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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>LISTEN</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-insert.html" title="INSERT" /><link rel="next" href="sql-load.html" title="LOAD" /></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">LISTEN</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="sql-insert.html" title="INSERT">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-load.html" title="LOAD">Next</a></td></tr></table><hr></hr></div><div class="refentry" id="SQL-LISTEN"><div class="titlepage"></div><a id="id-1.9.3.153.1" class="indexterm"></a><div class="refnamediv"><h2><span class="refentrytitle">LISTEN</span></h2><p>LISTEN — listen for a notification</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><pre class="synopsis">
LISTEN <em class="replaceable"><code>channel</code></em>
</pre></div><div class="refsect1" id="id-1.9.3.153.5"><h2>Description</h2><p>
<code class="command">LISTEN</code> registers the current session as a
listener on the notification channel named <em class="replaceable"><code>channel</code></em>.
If the current session is already registered as a listener for
this notification channel, nothing is done.
</p><p>
Whenever the command <code class="command">NOTIFY <em class="replaceable"><code>channel</code></em></code> is invoked, either
by this session or another one connected to the same database, all
the sessions currently listening on that notification channel are
notified, and each will in turn notify its connected client
application.
</p><p>
A session can be unregistered for a given notification channel with the
<code class="command">UNLISTEN</code> command. A session's listen
registrations are automatically cleared when the session ends.
</p><p>
The method a client application must use to detect notification events depends on
which <span class="productname">PostgreSQL</span> application programming interface it
uses. With the <span class="application">libpq</span> library, the application issues
<code class="command">LISTEN</code> as an ordinary SQL command, and then must
periodically call the function <code class="function">PQnotifies</code> to find out
whether any notification events have been received. Other interfaces such as
<span class="application">libpgtcl</span> provide higher-level methods for handling notify events; indeed,
with <span class="application">libpgtcl</span> the application programmer should not even issue
<code class="command">LISTEN</code> or <code class="command">UNLISTEN</code> directly. See the
documentation for the interface you are using for more details.
</p><p>
<a class="xref" href="sql-notify.html" title="NOTIFY"><span class="refentrytitle">NOTIFY</span></a>
contains a more extensive
discussion of the use of <code class="command">LISTEN</code> and
<code class="command">NOTIFY</code>.
</p></div><div class="refsect1" id="id-1.9.3.153.6"><h2>Parameters</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="replaceable"><code>channel</code></em></span></dt><dd><p>
Name of a notification channel (any identifier).
</p></dd></dl></div></div><div class="refsect1" id="id-1.9.3.153.7"><h2>Notes</h2><p>
<code class="command">LISTEN</code> takes effect at transaction commit.
If <code class="command">LISTEN</code> or <code class="command">UNLISTEN</code> is executed
within a transaction that later rolls back, the set of notification
channels being listened to is unchanged.
</p><p>
A transaction that has executed <code class="command">LISTEN</code> cannot be
prepared for two-phase commit.
</p></div><div class="refsect1" id="id-1.9.3.153.8"><h2>Examples</h2><p>
Configure and execute a listen/notify sequence from
<span class="application">psql</span>:
</p><pre class="programlisting">
LISTEN virtual;
NOTIFY virtual;
Asynchronous notification "virtual" received from server process with PID 8448.
</pre></div><div class="refsect1" id="id-1.9.3.153.9"><h2>Compatibility</h2><p>
There is no <code class="command">LISTEN</code> statement in the SQL
standard.
</p></div><div class="refsect1" id="id-1.9.3.153.10"><h2>See Also</h2><span class="simplelist"><a class="xref" href="sql-notify.html" title="NOTIFY"><span class="refentrytitle">NOTIFY</span></a>, <a class="xref" href="sql-unlisten.html" title="UNLISTEN"><span class="refentrytitle">UNLISTEN</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-insert.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-load.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">INSERT </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> LOAD</td></tr></table></div></body></html>
