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 a6b93e5910fc2ac11e5971c9816670ea1e51ab32..d410b2d47db9ca07e7b485af0a84a30dc558844e 100644 (file)
--- a/src/backend/commands/async.c
+++ b/src/backend/commands/async.c
@@ -277,6 +277,10 @@ 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);
 }
 
@@ -291,6 +295,10 @@ 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, "");
 }