From 04e72ed617be354a53a076b76c6644e364ed80a3 Mon Sep 17 00:00:00 2001 From: Tomas Vondra Date: Sun, 7 Apr 2024 00:24:12 +0200 Subject: [PATCH] BitmapHeapScan: Push skip_fetch optimization into table AM Commit 7c70996ebf0949b142 introduced an optimization to allow bitmap scans to operate like index-only scans by not fetching a block from the heap if none of the underlying data is needed and the block is marked all visible in the visibility map. With the introduction of table AMs, a FIXME was added to this code indicating that the skip_fetch logic should be pushed into the table AM-specific code, as not all table AMs may use a visibility map in the same way. This commit resolves this FIXME for the current block. The layering violation is still present in BitmapHeapScans's prefetching code, which uses the visibility map to decide whether or not to prefetch a block. However, this can be addressed independently. Author: Melanie Plageman Reviewed-by: Andres Freund, Heikki Linnakangas, Tomas Vondra, Mark Dilger Discussion: https://postgr.es/m/CAAKRu_ZwCwWFeL_H3ia26bP2e7HiKLWt0ZmGXPVwPO6uXq0vaA%40mail.gmail.com --- src/backend/access/heap/heapam.c | 15 +++ src/backend/access/heap/heapam_handler.c | 29 +++++ src/backend/executor/nodeBitmapHeapscan.c | 126 +++++++--------------- src/include/access/heapam.h | 10 ++ src/include/access/tableam.h | 12 ++- src/include/nodes/execnodes.h | 8 +- 6 files changed, 106 insertions(+), 94 deletions(-) diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c index dada2ecd1e..01bb2f4cc1 100644 --- a/src/backend/access/heap/heapam.c +++ b/src/backend/access/heap/heapam.c @@ -967,6 +967,8 @@ 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. @@ -1055,6 +1057,14 @@ 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; + } + + Assert(scan->rs_empty_tuples_pending == 0); + /* * reinitialize scan descriptor */ @@ -1074,6 +1084,11 @@ heap_endscan(TableScanDesc sscan) if (BufferIsValid(scan->rs_cbuf)) ReleaseBuffer(scan->rs_cbuf); + if (BufferIsValid(scan->rs_vmbuffer)) + ReleaseBuffer(scan->rs_vmbuffer); + + Assert(scan->rs_empty_tuples_pending == 0); + /* * decrement relation reference count and free scan descriptor storage */ diff --git a/src/backend/access/heap/heapam_handler.c b/src/backend/access/heap/heapam_handler.c index 3e7a6b5548..58de2c82a7 100644 --- a/src/backend/access/heap/heapam_handler.c +++ b/src/backend/access/heap/heapam_handler.c @@ -27,6 +27,7 @@ #include "access/syncscan.h" #include "access/tableam.h" #include "access/tsmapi.h" +#include "access/visibilitymap.h" #include "access/xact.h" #include "catalog/catalog.h" #include "catalog/index.h" @@ -2198,6 +2199,24 @@ heapam_scan_bitmap_next_block(TableScanDesc scan, hscan->rs_cindex = 0; hscan->rs_ntuples = 0; + /* + * 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; + } + /* * Ignore any claimed entries past what we think is the end of the * relation. It may have been extended after the start of our scan (we @@ -2310,6 +2329,16 @@ 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 2148a21531..5f768701a5 100644 --- a/src/backend/executor/nodeBitmapHeapscan.c +++ b/src/backend/executor/nodeBitmapHeapscan.c @@ -105,16 +105,6 @@ BitmapHeapNext(BitmapHeapScanState *node) */ if (!node->initialized) { - /* - * 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. - */ - node->can_skip_fetch = (node->ss.ps.plan->qual == NIL && - node->ss.ps.plan->targetlist == NIL); - if (!pstate) { tbm = (TIDBitmap *) MultiExecProcNode(outerPlanState(node)); @@ -195,10 +185,24 @@ 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); + NULL, + need_tuples); node->ss.ss_currentScanDesc = scan; } @@ -208,7 +212,7 @@ BitmapHeapNext(BitmapHeapScanState *node) for (;;) { - bool skip_fetch; + bool valid_block; CHECK_FOR_INTERRUPTS(); @@ -229,37 +233,14 @@ BitmapHeapNext(BitmapHeapScanState *node) BitmapAdjustPrefetchIterator(node, tbmres); + valid_block = table_scan_bitmap_next_block(scan, tbmres); + if (tbmres->ntuples >= 0) node->exact_pages++; else node->lossy_pages++; - /* - * We can skip fetching the heap page if we don't need any fields - * from the heap, and the bitmap entries don't need rechecking, - * and all tuples on the page are visible to our transaction. - * - * XXX: It's a layering violation that we do these checks above - * tableam, they should probably moved below it at some point. - */ - skip_fetch = (node->can_skip_fetch && - !tbmres->recheck && - VM_ALL_VISIBLE(node->ss.ss_currentRelation, - tbmres->blockno, - &node->vmbuffer)); - - if (skip_fetch) - { - /* can't be lossy in the skip_fetch case */ - Assert(tbmres->ntuples >= 0); - - /* - * The number of tuples on this page is put into - * node->return_empty_tuples. - */ - node->return_empty_tuples = tbmres->ntuples; - } - else if (!table_scan_bitmap_next_block(scan, tbmres)) + if (!valid_block) { /* AM doesn't think this block is valid, skip */ continue; @@ -302,51 +283,32 @@ BitmapHeapNext(BitmapHeapScanState *node) * should happen only when we have determined there is still something * to do on the current page, else we may uselessly prefetch the same * page we are just about to request for real. - * - * XXX: It's a layering violation that we do these checks above - * tableam, they should probably moved below it at some point. */ BitmapPrefetch(node, scan); - if (node->return_empty_tuples > 0) + /* + * Attempt to fetch tuple from AM. + */ + if (!table_scan_bitmap_next_tuple(scan, tbmres, slot)) { - /* - * If we don't have to fetch the tuple, just return nulls. - */ - ExecStoreAllNullTuple(slot); - - if (--node->return_empty_tuples == 0) - { - /* no more tuples to return in the next round */ - node->tbmres = tbmres = NULL; - } + /* nothing more to look at on this page */ + node->tbmres = tbmres = NULL; + continue; } - else - { - /* - * Attempt to fetch tuple from AM. - */ - if (!table_scan_bitmap_next_tuple(scan, tbmres, slot)) - { - /* nothing more to look at on this page */ - node->tbmres = tbmres = NULL; - continue; - } - /* - * If we are using lossy info, we have to recheck the qual - * conditions at every tuple. - */ - if (tbmres->recheck) + /* + * If we are using lossy info, we have to recheck the qual conditions + * at every tuple. + */ + if (tbmres->recheck) + { + econtext->ecxt_scantuple = slot; + if (!ExecQualAndReset(node->bitmapqualorig, econtext)) { - econtext->ecxt_scantuple = slot; - if (!ExecQualAndReset(node->bitmapqualorig, econtext)) - { - /* Fails recheck, so drop it and loop back for another */ - InstrCountFiltered2(node, 1); - ExecClearTuple(slot); - continue; - } + /* Fails recheck, so drop it and loop back for another */ + InstrCountFiltered2(node, 1); + ExecClearTuple(slot); + continue; } } @@ -519,7 +481,7 @@ BitmapPrefetch(BitmapHeapScanState *node, TableScanDesc scan) * it did for the current heap page; which is not a certainty * but is true in many cases. */ - skip_fetch = (node->can_skip_fetch && + skip_fetch = (!(scan->rs_flags & SO_NEED_TUPLES) && (node->tbmres ? !node->tbmres->recheck : false) && VM_ALL_VISIBLE(node->ss.ss_currentRelation, tbmpre->blockno, @@ -570,7 +532,7 @@ BitmapPrefetch(BitmapHeapScanState *node, TableScanDesc scan) } /* As above, skip prefetch if we expect not to need page */ - skip_fetch = (node->can_skip_fetch && + skip_fetch = (!(scan->rs_flags & SO_NEED_TUPLES) && (node->tbmres ? !node->tbmres->recheck : false) && VM_ALL_VISIBLE(node->ss.ss_currentRelation, tbmpre->blockno, @@ -640,8 +602,6 @@ ExecReScanBitmapHeapScan(BitmapHeapScanState *node) tbm_end_shared_iterate(node->shared_prefetch_iterator); if (node->tbm) tbm_free(node->tbm); - if (node->vmbuffer != InvalidBuffer) - ReleaseBuffer(node->vmbuffer); if (node->pvmbuffer != InvalidBuffer) ReleaseBuffer(node->pvmbuffer); node->tbm = NULL; @@ -651,7 +611,6 @@ ExecReScanBitmapHeapScan(BitmapHeapScanState *node) node->initialized = false; node->shared_tbmiterator = NULL; node->shared_prefetch_iterator = NULL; - node->vmbuffer = InvalidBuffer; node->pvmbuffer = InvalidBuffer; ExecScanReScan(&node->ss); @@ -696,8 +655,6 @@ ExecEndBitmapHeapScan(BitmapHeapScanState *node) tbm_end_shared_iterate(node->shared_tbmiterator); if (node->shared_prefetch_iterator) tbm_end_shared_iterate(node->shared_prefetch_iterator); - if (node->vmbuffer != InvalidBuffer) - ReleaseBuffer(node->vmbuffer); if (node->pvmbuffer != InvalidBuffer) ReleaseBuffer(node->pvmbuffer); @@ -741,8 +698,6 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags) scanstate->tbm = NULL; scanstate->tbmiterator = NULL; scanstate->tbmres = NULL; - scanstate->return_empty_tuples = 0; - scanstate->vmbuffer = InvalidBuffer; scanstate->pvmbuffer = InvalidBuffer; scanstate->exact_pages = 0; scanstate->lossy_pages = 0; @@ -753,7 +708,6 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags) scanstate->shared_tbmiterator = NULL; scanstate->shared_prefetch_iterator = NULL; scanstate->pstate = NULL; - scanstate->can_skip_fetch = false; /* * Miscellaneous initialization diff --git a/src/include/access/heapam.h b/src/include/access/heapam.h index 2765efc4e5..750ea30852 100644 --- a/src/include/access/heapam.h +++ b/src/include/access/heapam.h @@ -76,6 +76,16 @@ typedef struct HeapScanDescData */ ParallelBlockTableScanWorkerData *rs_parallelworkerdata; + /* + * These fields are only used for bitmap scans for the "skip fetch" + * optimization. Bitmap scans needing no fields from the heap may skip + * fetching an all visible block, instead using the number of tuples per + * 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 */ int rs_cindex; /* current tuple's index in vistuples */ int rs_ntuples; /* number of visible tuples on page */ diff --git a/src/include/access/tableam.h b/src/include/access/tableam.h index e7eeb75409..be198fa315 100644 --- a/src/include/access/tableam.h +++ b/src/include/access/tableam.h @@ -63,6 +63,13 @@ 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; /* @@ -937,10 +944,13 @@ table_beginscan_strat(Relation rel, Snapshot snapshot, */ static inline TableScanDesc table_beginscan_bm(Relation rel, Snapshot snapshot, - int nkeys, struct ScanKeyData *key) + int nkeys, struct ScanKeyData *key, bool need_tuple) { 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/include/nodes/execnodes.h b/src/include/nodes/execnodes.h index e57ebd7288..fa2f70b7a4 100644 --- a/src/include/nodes/execnodes.h +++ b/src/include/nodes/execnodes.h @@ -1794,10 +1794,7 @@ typedef struct ParallelBitmapHeapState * tbm bitmap obtained from child index scan(s) * tbmiterator iterator for scanning current pages * tbmres current-page data - * can_skip_fetch can we potentially skip tuple fetches in this scan? - * return_empty_tuples number of empty tuples to return - * vmbuffer buffer for visibility-map lookups - * pvmbuffer ditto, for prefetched pages + * pvmbuffer buffer for visibility-map lookups of prefetched pages * exact_pages total number of exact pages retrieved * lossy_pages total number of lossy pages retrieved * prefetch_iterator iterator for prefetching ahead of current page @@ -1817,9 +1814,6 @@ typedef struct BitmapHeapScanState TIDBitmap *tbm; TBMIterator *tbmiterator; TBMIterateResult *tbmres; - bool can_skip_fetch; - int return_empty_tuples; - Buffer vmbuffer; Buffer pvmbuffer; long exact_pages; long lossy_pages;