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<title>CREATE RULE</title>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="refentry" lang="en">
<a name="sql-createrule"></a><div class="titlepage"></div>
<div class="refnamediv">
<h2>Name</h2>
<p>CREATE RULE — define a new rewrite rule</p>
</div>
<a name="id760286"></a><div class="refsynopsisdiv">
<h2>Synopsis</h2>
<pre class="synopsis">CREATE [ OR REPLACE ] RULE <em class="replaceable"><code>name</code></em> AS ON <em class="replaceable"><code>event</code></em>
TO <em class="replaceable"><code>table</code></em> [ WHERE <em class="replaceable"><code>condition</code></em> ]
DO [ ALSO | INSTEAD ] { NOTHING | <em class="replaceable"><code>command</code></em> | ( <em class="replaceable"><code>command</code></em> ; <em class="replaceable"><code>command</code></em> ... ) }</pre>
</div>
<div class="refsect1" lang="en">
<a name="id760351"></a><h2>Description</h2>
<p> <code class="command">CREATE RULE</code> defines a new rule applying to a specified
table or view.
<code class="command">CREATE OR REPLACE RULE</code> will either create a
new rule, or replace an existing rule of the same name for the same
table.
</p>
<p> The <span class="productname">PostgreSQL</span> rule system allows one to
define an alternate action to be performed on insertions, updates,
or deletions in database tables. Roughly speaking, a rule causes
additional commands to be executed when a given command on a given
table is executed. Alternatively, an <code class="literal">INSTEAD</code>
rule can replace a given command by another, or cause a command
not to be executed at all. Rules are used to implement table
views as well. It is important to realize that a rule is really
a command transformation mechanism, or command macro. The
transformation happens before the execution of the commands starts.
If you actually want an operation that fires independently for each
physical row, you probably want to use a trigger, not a rule.
More information about the rules system is in <a href="rules.html" title="Chapter34.The Rule System">Chapter34, <i>The Rule System</i></a>.
</p>
<p> Presently, <code class="literal">ON SELECT</code> rules must be unconditional
<code class="literal">INSTEAD</code> rules and must have actions that consist
of a single <code class="command">SELECT</code> command. Thus, an
<code class="literal">ON SELECT</code> rule effectively turns the table into
a view, whose visible contents are the rows returned by the rule's
<code class="command">SELECT</code> command rather than whatever had been
stored in the table (if anything). It is considered better style
to write a <code class="command">CREATE VIEW</code> command than to create a
real table and define an <code class="literal">ON SELECT</code> rule for it.
</p>
<p> You can create the illusion of an updatable view by defining
<code class="literal">ON INSERT</code>, <code class="literal">ON UPDATE</code>, and
<code class="literal">ON DELETE</code> 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.
</p>
<p> There is a catch if you try to use conditional rules for view
updates: there <span class="emphasis"><em>must</em></span> be an unconditional
<code class="literal">INSTEAD</code> rule for each action you wish to allow
on the view. If the rule is conditional, or is not
<code class="literal">INSTEAD</code>, 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 of the view
in some cases. If you want to handle all the useful cases in
conditional rules, add an unconditional <code class="literal">DO
INSTEAD NOTHING</code> rule to ensure that the system
understands it will never be called on to update the dummy table.
Then make the conditional rules non-<code class="literal">INSTEAD</code>; in
the cases where they are applied, they add to the default
<code class="literal">INSTEAD NOTHING</code> action.
</p>
</div>
<div class="refsect1" lang="en">
<a name="id760542"></a><h2>Parameters</h2>
<div class="variablelist"><dl>
<dt><span class="term"><em class="replaceable"><code>name</code></em></span></dt>
<dd><p> The name of a rule to create. This must be distinct from the
name of any other rule for the same table. Multiple rules on
the same table and same event type are applied in alphabetical
name order.
</p></dd>
<dt><span class="term"><em class="replaceable"><code>event</code></em></span></dt>
<dd><p> The event is one of <code class="literal">SELECT</code>,
<code class="literal">INSERT</code>, <code class="literal">UPDATE</code>, or
<code class="literal">DELETE</code>.
</p></dd>
<dt><span class="term"><em class="replaceable"><code>table</code></em></span></dt>
<dd><p> The name (optionally schema-qualified) of the table or view the
rule applies to.
</p></dd>
<dt><span class="term"><em class="replaceable"><code>condition</code></em></span></dt>
<dd><p> Any <acronym class="acronym">SQL</acronym> conditional expression (returning
<code class="type">boolean</code>). The condition expression may not refer
to any tables except <code class="literal">NEW</code> and <code class="literal">OLD</code>, and
may not contain aggregate functions.
</p></dd>
<dt><span class="term"><code class="option">INSTEAD</code></span></dt>
<dd><p> <code class="literal">INSTEAD</code> indicates that the commands should be
executed <span class="emphasis"><em>instead of</em></span> the original command.
</p></dd>
<dt><span class="term"><code class="option">ALSO</code></span></dt>
<dd>
<p> <code class="literal">ALSO</code> indicates that the commands should be
executed <span class="emphasis"><em>in addition to</em></span> the original
command.
</p>
<p> If neither <code class="literal">ALSO</code> nor
<code class="literal">INSTEAD</code> is specified, <code class="literal">ALSO</code>
is the default.
</p>
</dd>
<dt><span class="term"><em class="replaceable"><code>command</code></em></span></dt>
<dd><p> The command or commands that make up the rule action. Valid
commands are <code class="command">SELECT</code>,
<code class="command">INSERT</code>, <code class="command">UPDATE</code>,
<code class="command">DELETE</code>, or <code class="command">NOTIFY</code>.
</p></dd>
</dl></div>
<p> Within <em class="replaceable"><code>condition</code></em> and
<em class="replaceable"><code>command</code></em>, the special
table names <code class="literal">NEW</code> and <code class="literal">OLD</code> may
be used to refer to values in the referenced table.
<code class="literal">NEW</code> is valid in <code class="literal">ON INSERT</code> and
<code class="literal">ON UPDATE</code> rules to refer to the new row being
inserted or updated. <code class="literal">OLD</code> is valid in
<code class="literal">ON UPDATE</code> and <code class="literal">ON DELETE</code> rules
to refer to the existing row being updated or deleted.
</p>
</div>
<div class="refsect1" lang="en">
<a name="id760831"></a><h2>Notes</h2>
<p> You must have the privilege <code class="literal">RULE</code> on a table to
be allowed to define a rule on it.
</p>
<p> It is very important to take care to avoid circular rules. For
example, though each of the following two rule definitions are
accepted by <span class="productname">PostgreSQL</span>, the
<code class="command">SELECT</code> command would cause
<span class="productname">PostgreSQL</span> to report an error because
the query cycled too many times:
</p>
<pre class="programlisting">CREATE RULE "_RETURN" AS
ON SELECT TO t1
DO INSTEAD
SELECT * FROM t2;
CREATE RULE "_RETURN" AS
ON SELECT TO t2
DO INSTEAD
SELECT * FROM t1;
SELECT * FROM t1;</pre>
<p>
</p>
<p> Presently, if a rule action contains a <code class="command">NOTIFY</code>
command, the <code class="command">NOTIFY</code> command will be executed
unconditionally, that is, the <code class="command">NOTIFY</code> will be
issued even if there are not any rows that the rule should apply
to. For example, in
</p>
<pre class="programlisting">CREATE RULE notify_me AS ON UPDATE TO mytable DO ALSO NOTIFY mytable;
UPDATE mytable SET name = 'foo' WHERE id = 42;</pre>
<p>
one <code class="command">NOTIFY</code> event will be sent during the
<code class="command">UPDATE</code>, whether or not there are any rows that
match the condition <code class="literal">id = 42</code>. This is an
implementation restriction that may be fixed in future releases.
</p>
</div>
<div class="refsect1" lang="en">
<a name="id760938"></a><h2>Compatibility</h2>
<p> <code class="command">CREATE RULE</code> is a
<span class="productname">PostgreSQL</span> language extension, as is the
entire query rewrite system.
</p>
</div>
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