<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> <REFSYNOPSISDIV> <REFSYNOPSISDIVINFO> <DATE>1998-04-15</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>1998-04-15</DATE> </REFSECT2INFO> <TITLE> Inputs </TITLE> <PARA> </PARA> <VARIABLELIST> <VARLISTENTRY> <TERM> </TERM> <LISTITEM> <PARA> <VARIABLELIST> <VARLISTENTRY> <TERM> <ReturnValue><replaceable class="parameter">name</replaceable></ReturnValue> </TERM> <LISTITEM> <PARA> The name of a rule to create. </PARA> </LISTITEM> </VARLISTENTRY> <VARLISTENTRY> <TERM> <ReturnValue><replaceable class="parameter">event</replaceable></ReturnValue> </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> <ReturnValue><replaceable class="parameter">object</replaceable></ReturnValue> </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> <ReturnValue><replaceable class="parameter">condition</replaceable></ReturnValue> </TERM> <LISTITEM> <PARA> Any SQL <literal>where</literal> 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> <ReturnValue><replaceable class="parameter">action</replaceable></ReturnValue> </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> </LISTITEM> </VARLISTENTRY> </VARIABLELIST> </REFSECT2> <REFSECT2 ID="R2-SQL-CREATERULE-2"> <REFSECT2INFO> <DATE>1998-04-15</DATE> </REFSECT2INFO> <TITLE> Outputs </TITLE> <PARA> </PARA> <VARIABLELIST> <VARLISTENTRY> <TERM> </TERM> <LISTITEM> <PARA> <VARIABLELIST> <VARLISTENTRY> <TERM> <ReturnValue>CREATE</ReturnValue> </TERM> <LISTITEM> <PARA> Message returned if the rule is successfully created. </PARA> </LISTITEM> </VARLISTENTRY> </variablelist> </LISTITEM> </VARLISTENTRY> </VARIABLELIST> </REFSECT2> </REFSYNOPSISDIV> <REFSECT1 ID="R1-SQL-CREATERULE-1"> <REFSECT1INFO> <DATE>1998-04-15</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 <literal>on</literal> clause and the <replaceable class="parameter">condition</replaceable> specified in the <literal>where</literal> 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>1998-04-15</DATE> </REFSECT2INFO> <TITLE> Notes </TITLE> <para> A note of 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 ... on update to EMP-1.salary where EMP-2.name = "Joe" do update EMP-3 ( ... ) where ... </programlisting> Each rule can have the optional tag <literal>instead</literal>. 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 Postgres, the retrieve command will cause Postgres to crash: <example> <title>Example of a circular rewrite rule combination.</title> <programlisting> create rule bad_rule_combination_1 is on select to EMP do instead select to TOYEMP create rule bad_rule_combination_2 is on select to TOYEMP do instead select to EMP </programlisting> <para> This attempt to retrieve from EMP will cause Postgres 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 (see change acl(l)). <comment> There is no manpage change or change_acl. What is intended? </comment> </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 is 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 is 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>pg_username()</function> returns the name of the current user) <programlisting> create rule example_3 is on select to EMP.salary where current.dept = "shoe" and pg_username() = "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 is 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 is on insert to EMP where new.salary > 5000 do update newset salary = 5000 </programlisting> </PARA> </REFSECT1> <REFSECT1 ID="R1-SQL-CREATERULE-3"> <TITLE> Bugs </TITLE> <PARA> <literal>instead</literal> rules do not work properly. </para> <para> The object in a SQL 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 ID="R1-SQL-CREATERULE-4"> <TITLE> Compatibility </TITLE> <PARA> CREATE RULE statement is a PostgreSQL language extension. </PARA> <REFSECT2 ID="R2-SQL-CREATERULE-4"> <REFSECT2INFO> <DATE>1998-04-15</DATE> </REFSECT2INFO> <TITLE> SQL92 </TITLE> <para> There is no CREATE RULE statement in SQL92. </para> </refsect2> </refsect1> </REFENTRY> <!-- Keep this comment at the end of the file Local variables: mode: sgml sgml-omittag:t sgml-shorttag:t sgml-minimize-attributes:nil sgml-always-quote-attributes:t sgml-indent-step:1 sgml-indent-data:t sgml-parent-document:nil sgml-default-dtd-file:"../reference.ced" sgml-exposed-tags:nil sgml-local-catalogs:"/usr/lib/sgml/catalog" sgml-local-ecat-files:nil End: -->