| PostgreSQL 7.4.7 Documentation | ||||
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   pg_listen creates, changes, or cancels a
   request to listen for asynchronous notification messages from the
   PostgreSQL server.  With a
   callbackCommand parameter, the request is
   established, or the command string of an already existing request
   is replaced.  With no callbackCommand parameter, a
   prior request is canceled.
  
   After a pg_listen request is established, the
   specified command string is executed whenever a notification
   message bearing the given name arrives from the server.  This
   occurs when any PostgreSQL client
   application issues a
   NOTIFY command referencing that name.  The
   command string is executed from the Tcl idle loop.  That is the
   normal idle state of an application written with Tk.  In non-Tk Tcl
   shells, you can execute update or
   vwait to cause the idle loop to be entered.
  
   You should not invoke the SQL statements LISTEN
   or UNLISTEN directly when using
   pg_listen.  pgtcl
   takes care of issuing those statements for you.  But if you want to
   send a notification message yourself, invoke the SQL
   NOTIFY statement using
   pg_exec.