From 1a621864420cd17db1bef0546990d8298e3a3d34 Mon Sep 17 00:00:00 2001 From: Melanie Plageman Date: Wed, 6 Sep 2023 16:54:41 -0400 Subject: [PATCH v4 3/3] Reuse heap_page_prune() tuple visibility statuses heap_page_prune() obtains the HTSV_Result (tuple visibility status) returned from HeapTupleSatisfiesVacuum() for every tuple on the page and stores them in an array. By making this array available to heap_page_prune()'s caller lazy_scan_prune(), we can avoid an additional call to HeapTupleSatisfiesVacuum() when freezing the tuples and recording LP_DEAD items for vacuum. This saves resources and eliminates the possibility that vacuuming corrupted data results in a hang due to endless retry looping. This replaces the retry mechanism introduced in 8523492d4 to handle cases in which a tuple's inserting transaction aborted between the visibility check in heap_page_prune() and lazy_scan_prune()'s call to HeapTupleSatisfiesVacuum() -- rendering it dead but without a dead line pointer. We can instead reuse the tuple's original visibility status, circumventing any disagreements. Discussion: https://postgr.es/m/CAAKRu_br124qsGJieuYA0nGjywEukhK1dKBfRdby_4yY3E9SXA%40mail.gmail.com --- src/backend/access/heap/pruneheap.c | 37 ++++++++++++------------ src/backend/access/heap/vacuumlazy.c | 42 ++++++++-------------------- src/include/access/heapam.h | 11 ++++++++ 3 files changed, 41 insertions(+), 49 deletions(-) diff --git a/src/backend/access/heap/pruneheap.c b/src/backend/access/heap/pruneheap.c index 5ac286e152..256039c6be 100644 --- a/src/backend/access/heap/pruneheap.c +++ b/src/backend/access/heap/pruneheap.c @@ -53,16 +53,6 @@ typedef struct * 1. Otherwise every access would need to subtract 1. */ bool marked[MaxHeapTuplesPerPage + 1]; - - /* - * Tuple visibility is only computed once for each tuple, for correctness - * and efficiency reasons; see comment in heap_page_prune() for details. - * This is of type int8[], instead of HTSV_Result[], so we can use -1 to - * indicate no visibility has been computed, e.g. for LP_DEAD items. - * - * Same indexing as ->marked. - */ - int8 htsv[MaxHeapTuplesPerPage + 1]; } PruneState; /* Local functions */ @@ -71,6 +61,7 @@ static HTSV_Result heap_prune_satisfies_vacuum(PruneState *prstate, Buffer buffer); static int heap_prune_chain(Buffer buffer, OffsetNumber rootoffnum, + int8 *htsv, PruneState *prstate); static void heap_prune_record_prunable(PruneState *prstate, TransactionId xid); static void heap_prune_record_redirect(PruneState *prstate, @@ -243,6 +234,10 @@ heap_page_prune(Relation relation, Buffer buffer, prstate.nredirected = prstate.ndead = prstate.nunused = 0; memset(prstate.marked, 0, sizeof(prstate.marked)); + /* + * presult->htsv is not initialized here because all ntuple spots in the + * array will be set either to a valid HTSV_Result value or -1. + */ presult->ndeleted = 0; presult->nnewlpdead = 0; @@ -279,7 +274,7 @@ heap_page_prune(Relation relation, Buffer buffer, /* Nothing to do if slot doesn't contain a tuple */ if (!ItemIdIsNormal(itemid)) { - prstate.htsv[offnum] = -1; + presult->htsv[offnum] = -1; continue; } @@ -295,8 +290,8 @@ heap_page_prune(Relation relation, Buffer buffer, if (off_loc) *off_loc = offnum; - prstate.htsv[offnum] = heap_prune_satisfies_vacuum(&prstate, &tup, - buffer); + presult->htsv[offnum] = heap_prune_satisfies_vacuum(&prstate, &tup, + buffer); } /* Scan the page */ @@ -320,7 +315,8 @@ heap_page_prune(Relation relation, Buffer buffer, continue; /* Process this item or chain of items */ - presult->ndeleted += heap_prune_chain(buffer, offnum, &prstate); + presult->ndeleted += heap_prune_chain(buffer, offnum, + presult->htsv, &prstate); } /* Clear the offset information once we have processed the given page. */ @@ -449,6 +445,8 @@ heap_prune_satisfies_vacuum(PruneState *prstate, HeapTuple tup, Buffer buffer) /* * Prune specified line pointer or a HOT chain originating at line pointer. * + * Tuple visibility information is provided in htsv. + * * If the item is an index-referenced tuple (i.e. not a heap-only tuple), * the HOT chain is pruned by removing all DEAD tuples at the start of the HOT * chain. We also prune any RECENTLY_DEAD tuples preceding a DEAD tuple. @@ -476,7 +474,8 @@ heap_prune_satisfies_vacuum(PruneState *prstate, HeapTuple tup, Buffer buffer) * Returns the number of tuples (to be) deleted from the page. */ static int -heap_prune_chain(Buffer buffer, OffsetNumber rootoffnum, PruneState *prstate) +heap_prune_chain(Buffer buffer, OffsetNumber rootoffnum, + int8 *htsv, PruneState *prstate) { int ndeleted = 0; Page dp = (Page) BufferGetPage(buffer); @@ -497,7 +496,7 @@ heap_prune_chain(Buffer buffer, OffsetNumber rootoffnum, PruneState *prstate) */ if (ItemIdIsNormal(rootlp)) { - Assert(prstate->htsv[rootoffnum] != -1); + Assert(htsv[rootoffnum] != -1); htup = (HeapTupleHeader) PageGetItem(dp, rootlp); if (HeapTupleHeaderIsHeapOnly(htup)) @@ -520,7 +519,7 @@ heap_prune_chain(Buffer buffer, OffsetNumber rootoffnum, PruneState *prstate) * either here or while following a chain below. Whichever path * gets there first will mark the tuple unused. */ - if (prstate->htsv[rootoffnum] == HEAPTUPLE_DEAD && + if (htsv[rootoffnum] == HEAPTUPLE_DEAD && !HeapTupleHeaderIsHotUpdated(htup)) { heap_prune_record_unused(prstate, rootoffnum); @@ -588,7 +587,7 @@ heap_prune_chain(Buffer buffer, OffsetNumber rootoffnum, PruneState *prstate) break; Assert(ItemIdIsNormal(lp)); - Assert(prstate->htsv[offnum] != -1); + Assert(htsv[offnum] != -1); htup = (HeapTupleHeader) PageGetItem(dp, lp); /* @@ -608,7 +607,7 @@ heap_prune_chain(Buffer buffer, OffsetNumber rootoffnum, PruneState *prstate) */ tupdead = recent_dead = false; - switch ((HTSV_Result) prstate->htsv[offnum]) + switch ((HTSV_Result) htsv[offnum]) { case HEAPTUPLE_DEAD: tupdead = true; diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c index 7ead9cfe9d..6c328f3ed5 100644 --- a/src/backend/access/heap/vacuumlazy.c +++ b/src/backend/access/heap/vacuumlazy.c @@ -1524,12 +1524,12 @@ lazy_scan_new_or_empty(LVRelState *vacrel, Buffer buf, BlockNumber blkno, * of complexity just so we could deal with tuples that were DEAD to VACUUM, * but nevertheless were left with storage after pruning. * - * The approach we take now is to restart pruning when the race condition is - * detected. This allows heap_page_prune() to prune the tuples inserted by - * the now-aborted transaction. This is a little crude, but it guarantees - * that any items that make it into the dead_items array are simple LP_DEAD - * line pointers, and that every remaining item with tuple storage is - * considered as a candidate for freezing. + * As of Postgres 17, we circumvent this problem altogether by reusing the + * result of heap_page_prune()'s visibility check. Without the second call to + * HeapTupleSatisfiesVacuum(), there is no new HTSV_Result and there can be no + * disagreement. The tuple's TID won't be added to the array of dead tuple TIDs + * for vacuum, thus vacuum will never be tasked with reaping a tuple with + * storage. */ static void lazy_scan_prune(LVRelState *vacrel, @@ -1542,8 +1542,6 @@ lazy_scan_prune(LVRelState *vacrel, OffsetNumber offnum, maxoff; ItemId itemid; - HeapTupleData tuple; - HTSV_Result res; PruneResult presult; int tuples_frozen, lpdead_items, @@ -1563,8 +1561,6 @@ lazy_scan_prune(LVRelState *vacrel, */ maxoff = PageGetMaxOffsetNumber(page); -retry: - /* Initialize (or reset) page-level state */ pagefrz.freeze_required = false; pagefrz.FreezePageRelfrozenXid = vacrel->NewRelfrozenXid; @@ -1604,6 +1600,7 @@ retry: offnum <= maxoff; offnum = OffsetNumberNext(offnum)) { + HeapTupleHeader htup; bool totally_frozen; /* @@ -1646,22 +1643,7 @@ retry: Assert(ItemIdIsNormal(itemid)); - ItemPointerSet(&(tuple.t_self), blkno, offnum); - tuple.t_data = (HeapTupleHeader) PageGetItem(page, itemid); - tuple.t_len = ItemIdGetLength(itemid); - tuple.t_tableOid = RelationGetRelid(rel); - - /* - * DEAD tuples are almost always pruned into LP_DEAD line pointers by - * heap_page_prune(), but it's possible that the tuple state changed - * since heap_page_prune() looked. Handle that here by restarting. - * (See comments at the top of function for a full explanation.) - */ - res = HeapTupleSatisfiesVacuum(&tuple, vacrel->cutoffs.OldestXmin, - buf); - - if (unlikely(res == HEAPTUPLE_DEAD)) - goto retry; + htup = (HeapTupleHeader) PageGetItem(page, itemid); /* * The criteria for counting a tuple as live in this block need to @@ -1682,7 +1664,7 @@ retry: * (Cases where we bypass index vacuuming will violate this optimistic * assumption, but the overall impact of that should be negligible.) */ - switch (res) + switch ((HTSV_Result) presult.htsv[offnum]) { case HEAPTUPLE_LIVE: @@ -1704,7 +1686,7 @@ retry: { TransactionId xmin; - if (!HeapTupleHeaderXminCommitted(tuple.t_data)) + if (!HeapTupleHeaderXminCommitted(htup)) { prunestate->all_visible = false; break; @@ -1714,7 +1696,7 @@ retry: * The inserter definitely committed. But is it old enough * that everyone sees it as committed? */ - xmin = HeapTupleHeaderGetXmin(tuple.t_data); + xmin = HeapTupleHeaderGetXmin(htup); if (!TransactionIdPrecedes(xmin, vacrel->cutoffs.OldestXmin)) { @@ -1768,7 +1750,7 @@ retry: prunestate->hastup = true; /* page makes rel truncation unsafe */ /* Tuple with storage -- consider need to freeze */ - if (heap_prepare_freeze_tuple(tuple.t_data, &vacrel->cutoffs, &pagefrz, + if (heap_prepare_freeze_tuple(htup, &vacrel->cutoffs, &pagefrz, &frozen[tuples_frozen], &totally_frozen)) { /* Save prepared freeze plan for later */ diff --git a/src/include/access/heapam.h b/src/include/access/heapam.h index 2d3f149e4f..864ae561c4 100644 --- a/src/include/access/heapam.h +++ b/src/include/access/heapam.h @@ -198,6 +198,17 @@ typedef struct PruneResult { int ndeleted; /* Number of tuples deleted from the page */ int nnewlpdead; /* Number of newly LP_DEAD items */ + + /* + * Tuple visibility is only computed once for each tuple, for correctness + * and efficiency reasons; see comment in heap_page_prune() for details. + * This is of type int8[], instead of HTSV_Result[], so we can use -1 to + * indicate no visibility has been computed, e.g. for LP_DEAD items. + * + * This needs to be MaxHeapTuplesPerPage + 1 long as FirstOffsetNumber is + * 1. Otherwise every access would need to subtract 1. + */ + int8 htsv[MaxHeapTuplesPerPage + 1]; } PruneResult; /* ---------------- -- 2.37.2