<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >LISTEN</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK REV="MADE" HREF="mailto:pgsql-docs@postgresql.org"><LINK REL="HOME" TITLE="PostgreSQL 8.0.11 Documentation" HREF="index.html"><LINK REL="UP" TITLE="SQL Commands" HREF="sql-commands.html"><LINK REL="PREVIOUS" TITLE="INSERT" HREF="sql-insert.html"><LINK REL="NEXT" TITLE="LOAD" HREF="sql-load.html"><LINK REL="STYLESHEET" TYPE="text/css" HREF="stylesheet.css"><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"><META NAME="creation" CONTENT="2007-02-02T03:57:22"></HEAD ><BODY CLASS="REFENTRY" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="5" ALIGN="center" VALIGN="bottom" >PostgreSQL 8.0.11 Documentation</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="sql-insert.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="sql-insert.html" >Fast Backward</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="top" ><A HREF="sql-load.html" >Fast Forward</A ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="top" ><A HREF="sql-load.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><H1 ><A NAME="SQL-LISTEN" ></A >LISTEN</H1 ><DIV CLASS="REFNAMEDIV" ><A NAME="AEN43677" ></A ><H2 >Name</H2 >LISTEN -- listen for a notification</DIV ><A NAME="AEN43680" ></A ><DIV CLASS="REFSYNOPSISDIV" ><A NAME="AEN43682" ></A ><H2 >Synopsis</H2 ><PRE CLASS="SYNOPSIS" >LISTEN <TT CLASS="REPLACEABLE" ><I >name</I ></TT ></PRE ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN43685" ></A ><H2 >Description</H2 ><P > <TT CLASS="COMMAND" >LISTEN</TT > registers the current session as a listener on the notification condition <TT CLASS="REPLACEABLE" ><I >name</I ></TT >. If the current session is already registered as a listener for this notification condition, nothing is done. </P ><P > Whenever the command <TT CLASS="COMMAND" >NOTIFY <TT CLASS="REPLACEABLE" ><I >name</I ></TT ></TT > is invoked, either by this session or another one connected to the same database, all the sessions currently listening on that notification condition are notified, and each will in turn notify its connected client application. See the discussion of <TT CLASS="COMMAND" >NOTIFY</TT > for more information. </P ><P > A session can be unregistered for a given notify condition with the <TT CLASS="COMMAND" >UNLISTEN</TT > 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 <TT CLASS="COMMAND" >LISTEN</TT > 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 <TT CLASS="COMMAND" >LISTEN</TT > or <TT CLASS="COMMAND" >UNLISTEN</TT > directly. See the documentation for the interface you are using for more details. </P ><P > <A HREF="sql-notify.html" ><I >NOTIFY</I ></A > contains a more extensive discussion of the use of <TT CLASS="COMMAND" >LISTEN</TT > and <TT CLASS="COMMAND" >NOTIFY</TT >. </P ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN43709" ></A ><H2 >Parameters</H2 ><P ></P ><DIV CLASS="VARIABLELIST" ><DL ><DT ><TT CLASS="REPLACEABLE" ><I >name</I ></TT ></DT ><DD ><P > Name of a notify condition (any identifier). </P ></DD ></DL ></DIV ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN43717" ></A ><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 ><P> </P ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN43722" ></A ><H2 >Compatibility</H2 ><P > There is no <TT CLASS="COMMAND" >LISTEN</TT > statement in the SQL standard. </P ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN43726" ></A ><H2 >See Also</H2 ><A HREF="sql-notify.html" ><I >NOTIFY</I ></A >, <A HREF="sql-unlisten.html" ><I >UNLISTEN</I ></A ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="sql-insert.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="sql-load.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >INSERT</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="sql-commands.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >LOAD</TD ></TR ></TABLE ></DIV ></BODY ></HTML >