BitmapHeapScan: Use correct recheck flag for skip_fetch

tvondra · tvondra · commit 1fdb0ce9b109 · 2024-04-07T00:51:03.000+02:00
As of 7c70996, BitmapPrefetch() used the recheck flag for the current block to determine whether or not it should skip prefetching the proposed prefetch block. As explained in the comment, this assumed the index AM will report the same recheck value for the future page as it did for the current page - but there's no guarantee. This only affects prefetching - if the recheck flag changes, we may prefetch blocks unecessarily and not prefetch blocks that will be needed. But we don't need to rely on that assumption - we know the recheck flag for the block we're considering prefetching, so we can use that. The impact is very limited in practice - the opclass would need to assign different recheck flags to different blocks, but none of the built-in opclasses seems to do that. Author: Melanie Plageman Reviewed-by: Tomas Vondra, Andres Freund, Tom Lane Discussion: https://postgr.es/m/1939305.1712415547%40sss.pgh.pa.us
diff --git a/src/backend/executor/nodeBitmapHeapscan.c b/src/backend/executor/nodeBitmapHeapscan.c
@@ -475,14 +475,9 @@ BitmapPrefetch(BitmapHeapScanState *node, TableScanDesc scan)
 				 * skip this prefetch call, but continue to run the prefetch
 				 * logic normally.  (Would it be better not to increment
 				 * prefetch_pages?)
-				 *
-				 * This depends on the assumption that the index AM will
-				 * report the same recheck flag for this future heap page as
-				 * it did for the current heap page; which is not a certainty
-				 * but is true in many cases.
 				 */
 				skip_fetch = (!(scan->rs_flags & SO_NEED_TUPLES) &&
-							  (node->tbmres ? !node->tbmres->recheck : false) &&
+							  !tbmpre->recheck &&
 							  VM_ALL_VISIBLE(node->ss.ss_currentRelation,
 											 tbmpre->blockno,
 											 &node->pvmbuffer));
@@ -533,7 +528,7 @@ BitmapPrefetch(BitmapHeapScanState *node, TableScanDesc scan)
 
 				/* As above, skip prefetch if we expect not to need page */
 				skip_fetch = (!(scan->rs_flags & SO_NEED_TUPLES) &&
-							  (node->tbmres ? !node->tbmres->recheck : false) &&
+							  !tbmpre->recheck &&
 							  VM_ALL_VISIBLE(node->ss.ss_currentRelation,
 											 tbmpre->blockno,
 											 &node->pvmbuffer));
