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Postgres documentation
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<refentry id="SQL-LISTEN">
 <refmeta>
  <refentrytitle id="SQL-LISTEN-TITLE">
   LISTEN
  </refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>
 <refnamediv>
  <refname>
   LISTEN
  </refname>
  <refpurpose>
   Listen for a response on a notify condition
  </refpurpose>
 </refnamediv>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>1999-07-20</date>
  </refsynopsisdivinfo>
  <synopsis>
LISTEN <replaceable class="PARAMETER">name</replaceable>
  </synopsis>

  <refsect2 id="R2-SQL-LISTEN-1">
   <refsect2info>
    <date>1998-10-07</date>
   </refsect2info>
   <title>
    Inputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term><replaceable class="PARAMETER">name</replaceable></term>
      <listitem>
       <para>
	Name of notify condition.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>

  <refsect2 id="R2-SQL-LISTEN-2">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title>
    Outputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term><computeroutput>
LISTEN
       </computeroutput></term>
      <listitem>
       <para>
	Message returned upon successful completion of registration.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><computeroutput>
NOTICE Async_Listen: We are already listening on <replaceable class="PARAMETER">name</replaceable>
       </computeroutput></term>
      <listitem>
       <para>
	If this backend is already registered for that notify condition.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>		
  </refsect2>
 </refsynopsisdiv>

 <refsect1 id="R1-SQL-LISTEN-1">
  <refsect1info>
   <date>1998-10-07</date>
  </refsect1info>
  <title>
   Description
  </title>

  <para>
   <command>LISTEN</command> registers the current 
   <productname>Postgres</productname> backend as a
   listener on the notify condition
   <replaceable class="PARAMETER">name</replaceable>.
  </para>

  <para>
   Whenever the command 
   <command>NOTIFY <replaceable class="PARAMETER">name</replaceable></command>
   is invoked, either by this backend or another one connected to
   the same database, all the backends currently listening on that notify
   condition are notified, and each will in turn notify its connected
   frontend application.  See the discussion of <command>NOTIFY</command>
   for more information.
  </para>

  <para>
   A backend can be unregistered for a given notify condition with the
   <command>UNLISTEN</command> command.  Also, a backend's listen registrations
   are automatically cleared when the backend process exits.
  </para>

  <para>
   The method a frontend application must use to detect notify events depends on
   which <productname>Postgres</productname> application programming interface it
   uses.  With the basic libpq library, the application issues
   <command>LISTEN</command> as an ordinary SQL command, and then must
   periodically call the routine <function>PQnotifies</function> to find out
   whether any notify events have been received.  Other interfaces such as
   libpgtcl provide higher-level methods for handling notify events; indeed,
   with libpgtcl the application programmer should not even issue
   <command>LISTEN</command> or <command>UNLISTEN</command> directly.  See the
   documentation for the library you are using for more details.
  </para>

  <para>
   <xref linkend="sql-notify" endterm="sql-notify-title">
   contains a more extensive
   discussion of the use of <command>LISTEN</command> and
   <command>NOTIFY</command>.
  </para>

  <refsect2 id="R2-SQL-LISTEN-3">
   <refsect2info>
    <date>1998-10-07</date>
   </refsect2info>
   <title>
    Notes
   </title>

   <para>
    <replaceable class="PARAMETER">name</replaceable>
    can be any string valid as a name;
    it need not correspond to the name of any actual table.  If
    <replaceable class="PARAMETER">notifyname</replaceable>
    is enclosed in double-quotes, it need not even be a syntactically
    valid name, but can be any string up to 31 characters long.
   </para>
   <para>
    In some previous releases of
    <productname>Postgres</productname>,
    <replaceable class="PARAMETER">name</replaceable>
    had to be enclosed in double-quotes when it did not correspond to any existing
    table name, even if syntactically valid as a name.  That is no longer required.
   </para>
  </refsect2>
 </refsect1>

 <refsect1 id="R1-SQL-LISTEN-2">
  <title>
   Usage
  </title>
  <para>
   Configure and execute a listen/notify sequence from <application>psql</application>:
<programlisting>
LISTEN virtual;
NOTIFY virtual;

Asynchronous NOTIFY 'virtual' from backend with pid '8448' received.
</programlisting>
  </para>
 </refsect1>

 <refsect1 id="R1-SQL-LISTEN-3">
  <title>
   Compatibility
  </title>

  <refsect2 id="R2-SQL-LISTEN-4">
   <refsect2info>
    <date>1998-09-01</date>
   </refsect2info>
   <title>
    SQL92
   </title>
   <para>
    There is no <command>LISTEN</command> in <acronym>SQL92</acronym>.
   </para>
  </refsect2>
 </refsect1>
</refentry>

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