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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<HTML
><HEAD
><TITLE
>LISTEN</TITLE
><META
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>PostgreSQL 9.1.15 Documentation</A
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><H1
><A
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></A
>LISTEN</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN73150"
></A
><H2
>Name</H2
>LISTEN -- listen for a notification</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN73155"
></A
><H2
>Synopsis</H2
><PRE
CLASS="SYNOPSIS"
>LISTEN <TT
CLASS="REPLACEABLE"
><I
>channel</I
></TT
></PRE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN73158"
></A
><H2
>Description</H2
><P
> <TT
CLASS="COMMAND"
>LISTEN</TT
> registers the current session as a
listener on the notification channel named <TT
CLASS="REPLACEABLE"
><I
>channel</I
></TT
>.
If the current session is already registered as a listener for
this notification channel, nothing is done.
</P
><P
> Whenever the command <TT
CLASS="COMMAND"
>NOTIFY <TT
CLASS="REPLACEABLE"
><I
>channel</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 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
<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"
>NOTIFY</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="AEN73181"
></A
><H2
>Parameters</H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="REPLACEABLE"
><I
>channel</I
></TT
></DT
><DD
><P
> Name of a notification channel (any identifier).
</P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN73189"
></A
><H2
>Notes</H2
><P
> <TT
CLASS="COMMAND"
>LISTEN</TT
> takes effect at transaction commit.
If <TT
CLASS="COMMAND"
>LISTEN</TT
> or <TT
CLASS="COMMAND"
>UNLISTEN</TT
> 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 <TT
CLASS="COMMAND"
>LISTEN</TT
> cannot be
prepared for two-phase commit.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN73197"
></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="AEN73202"
></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="AEN73206"
></A
><H2
>See Also</H2
><A
HREF="sql-notify.html"
>NOTIFY</A
>, <A
HREF="sql-unlisten.html"
>UNLISTEN</A
></DIV
><DIV
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