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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>CREATE RULE</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="sql-createrole.html" title="CREATE ROLE" /><link rel="next" href="sql-createschema.html" title="CREATE SCHEMA" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">CREATE RULE</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="sql-createrole.html" title="CREATE ROLE">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><th width="60%" align="center">SQL Commands</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="sql-createschema.html" title="CREATE SCHEMA">Next</a></td></tr></table><hr></hr></div><div class="refentry" id="SQL-CREATERULE"><div class="titlepage"></div><a id="id-1.9.3.79.1" class="indexterm"></a><div class="refnamediv"><h2><span class="refentrytitle">CREATE RULE</span></h2><p>CREATE RULE — define a new rewrite rule</p></div><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_name</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> ... ) }
<span class="phrase">where <em class="replaceable"><code>event</code></em> can be one of:</span>
SELECT | INSERT | UPDATE | DELETE
</pre></div><div class="refsect1" id="id-1.9.3.79.5"><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 alternative 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 SQL
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 command 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 class="xref" href="rules.html" title="Chapter 41. The Rule System">Chapter 41</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. If you want to support
<code class="command">INSERT RETURNING</code> and so on, then be sure to put a suitable
<code class="literal">RETURNING</code> clause into each of these rules.
</p><p>
There is a catch if you try to use conditional rules for complex 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. (This method does not
currently work to support <code class="literal">RETURNING</code> queries, however.)
</p><div class="note"><h3 class="title">Note</h3><p>
A view that is simple enough to be automatically updatable (see <a class="xref" href="sql-createview.html" title="CREATE VIEW"><span class="refentrytitle">CREATE VIEW</span></a>) does not require a user-created rule in
order to be updatable. While you can create an explicit rule anyway,
the automatic update transformation will generally outperform an
explicit rule.
</p><p>
Another alternative worth considering is to use <code class="literal">INSTEAD OF</code>
triggers (see <a class="xref" href="sql-createtrigger.html" title="CREATE TRIGGER"><span class="refentrytitle">CREATE TRIGGER</span></a>) in place of rules.
</p></div></div><div class="refsect1" id="id-1.9.3.79.6"><h2>Parameters</h2><div class="variablelist"><dl class="variablelist"><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>. Note that an
<code class="command">INSERT</code> containing an <code class="literal">ON
CONFLICT</code> clause cannot be used on tables that have
either <code class="literal">INSERT</code> or <code class="literal">UPDATE</code>
rules. Consider using an updatable view instead.
</p></dd><dt><span class="term"><em class="replaceable"><code>table_name</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 cannot refer
to any tables except <code class="literal">NEW</code> and <code class="literal">OLD</code>, and
cannot 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> can
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" id="id-1.9.3.79.7"><h2>Notes</h2><p>
You must be the owner of a table to create or change rules for it.
</p><p>
In a rule for <code class="literal">INSERT</code>, <code class="literal">UPDATE</code>, or
<code class="literal">DELETE</code> on a view, you can add a <code class="literal">RETURNING</code>
clause that emits the view's columns. This clause will be used to compute
the outputs if the rule is triggered by an <code class="command">INSERT RETURNING</code>,
<code class="command">UPDATE RETURNING</code>, or <code class="command">DELETE RETURNING</code> command
respectively. When the rule is triggered by a command without
<code class="literal">RETURNING</code>, the rule's <code class="literal">RETURNING</code> clause will be
ignored. The current implementation allows only unconditional
<code class="literal">INSTEAD</code> rules to contain <code class="literal">RETURNING</code>; furthermore
there can be at most one <code class="literal">RETURNING</code> clause among all the rules
for the same event. (This ensures that there is only one candidate
<code class="literal">RETURNING</code> clause to be used to compute the results.)
<code class="literal">RETURNING</code> queries on the view will be rejected if
there is no <code class="literal">RETURNING</code> clause in any available rule.
</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
of recursive expansion of a rule:
</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 might be fixed in future releases.
</p></div><div class="refsect1" id="id-1.9.3.79.8"><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><div class="refsect1" id="id-1.9.3.79.9"><h2>See Also</h2><span class="simplelist"><a class="xref" href="sql-alterrule.html" title="ALTER RULE"><span class="refentrytitle">ALTER RULE</span></a>, <a class="xref" href="sql-droprule.html" title="DROP RULE"><span class="refentrytitle">DROP RULE</span></a></span></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="sql-createrole.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sql-commands.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-createschema.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">CREATE ROLE </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> CREATE SCHEMA</td></tr></table></div></body></html>
