From f5e90fc3ac68b8606cc14bba42d72d062c25eab1 Mon Sep 17 00:00:00 2001 From: Matthias van de Meent Date: Mon, 2 Dec 2024 15:38:14 +0100 Subject: [PATCH v3 1/2] Remove BitmapScan's skip_fetch optimization The optimization doesn't take concurrent vacuum's removal of TIDs into account, which can remove dead TIDs and make ALL_VISIBLE pages for which we have pointers to those dead TIDs in the bitmap. The optimization itself isn't impossible, but would require more work figuring out that vacuum started removing TIDs and then stop using the optimization. However, we currently don't have such a system in place, making the optimization unsound to keep around. Reported-By: Konstantin Knizhnik Backpatch-through: 13 --- src/include/access/heapam.h | 1 - src/include/access/tableam.h | 12 +------ src/backend/access/heap/heapam.c | 18 ---------- src/backend/access/heap/heapam_handler.c | 28 --------------- src/backend/executor/nodeBitmapHeapscan.c | 43 ++--------------------- 5 files changed, 4 insertions(+), 98 deletions(-) diff --git a/src/include/access/heapam.h b/src/include/access/heapam.h index 7d06dad83fc..84a1f30aecb 100644 --- a/src/include/access/heapam.h +++ b/src/include/access/heapam.h @@ -99,7 +99,6 @@ typedef struct HeapScanDescData * block reported by the bitmap to determine how many NULL-filled tuples * to return. */ - Buffer rs_vmbuffer; int rs_empty_tuples_pending; /* these fields only used in page-at-a-time mode and for bitmap scans */ diff --git a/src/include/access/tableam.h b/src/include/access/tableam.h index 09b9b394e0e..74c8befef6b 100644 --- a/src/include/access/tableam.h +++ b/src/include/access/tableam.h @@ -62,13 +62,6 @@ typedef enum ScanOptions /* unregister snapshot at scan end? */ SO_TEMP_SNAPSHOT = 1 << 9, - - /* - * At the discretion of the table AM, bitmap table scans may be able to - * skip fetching a block from the table if none of the table data is - * needed. If table data may be needed, set SO_NEED_TUPLES. - */ - SO_NEED_TUPLES = 1 << 10, } ScanOptions; /* @@ -955,13 +948,10 @@ table_beginscan_strat(Relation rel, Snapshot snapshot, */ static inline TableScanDesc table_beginscan_bm(Relation rel, Snapshot snapshot, - int nkeys, struct ScanKeyData *key, bool need_tuple) + int nkeys, struct ScanKeyData *key) { uint32 flags = SO_TYPE_BITMAPSCAN | SO_ALLOW_PAGEMODE; - if (need_tuple) - flags |= SO_NEED_TUPLES; - return rel->rd_tableam->scan_begin(rel, snapshot, nkeys, key, NULL, flags); } diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c index 485525f4d64..905dd9d04a5 100644 --- a/src/backend/access/heap/heapam.c +++ b/src/backend/access/heap/heapam.c @@ -1056,8 +1056,6 @@ heap_beginscan(Relation relation, Snapshot snapshot, scan->rs_base.rs_flags = flags; scan->rs_base.rs_parallel = parallel_scan; scan->rs_strategy = NULL; /* set in initscan */ - scan->rs_vmbuffer = InvalidBuffer; - scan->rs_empty_tuples_pending = 0; /* * Disable page-at-a-time mode if it's not a MVCC-safe snapshot. @@ -1173,19 +1171,6 @@ heap_rescan(TableScanDesc sscan, ScanKey key, bool set_params, if (BufferIsValid(scan->rs_cbuf)) ReleaseBuffer(scan->rs_cbuf); - if (BufferIsValid(scan->rs_vmbuffer)) - { - ReleaseBuffer(scan->rs_vmbuffer); - scan->rs_vmbuffer = InvalidBuffer; - } - - /* - * Reset rs_empty_tuples_pending, a field only used by bitmap heap scan, - * to avoid incorrectly emitting NULL-filled tuples from a previous scan - * on rescan. - */ - scan->rs_empty_tuples_pending = 0; - /* * The read stream is reset on rescan. This must be done before * initscan(), as some state referred to by read_stream_reset() is reset @@ -1213,9 +1198,6 @@ heap_endscan(TableScanDesc sscan) if (BufferIsValid(scan->rs_cbuf)) ReleaseBuffer(scan->rs_cbuf); - if (BufferIsValid(scan->rs_vmbuffer)) - ReleaseBuffer(scan->rs_vmbuffer); - /* * Must free the read stream before freeing the BufferAccessStrategy. */ diff --git a/src/backend/access/heap/heapam_handler.c b/src/backend/access/heap/heapam_handler.c index e817f8f8f84..0ea4f029793 100644 --- a/src/backend/access/heap/heapam_handler.c +++ b/src/backend/access/heap/heapam_handler.c @@ -2155,24 +2155,6 @@ heapam_scan_bitmap_next_block(TableScanDesc scan, *blockno = tbmres->blockno; *recheck = tbmres->recheck; - /* - * We can skip fetching the heap page if we don't need any fields from the - * heap, the bitmap entries don't need rechecking, and all tuples on the - * page are visible to our transaction. - */ - if (!(scan->rs_flags & SO_NEED_TUPLES) && - !tbmres->recheck && - VM_ALL_VISIBLE(scan->rs_rd, tbmres->blockno, &hscan->rs_vmbuffer)) - { - /* can't be lossy in the skip_fetch case */ - Assert(tbmres->ntuples >= 0); - Assert(hscan->rs_empty_tuples_pending >= 0); - - hscan->rs_empty_tuples_pending += tbmres->ntuples; - - return true; - } - block = tbmres->blockno; /* @@ -2287,16 +2269,6 @@ heapam_scan_bitmap_next_tuple(TableScanDesc scan, Page page; ItemId lp; - if (hscan->rs_empty_tuples_pending > 0) - { - /* - * If we don't have to fetch the tuple, just return nulls. - */ - ExecStoreAllNullTuple(slot); - hscan->rs_empty_tuples_pending--; - return true; - } - /* * Out of range? If so, nothing more to look at on this page */ diff --git a/src/backend/executor/nodeBitmapHeapscan.c b/src/backend/executor/nodeBitmapHeapscan.c index be616683f98..e5d7fab4dbe 100644 --- a/src/backend/executor/nodeBitmapHeapscan.c +++ b/src/backend/executor/nodeBitmapHeapscan.c @@ -158,24 +158,10 @@ BitmapHeapNext(BitmapHeapScanState *node) */ if (!scan) { - bool need_tuples = false; - - /* - * 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 returning data. This test is a bit simplistic, as - * it checks the stronger condition that there's no qual or return - * tlist at all. But in most cases it's probably not worth working - * harder than that. - */ - need_tuples = (node->ss.ps.plan->qual != NIL || - node->ss.ps.plan->targetlist != NIL); - scan = table_beginscan_bm(node->ss.ss_currentRelation, node->ss.ps.state->es_snapshot, 0, - NULL, - need_tuples); + NULL); node->ss.ss_currentScanDesc = scan; } @@ -434,7 +420,6 @@ BitmapPrefetch(BitmapHeapScanState *node, TableScanDesc scan) while (node->prefetch_pages < node->prefetch_target) { TBMIterateResult *tbmpre = tbm_iterate(prefetch_iterator); - bool skip_fetch; if (tbmpre == NULL) { @@ -445,20 +430,7 @@ BitmapPrefetch(BitmapHeapScanState *node, TableScanDesc scan) node->prefetch_pages++; node->prefetch_blockno = tbmpre->blockno; - /* - * If we expect not to have to actually read this heap page, - * skip this prefetch call, but continue to run the prefetch - * logic normally. (Would it be better not to increment - * prefetch_pages?) - */ - skip_fetch = (!(scan->rs_flags & SO_NEED_TUPLES) && - !tbmpre->recheck && - VM_ALL_VISIBLE(node->ss.ss_currentRelation, - tbmpre->blockno, - &node->pvmbuffer)); - - if (!skip_fetch) - PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, tbmpre->blockno); + PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, tbmpre->blockno); } } @@ -475,7 +447,6 @@ BitmapPrefetch(BitmapHeapScanState *node, TableScanDesc scan) { TBMIterateResult *tbmpre; bool do_prefetch = false; - bool skip_fetch; /* * Recheck under the mutex. If some other process has already @@ -502,15 +473,7 @@ BitmapPrefetch(BitmapHeapScanState *node, TableScanDesc scan) node->prefetch_blockno = tbmpre->blockno; - /* As above, skip prefetch if we expect not to need page */ - skip_fetch = (!(scan->rs_flags & SO_NEED_TUPLES) && - !tbmpre->recheck && - VM_ALL_VISIBLE(node->ss.ss_currentRelation, - tbmpre->blockno, - &node->pvmbuffer)); - - if (!skip_fetch) - PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, tbmpre->blockno); + PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, tbmpre->blockno); } } } -- 2.45.2