Remove queryids_lock

A normal backend should only write in a dedicated array position at the very
beginning of the query execution, and the underlying parallel workers will only
read from the same array position during the end of query execution.  As a
consequence there's no need to protect array access with a lock.

Note that this locking approach can lead to severe performance degradation in
some light OLTP workload with a high number of clients.

pg_stat_kcache.c

@@ -144,7 +144,6 @@ typedef struct pgskSharedState
 {
 	LWLock	   *lock;					/* protects hashtable search/modification */
 #if PG_VERSION_NUM >= 90600
-	LWLock	   *queryids_lock;			/* protects queryids array */
 	pgsk_queryid	queryids[FLEXIBLE_ARRAY_MEMBER]; /* queryid info for
 														parallel leaders */
 #endif
@@ -332,7 +331,7 @@ _PG_init(void)
 #if PG_VERSION_NUM < 150000
 	RequestAddinShmemSpace(pgsk_memsize());
 #if PG_VERSION_NUM >= 90600
-	RequestNamedLWLockTranche("pg_stat_kcache", 2);
+	RequestNamedLWLockTranche("pg_stat_kcache", 1);
 #else
 	RequestAddinLWLocks(1);
 #endif		/* pg 9.6+ */
@@ -432,9 +431,7 @@ pgsk_set_queryid(pgsk_queryid queryid)
 	/* Only the leader knows the queryid. */
 	Assert(!IsParallelWorker());
 
-	LWLockAcquire(pgsk->queryids_lock, LW_EXCLUSIVE);
 	pgsk->queryids[MyBackendId] = queryid;
-	LWLockRelease(pgsk->queryids_lock);
 }
 #endif
 
@@ -452,7 +449,7 @@ pgsk_shmem_request(void)
 		prev_shmem_request_hook();
 
 	RequestAddinShmemSpace(pgsk_memsize());
-	RequestNamedLWLockTranche("pg_stat_kcache", 2);
+	RequestNamedLWLockTranche("pg_stat_kcache", 1);
 }
 #endif
 
@@ -489,9 +486,7 @@ pgsk_shmem_startup(void)
 	{
 		/* First time through ... */
 #if PG_VERSION_NUM >= 90600
-		LWLockPadded *locks = GetNamedLWLockTranche("pg_stat_kcache");
-		pgsk->lock = &(locks[0]).lock;
-		pgsk->queryids_lock = &(locks[1]).lock;
+		pgsk->lock = &(GetNamedLWLockTranche("pg_stat_kcache"))->lock;
 #else
 		pgsk->lock = LWLockAssign();
 #endif
@@ -1087,11 +1082,7 @@ pgsk_ExecutorEnd (QueryDesc *queryDesc)
 
 #if PG_VERSION_NUM >= 90600
 		if (IsParallelWorker())
-		{
-			LWLockAcquire(pgsk->queryids_lock, LW_SHARED);
 			queryId = pgsk->queryids[ParallelLeaderBackendId];
-			LWLockRelease(pgsk->queryids_lock);
-		}
 		else
 #endif
 		queryId = queryDesc->plannedstmt->queryId;