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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 id="docContent" class="container-fluid col-10"><div 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 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="sql-load.html" title="LOAD">Next</a></td></tr></table><hr /></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></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><p>
+   There is a race condition when first setting up a listening session:
+   if concurrently-committing transactions are sending notify events,
+   exactly which of those will the newly listening session receive?
+   The answer is that the session will receive all events committed after
+   an instant during the transaction's commit step.  But that is slightly
+   later than any database state that the transaction could have observed
+   in queries.  This leads to the following rule for
+   using <code class="command">LISTEN</code>: first execute (and commit!) that
+   command, then in a new transaction inspect the database state as needed
+   by the application logic, then rely on notifications to find out about
+   subsequent changes to the database state.  The first few received
+   notifications might refer to updates already observed in the initial
+   database inspection, but this is usually harmless.
+  </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-1.9.3.153.8"><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" title="INSERT">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-load.html" title="LOAD">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" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> LOAD</td></tr></table></div></body></html>
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