create_rule.sgml

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$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_rule.sgml,v 1.17 2000/07/21 04:22:54 momjian 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>1999-07-20</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> | NOTHING ]
  </synopsis>

  <refsect2 id="R2-SQL-CREATERULE-1">
   <refsect2info>
    <date>1998-09-11</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>.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><replaceable class="parameter">condition</replaceable></term>
      <listitem>
       <para>
	Any SQL WHERE clause, <literal>new</literal> or
	<literal>old</literal> can appear instead of an instance
	variable whenever an instance variable is permissible in SQL.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><replaceable class="parameter">action</replaceable></term>
      <listitem>
       <para>
	Any SQL statement, <literal>new</literal> or
	<literal>old</literal> can appear instead of an instance
	variable whenever an instance variable is permissible in SQL.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </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 or classes. Currently, rules are used to
   implement table views.
  </para>

  <para>
   The semantics of a rule is that at the time an individual instance is
   accessed, inserted, updated, or deleted, there is a old instance (for
   selects, updates and deletes) and a new instance (for inserts and
  updates).
  If the <replaceable class="parameter">event</replaceable>
   specified in the ON clause and the
   <replaceable class="parameter">condition</replaceable> specified in the
   WHERE clause are true for the old instance, the
   <replaceable class="parameter">action</replaceable> part of the rule is
   executed. First, however, 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
   executes with the same command and transaction identifier as the user
   command that caused activation.
  </para>
  
  <refsect2 id="R2-SQL-CREATERULE-3">
   <refsect2info>
    <date>1998-09-11</date>
   </refsect2info>
   <title>
    Notes
   </title>
   <para>
    A caution about SQL rules is in  order.  If the same class name
    or instance variable appears in the
    <replaceable class="parameter">event</replaceable>,
    <replaceable class="parameter">condition</replaceable> and
    <replaceable class="parameter">action</replaceable> parts of a rule,
    they are all considered different tuple variables. More accurately,
    <literal>new</literal> and <literal>old</literal> are the only tuple
    variables that are shared between these clauses. For example, the following
    two rules have the same semantics:
    <programlisting>
ON UPDATE TO emp.salary WHERE emp.name = "Joe"
    DO 
	UPDATE emp SET ... WHERE ...
    </programlisting>

    <programlisting>
ON UPDATE TO emp-1.salary WHERE emp-2.name = "Joe"
    DO 
	UPDATE emp-3 SET ...  WHERE ...
    </programlisting>

    Each rule can have the optional tag INSTEAD.
    Without
    this tag, <replaceable class="parameter">action</replaceable> will be
    performed in addition to the user command when the
    <replaceable class="parameter">event</replaceable> in the
    <replaceable class="parameter">condition</replaceable> part of the rule
    occurs. Alternately, the
    <replaceable class="parameter">action</replaceable> part will be done
    instead of the user command. In this later case, the
    <replaceable class="parameter">action</replaceable> can be the keyword
    <literal>NOTHING</literal>.
   </para>
   <para>
    It is very important to note 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 TO toyemp;
     </programlisting>

     <programlisting>
CREATE RULE bad_rule_combination_2 AS
    ON SELECT TO toyemp
    DO INSTEAD 
	SELECT TO 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>

   <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>
    The object in a <acronym>SQL</acronym> rule cannot be an array reference and
    cannot have parameters.
   </para>

   <para>
    Aside from the "oid" field, system attributes cannot be
    referenced anywhere in a rule. Among other things, this
    means that functions of instances (e.g., <literal>foo(emp)</literal> where
    <literal>emp</literal> is a class) cannot be called anywhere in a rule.
   </para>

   <para>
    The rule system stores the rule text and query plans as
    text attributes. This implies that creation of rules may
    fail if the rule plus its various internal representations
    exceed some value that is on the order of one page (8KB).
   </para>
  </refsect2>
 </refsect1>
 
 <refsect1 id="R1-SQL-CREATERULE-2">
  <title>
   Usage
  </title>
  <para>
   Make Sam get the same salary adjustment as Joe:
   
   <programlisting>
CREATE RULE example_1 AS
    ON UPDATE emp.salary WHERE old.name = "Joe"
    DO 
	UPDATE emp 
	SET salary = new.salary
	WHERE emp.name = "Sam";
   </programlisting>

   At the time Joe receives a salary adjustment, the event
   will become true and Joe's old instance and proposed
   new instance are available to the execution routines.
   Hence, his new salary is substituted into the action part
   of the rule which is subsequently executed.  This propagates
   Joe's salary on to Sam.
  </para>
  <para>
   Make Bill get Joe's salary when it is accessed:
   <programlisting>
CREATE RULE example_2 AS
    ON SELECT TO EMP.salary
    WHERE old.name = "Bill"
    DO INSTEAD
	SELECT emp.salary
	FROM emp
	WHERE emp.name = "Joe";
   </programlisting>
  </para>
  <para>
   Deny Joe access to the salary of employees in the shoe
   department (<function>current_user</function> returns the name of
   the current user):
   <programlisting>  
CREATE RULE example_3 AS
    ON 
	SELECT TO emp.salary
   	WHERE old.dept = "shoe" AND current_user = "Joe"
    DO INSTEAD NOTHING;
   </programlisting>
  </para>
  <para>
   Create a view of the employees working in the toy department:
   <programlisting>
CREATE toyemp(name = char16, salary = int4);

CREATE RULE example_4 AS
    ON SELECT TO toyemp
    DO INSTEAD
    	SELECT emp.name, emp.salary
	FROM emp
        WHERE emp.dept = "toy";
   </programlisting>
  </para>
  <para>
   All new employees must make 5,000 or less:
   <programlisting>
CREATE RULE example_5 AS
    ON INERT TO emp WHERE new.salary > 5000
    DO 
	UPDATE emp SET salary = 5000
	WHERE emp.oid = new.oid;
   </programlisting>
  </para>
 </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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