events are not delivered until and unless the
   transaction is committed.  This is appropriate, since if the transaction
   is aborted, all the commands within it have had no
   effect, including <command>NOTIFY</command>.  But it can be disconcerting if one
   is expecting the notification events to be delivered immediately.  Secondly, if
   a listening session receives a notification signal while it is within a transaction,
   the notification event will not be delivered to its connected client until just
   after the transaction is completed (either committed or aborted).  Again, the
   reasoning is that if a notification were delivered within a transaction that was
   later aborted, one would want the notification to be undone somehow &mdash;
   but
   the server cannot <quote>take back</quote> a notification once it has sent it to the client.
   So notification events are only delivered between transactions.  The upshot of this
   is that applications using <command>NOTIFY</command> for real-time signaling
   should try to keep their transactions short.
  </para>

  <para>
   If the same channel name is signaled multiple times with identical
   payload strings within the same transaction, only one instance of the
   notification event is delivered to listeners.
   On the other hand, notifications with distinct payload strings will
   always be delivered as distinct notifications. Similarly, notifications from
   different transactions will never get folded into one notification.
   Except for dropping later instances of duplicate notifications,
   <command>NOTIFY</command> guarantees that notifications from the same
   transaction get delivered in the order they were sent.  It is also
   guaranteed that messages from different transactions are delivered in
   the order in which the transactions committed.
  </para>

  <para>
   It is common for a client that executes <command>NOTIFY</command>
   to be listening on the same notification channel itself.  In that case
   it will get back a notification event, just like all the other
   listening sessions.  Depending on the application logic, this could
   result in useless work, for example, reading a database table to
   find the same updates that that session just wrote out.  It is
   possible to avoid such extra work by noticing whether the notifying
   session's server process <acronym>PID</acronym> (supplied in the
   notification event message) is the same as one's own session's
   <acronym>PID</acronym> (available from <application>libpq</application>).  When they
   are the same, the notification event is one's own work bouncing
   back, and can be ignored.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><replaceable class="parameter">channel</replaceable></term>
    <listitem>
     <para>
      Name of the notification channel to be signaled (any identifier).
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><replaceable class="parameter">payload</replaceable></term>
    <listitem>
     <para>
      The <quote>payload</quote> string to be communicated