Skip searching for subxact locks at commit.

At commit all standby locks are released
for the top-level transaction, so searching
for locks for each subtransaction is both
pointless and costly (N^2) in the presence
of many AccessExclusiveLocks.

Author: simonat2ndQuadrant

src/backend/access/transam/xact.c

@@ -4609,9 +4609,11 @@ xact_redo_commit_internal(TransactionId xid, XLogRecPtr lsn,
 		/*
 		 * Release locks, if any. We do this for both two phase and normal one
 		 * phase transactions. In effect we are ignoring the prepare phase and
-		 * just going straight to lock release.
+		 * just going straight to lock release. At commit we release all locks
+		 * via their top-level xid only, so no need to provide subxact list,
+		 * which will save time when replaying commits.
 		 */
-		StandbyReleaseLockTree(xid, nsubxacts, sub_xids);
+		StandbyReleaseLockTree(xid, 0, NULL);
 	}
 
 	/* Make sure files supposed to be dropped are dropped */