create_rule.sgml

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$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_rule.sgml,v 1.21 2001/01/06 04:14:35 tgl Exp $
Postgres 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>
   Defines a new 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">object</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> ... )
|
[ <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.
       </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">object</replaceable></term>
      <listitem>
       <para>
	Object is either <replaceable class="parameter">table</replaceable>
	or <replaceable class="parameter">table</replaceable>.<replaceable
	 class="parameter">column</replaceable>.  (Currently, only the
	 <replaceable class="parameter">table</replaceable> form is
	 actually implemented.)
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><replaceable class="parameter">condition</replaceable></term>
      <listitem>
       <para>
	Any SQL boolean-condition expression.  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 (the
    <replaceable class="parameter">object</replaceable>).
    <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 SELECT, ON UPDATE, and ON DELETE
    rules to refer to the existing row being selected, 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>Postgres</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
   object (table) are examined, in an unspecified order.  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 before the original query is performed.
   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 either parentheses or square brackets.  Such queries will be
   performed in the specified order (whereas there are no guarantees about
   the execution order of multiple rules for an object).  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>
  
  <refsect2 id="R2-SQL-CREATERULE-3">
   <refsect2info>
    <date>2001-01-05</date>
   </refsect2info>
   <title>
    Notes
   </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 object 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 table and
    define an ON SELECT rule for it.
   </para>

   <para>
    You must have rule definition access to a class 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>Postgres</productname>, the
    select command will cause <productname>Postgres</productname> to 
    report an error because the query cycled too many times:

    <example>
     <title>Example of a circular rewrite rule combination:</title>
     <programlisting>
CREATE RULE bad_rule_combination_1 AS
    ON SELECT TO emp
    DO INSTEAD 
	SELECT * FROM toyemp;
     </programlisting>

     <programlisting>
CREATE RULE bad_rule_combination_2 AS
    ON SELECT TO toyemp
    DO INSTEAD 
	SELECT * FROM emp;
     </programlisting>
     <para>
      This attempt to select from EMP will cause
      <productname>Postgres</productname> to issue an error
      because the queries cycled too many times:
      <programlisting>
SELECT * FROM emp;
      </programlisting></para>
    </example>
   </para>
  </refsect2>
 </refsect1>

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

  <refsect2 id="R2-SQL-CREATERULE-4">
   <refsect2info>
    <date>1998-09-11</date>
   </refsect2info>
   <title>
    SQL92
   </title>

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

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