Remove HeapBitmapScan's skip_fetch optimization

anarazel · anarazel · commit 77d90d6d6334 · 2025-04-02T14:50:03.000-04:00
The optimization does not take the removal of TIDs by a concurrent vacuum into account. The concurrent vacuum can remove dead TIDs and make pages ALL_VISIBLE while those dead TIDs are referenced in the bitmap. This can lead to a skip_fetch scan returning too many tuples. It likely would be possible to implement this optimization safely, but we don't have the necessary infrastructure in place. Nor is it clear that it's worth building that infrastructure, given how limited the skip_fetch optimization is. In the backbranches we just disable the optimization by always passing need_tuples=true to table_beginscan_bm(). We can't perform API/ABI changes in the backbranches and we want to make the change as minimal as possible. Author: Matthias van de Meent <boekewurm+postgres@gmail.com> Reported-By: Konstantin Knizhnik <knizhnik@garret.ru> Discussion: https://postgr.es/m/CAEze2Wg3gXXZTr6_rwC+s4-o2ZVFB5F985uUSgJTsECx6AmGcQ@mail.gmail.com Backpatch-through: 13
diff --git a/src/backend/executor/nodeBitmapHeapscan.c b/src/backend/executor/nodeBitmapHeapscan.c
@@ -742,6 +742,20 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags)
 	scanstate->shared_prefetch_iterator = NULL;
 	scanstate->pstate = NULL;
 
+	/*
+	 * Unfortunately it turns out that the below optimization does not
+	 * take the removal of TIDs by a concurrent vacuum into
+	 * account. The concurrent vacuum can remove dead TIDs and make
+	 * pages ALL_VISIBLE while those dead TIDs are referenced in the
+	 * bitmap. This would lead to a !need_tuples scan returning too
+	 * many tuples.
+	 *
+	 * In the back-branches, we therefore simply disable the
+	 * optimization. Removing all the relevant code would be too
+	 * invasive (and a major backpatching pain).
+	 */
+	scanstate->can_skip_fetch = false;
+#ifdef NOT_ANYMORE
 	/*
 	 * We can potentially skip fetching heap pages if we do not need any
 	 * columns of the table, either for checking non-indexable quals or for
@@ -751,6 +765,7 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags)
 	 */
 	scanstate->can_skip_fetch = (node->scan.plan.qual == NIL &&
 								 node->scan.plan.targetlist == NIL);
+#endif
 
 	/*
 	 * Miscellaneous initialization
