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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 becauseDISCARD ALL invokes UNLISTEN. Connection-pooled applications making heavyuse of DISCARD ALL were seeing significant contention for pg_listener,as reported by Matteo Beccati. It seems unlikely that clients using LISTENwould use pooled connections, so this simple tweak seems sufficient,especially since the pg_listener implementation is slated to go away soonanyway.Back-patch to 8.3, where DISCARD ALL was introduced.

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src/backend/commands
- async.c

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`‎src/backend/commands/async.c`

Lines changed: 9 additions & 1 deletion

Original file line number	Diff line number	Diff line change
`@@ -7,7 +7,7 @@`
`7`	`7`	`* Portions Copyright (c) 1994, Regents of the University of California`
`8`	`8`	`*`
`9`	`9`	`* IDENTIFICATION`
`10`		`- * $PostgreSQL: pgsql/src/backend/commands/async.c,v 1.145 2009/01/01 17:23:37 momjian Exp $`
	`10`	`+ * $PostgreSQL: pgsql/src/backend/commands/async.c,v 1.146 2009/02/13 17:12:04 tgl Exp $`
`11`	`11`	`*`
`12`	`12`	`*-------------------------------------------------------------------------`
`13`	`13`	`*/`
`@@ -277,6 +277,10 @@ Async_Unlisten(const char *relname)`
`277`	`277`	`if (Trace_notify)`
`278`	`278`	`elog(DEBUG1,"Async_Unlisten(%s,%d)",relname,MyProcPid);`
`279`	`279`
	`280`	`+/* If we couldn't possibly be listening, no need to queue anything */`
	`281`	`+if (pendingActions==NIL&& !unlistenExitRegistered)`
	`282`	`+return;`
	`283`	`+`
`280`	`284`	`queue_listen(LISTEN_UNLISTEN,relname);`
`281`	`285`	`}`
`282`	`286`
`@@ -291,6 +295,10 @@ Async_UnlistenAll(void)`
`291`	`295`	`if (Trace_notify)`
`292`	`296`	`elog(DEBUG1,"Async_UnlistenAll(%d)",MyProcPid);`
`293`	`297`
	`298`	`+/* If we couldn't possibly be listening, no need to queue anything */`
	`299`	`+if (pendingActions==NIL&& !unlistenExitRegistered)`
	`300`	`+return;`
	`301`	`+`
`294`	`302`	`queue_listen(LISTEN_UNLISTEN_ALL,"");`
`295`	`303`	`}`
`296`	`304`

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Commitca5646f

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`‎src/backend/commands/async.c`

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