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diff --git a/doc/src/sgml/html/sql-listen.html b/doc/src/sgml/html/sql-listen.html new file mode 100644 index 0000000..9a61760 --- /dev/null +++ b/doc/src/sgml/html/sql-listen.html @@ -0,0 +1,70 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!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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