heapam: Only set tuple's block once per page in pagemode

hlinnaka · hlinnaka · commit 2904324a88f6 · 2025-04-01T13:24:27.000+03:00
Due to splitting the block id into two 16 bit integers, BlockIdSet() is more expensive than one might think. Doing it once per returned tuple shows up as a small but reliably reproducible cost. It's simple enough to set the block number just once per block in pagemode, so do so. Author: Andres Freund <andres@anarazel.de> Discussion: https://www.postgresql.org/message-id/lxzj26ga6ippdeunz6kuncectr5gfuugmm2ry22qu6hcx6oid6@lzx3sjsqhmt6
diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
@@ -1052,6 +1052,9 @@ heapgettup_pagemode(HeapScanDesc scan,
 		linesleft = scan->rs_ntuples;
 		lineindex = ScanDirectionIsForward(dir) ? 0 : linesleft - 1;
 
+		/* block is the same for all tuples, set it once outside the loop */
+		ItemPointerSetBlockNumber(&tuple->t_self, scan->rs_cblock);
+
 		/* lineindex now references the next or previous visible tid */
 continue_page:
 
@@ -1067,7 +1070,7 @@ heapgettup_pagemode(HeapScanDesc scan,
 
 			tuple->t_data = (HeapTupleHeader) PageGetItem(page, lpp);
 			tuple->t_len = ItemIdGetLength(lpp);
-			ItemPointerSet(&(tuple->t_self), scan->rs_cblock, lineoff);
+			ItemPointerSetOffsetNumber(&tuple->t_self, lineoff);
 
 			/* skip any tuples that don't match the scan key */
 			if (key != NULL &&
