Fix UNLISTEN to fall out quickly if the current backend has never executed

any LISTEN command.  This is more important than it used to be because
DISCARD ALL invokes UNLISTEN.  Connection-pooled applications making heavy
use of DISCARD ALL were seeing significant contention for pg_listener,
as reported by Matteo Beccati.  It seems unlikely that clients using LISTEN
would use pooled connections, so this simple tweak seems sufficient,
especially since the pg_listener implementation is slated to go away soon
anyway.

Back-patch to 8.3, where DISCARD ALL was introduced.

diff --git a/src/backend/commands/async.c b/src/backend/commands/async.c
index 06ba738fd47f8cbc7e5df6db2b705ba3f3643b48..38fc57393e66cebd003458b6e90e9a395f2154e4 100644 (file)
--- a/src/backend/commands/async.c
+++ b/src/backend/commands/async.c
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *       $PostgreSQL: pgsql/src/backend/commands/async.c,v 1.138.2.1 2008/03/12 20:11:54 tgl Exp $
+ *       $PostgreSQL: pgsql/src/backend/commands/async.c,v 1.138.2.2 2009/02/13 17:12:10 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -283,6 +283,10 @@ Async_Unlisten(const char *relname)
                if (Trace_notify)
                        elog(DEBUG1, "Async_Unlisten(%s,%d)", relname, MyProcPid);
 
+               /* If we couldn't possibly be listening, no need to queue anything */
+               if (pendingActions == NIL && !unlistenExitRegistered)
+                       return;
+
                queue_listen(LISTEN_UNLISTEN, relname);
        }
 }
@@ -298,6 +302,10 @@ Async_UnlistenAll(void)
        if (Trace_notify)
                elog(DEBUG1, "Async_UnlistenAll(%d)", MyProcPid);
 
+       /* If we couldn't possibly be listening, no need to queue anything */
+       if (pendingActions == NIL && !unlistenExitRegistered)
+               return;
+
        queue_listen(LISTEN_UNLISTEN_ALL, "");
 }