literal>new.</literal><replaceable class="parameter">attribute-name</replaceable>.
  </para>

  <para>
   The <replaceable class="parameter">action</replaceable> part of the
   rule can consist of one or more queries. To write multiple queries,
   surround them with parentheses. Such queries will be performed in the
   specified order. The <replaceable
   class="parameter">action</replaceable> can also be NOTHING indicating
   no action. Thus, a DO INSTEAD NOTHING rule suppresses the original
   query from executing (when its condition is true); a DO NOTHING rule
   is useless.
  </para>

  <para>
   The <replaceable class="parameter">action</replaceable> part of the rule
   executes with the same command and transaction identifier as the user
   command that caused activation.
  </para>

  <para>
   It is important to realize that a rule is really a query transformation
   mechanism, or query macro.  The entire query is processed to convert it
   into a series of queries that include the rule actions.  This occurs
   before evaluation of the query starts.  So, conditional rules are
   handled by adding the rule condition to the WHERE clause of the action(s)
   derived from the rule.  The above description of a rule as an operation
   that executes for each row is thus somewhat misleading.  If you actually
   want an operation that fires independently for each physical row, you
   probably want to use a trigger not a rule.  Rules are most useful for
   situations that call for transforming entire queries independently of
   the specific data being handled.
  </para>
  
  <refsect2 id="R2-SQL-CREATERULE-3">
   <refsect2info>
    <date>2001-11-06</date>
   </refsect2info>
   <title>
    Rules and Views
   </title>
   <para>
    Presently, ON SELECT rules must be unconditional INSTEAD rules and must
    have actions that consist of a single SELECT query.  Thus, an ON SELECT
    rule effectively turns the table into a view, whose visible
    contents are the rows returned by the rule's SELECT query 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.
   </para>

   <para>
    <xref linkend="sql-createview"> creates a dummy table (with no underlying
    storage) and associates an ON SELECT rule with it.  The system will not
    allow updates to the view, since it knows there is no real table there.
    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.
   </para>

   <para>
    There is a catch if you try to use conditional
    rules for view updates: there <emphasis>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 in some cases.
    If you want to
    handle all the useful cases in 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 the dummy table.  Then
    make the conditional rules non-INSTEAD; in the cases where they fire,
    they add to the default INSTEAD NOTHING action.
   </para>
  </refsect2>

  <refsect2 id="R2-SQL-CREATERULE-4">
   <refsect2info>
    <date>2001-01-05</date>
   </refsect2info>
   <title>
    Notes
   </title>
   <para>
    You must have rule definition access to a table in order
    to define a rule on it. Use <command>GRANT</command>
    and <command>REVOKE</command> to change permissions.
   </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
    select command will cause <productname>PostgreSQL</productname> to 
    report an error because the query cycled too many times:

<programlisting>
CREATE RULE "_RETemp" AS
    ON SELECT TO emp
    DO INSTEAD 
        SELECT * FROM toyemp;

CREATE RULE "_RETtoyemp" AS
    ON SELECT TO toyemp
    DO INSTEAD 
        SELECT * FROM emp;
</programlisting>

      This attempt to select from <literal>EMP</literal> will cause
      <productname>PostgreSQL</productname> to issue an error
      because the queries cycled too many times:

 <programlisting>
SELECT * FROM emp;
</programlisting>
   </para>

   <para>
    Presently, if a rule contains a NOTIFY query, the NOTIFY 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
      <programlisting>
CREATE RULE notify_me AS ON UPDATE TO mytable DO NOTIFY mytable;

UPDATE mytable SET name = 'foo' WHERE id = 42;
      </programlisting>
    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 future releases.
   </para>
  </refsect2>
 </refsect1>

 <refsect1 id="R1-SQL-CREATERULE-4">
  <title>
   Compatibility
  </title>

  <refsect2 id="R2-SQL-CREATERULE-5">
   <refsect2info>
    <date>1998-09-11</date>
   </refsect2info>
   <title>
    SQL92
   </title>

   <para>
    <command>CREATE RULE</command> statement is a <productname>PostgreSQL</productname>
    language extension.
    There is no <command>CREATE RULE</command> statement in <acronym>SQL92</acronym>.
   </para>
  </refsect2>
 </refsect1>
</refentry>

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