From 2cc41acd8fa3ebb8f0501c6102a253fb7053cf46 Mon Sep 17 00:00:00 2001 From: Kevin Grittner Date: Fri, 6 May 2016 07:47:12 -0500 Subject: [PATCH] Fix hash index vs "snapshot too old" problemms Hash indexes are not WAL-logged, and so do not maintain the LSN of index pages. Since the "snapshot too old" feature counts on detecting error conditions using the LSN of a table and all indexes on it, this makes it impossible to safely do early vacuuming on any table with a hash index, so add this to the tests for whether the xid used to vacuum a table can be adjusted based on old_snapshot_threshold. While at it, add a paragraph to the docs for old_snapshot_threshold which specifically mentions this and other aspects of the feature which may otherwise surprise users. Problem reported and patch reviewed by Amit Kapila --- doc/src/sgml/config.sgml | 13 ++++++++ src/backend/access/hash/hash.c | 1 - src/backend/access/hash/hashsearch.c | 4 --- src/backend/utils/cache/relcache.c | 46 ++++++++++++++++++++++++++++ src/backend/utils/time/snapmgr.c | 3 +- src/include/utils/rel.h | 1 + 6 files changed, 62 insertions(+), 6 deletions(-) diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml index 94001241fc..3d6baadaff 100644 --- a/doc/src/sgml/config.sgml +++ b/doc/src/sgml/config.sgml @@ -2077,6 +2077,19 @@ include_dir 'conf.d' allowed, please note that in many workloads extreme bloat or transaction ID wraparound may occur in much shorter time frames. + + + This setting does not attempt to guarantee that an error will be + generated under any particular circumstances. In fact, if the + correct results can be generated from (for example) a cursor which + has materialized a result set, no error will be generated even if the + underlying rows in the referenced table have been vacuumed away. + Some tables cannot safely be vacuumed early, and so will not be + affected by this setting. Examples include system catalogs and any + table which has a hash index. For such tables this setting will + neither reduce bloat nor create a possibility of a snapshot + too old error on scanning. + diff --git a/src/backend/access/hash/hash.c b/src/backend/access/hash/hash.c index 4fececeab8..49a6c816aa 100644 --- a/src/backend/access/hash/hash.c +++ b/src/backend/access/hash/hash.c @@ -279,7 +279,6 @@ hashgettuple(IndexScanDesc scan, ScanDirection dir) buf = so->hashso_curbuf; Assert(BufferIsValid(buf)); page = BufferGetPage(buf); - TestForOldSnapshot(scan->xs_snapshot, rel, page); maxoffnum = PageGetMaxOffsetNumber(page); for (offnum = ItemPointerGetOffsetNumber(current); offnum <= maxoffnum; diff --git a/src/backend/access/hash/hashsearch.c b/src/backend/access/hash/hashsearch.c index eb8c9cd3a4..48255584e1 100644 --- a/src/backend/access/hash/hashsearch.c +++ b/src/backend/access/hash/hashsearch.c @@ -189,7 +189,6 @@ _hash_first(IndexScanDesc scan, ScanDirection dir) /* Read the metapage */ metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ, LH_META_PAGE); page = BufferGetPage(metabuf); - TestForOldSnapshot(scan->xs_snapshot, rel, page); metap = HashPageGetMeta(page); /* @@ -243,7 +242,6 @@ _hash_first(IndexScanDesc scan, ScanDirection dir) /* Fetch the primary bucket page for the bucket */ buf = _hash_getbuf(rel, blkno, HASH_READ, LH_BUCKET_PAGE); page = BufferGetPage(buf); - TestForOldSnapshot(scan->xs_snapshot, rel, page); opaque = (HashPageOpaque) PageGetSpecialPointer(page); Assert(opaque->hasho_bucket == bucket); @@ -350,7 +348,6 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir) _hash_readnext(rel, &buf, &page, &opaque); if (BufferIsValid(buf)) { - TestForOldSnapshot(scan->xs_snapshot, rel, page); maxoff = PageGetMaxOffsetNumber(page); offnum = _hash_binsearch(page, so->hashso_sk_hash); } @@ -392,7 +389,6 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir) _hash_readprev(rel, &buf, &page, &opaque); if (BufferIsValid(buf)) { - TestForOldSnapshot(scan->xs_snapshot, rel, page); maxoff = PageGetMaxOffsetNumber(page); offnum = _hash_binsearch_last(page, so->hashso_sk_hash); } diff --git a/src/backend/utils/cache/relcache.c b/src/backend/utils/cache/relcache.c index 432feefa60..79cc3df590 100644 --- a/src/backend/utils/cache/relcache.c +++ b/src/backend/utils/cache/relcache.c @@ -5312,6 +5312,52 @@ RelationIdIsInInitFile(Oid relationId) return RelationSupportsSysCache(relationId); } +/* + * Tells whether any index for the relation is unlogged. + * + * Any index using the hash AM is implicitly unlogged. + * + * Note: There doesn't seem to be any way to have an unlogged index attached + * to a permanent table except to create a hash index, but it seems best to + * keep this general so that it returns sensible results even when they seem + * obvious (like for an unlogged table) and to handle possible future unlogged + * indexes on permanent tables. + */ +bool +RelationHasUnloggedIndex(Relation rel) +{ + List *indexoidlist; + ListCell *indexoidscan; + bool result = false; + + indexoidlist = RelationGetIndexList(rel); + + foreach(indexoidscan, indexoidlist) + { + Oid indexoid = lfirst_oid(indexoidscan); + HeapTuple tp; + Form_pg_class reltup; + + tp = SearchSysCache1(RELOID, ObjectIdGetDatum(indexoid)); + if (!HeapTupleIsValid(tp)) + elog(ERROR, "cache lookup failed for relation %u", indexoid); + reltup = (Form_pg_class) GETSTRUCT(tp); + + if (reltup->relpersistence == RELPERSISTENCE_UNLOGGED + || reltup->relam == HASH_AM_OID) + result = true; + + ReleaseSysCache(tp); + + if (result == true) + break; + } + + list_free(indexoidlist); + + return result; +} + /* * Invalidate (remove) the init file during commit of a transaction that * changed one or more of the relation cache entries that are kept in the diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 0a9a231f59..e1551a3aeb 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -1590,7 +1590,8 @@ TransactionIdLimitedForOldSnapshots(TransactionId recentXmin, && old_snapshot_threshold >= 0 && RelationNeedsWAL(relation) && !IsCatalogRelation(relation) - && !RelationIsAccessibleInLogicalDecoding(relation)) + && !RelationIsAccessibleInLogicalDecoding(relation) + && !RelationHasUnloggedIndex(relation)) { int64 ts = GetSnapshotCurrentTimestamp(); TransactionId xlimit = recentXmin; diff --git a/src/include/utils/rel.h b/src/include/utils/rel.h index b5d82d6004..a0ba417764 100644 --- a/src/include/utils/rel.h +++ b/src/include/utils/rel.h @@ -505,5 +505,6 @@ typedef struct ViewOptions /* routines in utils/cache/relcache.c */ extern void RelationIncrementReferenceCount(Relation rel); extern void RelationDecrementReferenceCount(Relation rel); +extern bool RelationHasUnloggedIndex(Relation rel); #endif /* REL_H */ -- 2.40.0