postgresql/doc/src/sgml/ref/create_rule.sgml

<refentry id="SQL-CREATERULE">
 <refmeta>
  <refentrytitle>
   CREATE RULE
  </refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>
 <refnamediv>
  <refname>
   CREATE RULE
  </refname>
  <refpurpose>
   Defines a new rule
  </refpurpose>
 </refnamediv>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>1998-09-11</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>current</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>current</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 semantics of a rule is that at the time an individual instance is
   accessed, updated, inserted or deleted, there is a current instance (for
   retrieves, updates and deletes) and a new instance (for updates and
   appends).  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 current instance, the
   <replaceable class="parameter">action</replaceable> part of the rule is
   executed. First, however, values from fields in the current instance
   and/or the new instance are substituted for
   <literal>current.</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>, the
    <replaceable class="parameter">condition</replaceable> and the
    <replaceable class="parameter">action</replaceable> parts of a rule,
    they are all considered different tuple variables. More accurately,
    <literal>new</literal> and <literal>current</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 ( ... ) WHERE ...
    </programlisting>

    <programlisting>
ON UPDATE TO emp-1.salary WHERE emp-2.name = "Joe"
    DO UPDATE emp-3 ( ... ) 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>
    When choosing between the rewrite and instance rule systems for a
    particular rule application, remember that in the rewrite system,
    <literal>current</literal> refers to a relation and some qualifiers
    whereas in the instance system it refers to an instance (tuple).
   </para>
   <para>
    It is very important to note that the rewrite rule system
    will neither detect nor process circular rules. For example, though each
    of the following two rule definitions are accepted by
    <productname>Postgres</productname>, the
    retrieve command will cause <productname>Postgres</productname> to crash:

    <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 retrieve from EMP will cause
      <productname>Postgres</productname> to crash.
      <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>
  </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 current.name = "Joe"
    DO UPDATE emp (salary = new.salary)
    WHERE emp.name = "Sam";
   </programlisting>

   At the time Joe receives a salary adjustment, the event
   will become true and Joe's current 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 current.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 current.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 NEWSET salary = 5000;
   </programlisting>
  </para>
 </refsect1>

 <refsect1 id="R1-SQL-CREATERULE-3">
  <title>
   Bugs
  </title>
  <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>
 </refsect1>

 <refsect1 id="R1-SQL-CREATERULE-4">
  <title>
   Compatibility
  </title>
  <para>
   <command>CREATE RULE</command> statement is a <productname>Postgres</productname>
   language extension.
  </para>

  <refsect2 id="R2-SQL-CREATERULE-4">
   <refsect2info>
    <date>1998-09-11</date>
   </refsect2info>
   <title>
    SQL92
   </title>
   <para>
    There is no <command>CREATE RULE</command> statement in <acronym>SQL92</acronym>.
   </para>
  </refsect2>
 </refsect1>
</refentry>

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