Slightly optimize ptrack_get_pagemapset

Probe the second slot only if the first one succeded.

Author: ololobus

ptrack.c

@@ -532,24 +532,29 @@ ptrack_get_pagemapset(PG_FUNCTION_ARGS)
 
 		hash = BID_HASH_FUNC(ctx->bid);
 		slot1 = hash % PtrackContentNblocks;
-		slot2 = ((hash << 32) | (hash >> 32)) % PtrackContentNblocks;
 
 		update_lsn1 = pg_atomic_read_u64(&ptrack_map->entries[slot1]);
-		update_lsn2 = pg_atomic_read_u64(&ptrack_map->entries[slot2]);
 
 		if (update_lsn1 != InvalidXLogRecPtr)
 			elog(DEBUG3, "ptrack: update_lsn1 %X/%X of blckno %u of file %s",
 				 (uint32) (update_lsn1 >> 32), (uint32) update_lsn1,
 				 ctx->bid.blocknum, ctx->relpath);
 
-		if (update_lsn2 != InvalidXLogRecPtr)
-			elog(DEBUG3, "ptrack: update_lsn2 %X/%X of blckno %u of file %s",
-				 (uint32) (update_lsn1 >> 32), (uint32) update_lsn2,
-				 ctx->bid.blocknum, ctx->relpath);
+		/* Only probe the second slot if the first one is marked */
+		if (update_lsn1 >= ctx->lsn)
+		{
+			slot2 = ((hash << 32) | (hash >> 32)) % PtrackContentNblocks;
+			update_lsn2 = pg_atomic_read_u64(&ptrack_map->entries[slot2]);
 
-		/* Block has been changed since specified LSN.  Mark it in the bitmap */
-		if (update_lsn1 >= ctx->lsn && update_lsn2 >= ctx->lsn)
-			datapagemap_add(&pagemap, ctx->bid.blocknum % ((BlockNumber) RELSEG_SIZE));
+			if (update_lsn2 != InvalidXLogRecPtr)
+				elog(DEBUG3, "ptrack: update_lsn2 %X/%X of blckno %u of file %s",
+					 (uint32) (update_lsn1 >> 32), (uint32) update_lsn2,
+					 ctx->bid.blocknum, ctx->relpath);
+
+			/* Block has been changed since specified LSN.  Mark it in the bitmap */
+			if (update_lsn2 >= ctx->lsn)
+				datapagemap_add(&pagemap, ctx->bid.blocknum % ((BlockNumber) RELSEG_SIZE));
+		}
 
 		ctx->bid.blocknum += 1;
 	}