From a494ff4b012b5ce89b2549d5dab6b66604083bf0 Mon Sep 17 00:00:00 2001 From: Andrew Gierth Date: Thu, 16 Mar 2017 22:33:38 +0000 Subject: [PATCH] Avoid having vacuum set reltuples to 0 on non-empty relations in the presence of page pins, which leads to serious estimation errors in the planner. This particularly affects small heavily-accessed tables, especially where locking (e.g. from FK constraints) forces frequent vacuums for mxid cleanup. Fix by keeping separate track of pages whose live tuples were actually counted vs. pages that were only scanned for freezing purposes. Thus, reltuples can only be set to 0 if all pages of the relation were actually counted. Backpatch to all supported versions. Per bug #14057 from Nicolas Baccelli, analyzed by me. Discussion: https://postgr.es/m/20160331103739.8956.94469@wrigleys.postgresql.org --- src/backend/commands/vacuumlazy.c | 15 +++++++++++---- 1 file changed, 11 insertions(+), 4 deletions(-) diff --git a/src/backend/commands/vacuumlazy.c b/src/backend/commands/vacuumlazy.c index 8aefa7aaa9..a33541a115 100644 --- a/src/backend/commands/vacuumlazy.c +++ b/src/backend/commands/vacuumlazy.c @@ -100,7 +100,8 @@ typedef struct LVRelStats BlockNumber old_rel_pages; /* previous value of pg_class.relpages */ BlockNumber rel_pages; /* total number of pages */ BlockNumber scanned_pages; /* number of pages we examined */ - double scanned_tuples; /* counts only tuples on scanned pages */ + BlockNumber tupcount_pages; /* pages whose tuples we counted */ + double scanned_tuples; /* counts only tuples on tupcount_pages */ double old_rel_tuples; /* previous value of pg_class.reltuples */ double new_rel_tuples; /* new estimated total # of tuples */ BlockNumber pages_removed; @@ -258,6 +259,10 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt, * density") with nonzero relpages and reltuples=0 (which means "zero * tuple density") unless there's some actual evidence for the latter. * + * It's important that we use tupcount_pages and not scanned_pages for the + * check described above; scanned_pages counts pages where we could not get + * cleanup lock, and which were processed only for frozenxid purposes. + * * We do update relallvisible even in the corner case, since if the table * is all-visible we'd definitely like to know that. But clamp the value * to be not more than what we're setting relpages to. @@ -267,7 +272,7 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt, */ new_rel_pages = vacrelstats->rel_pages; new_rel_tuples = vacrelstats->new_rel_tuples; - if (vacrelstats->scanned_pages == 0 && new_rel_pages > 0) + if (vacrelstats->tupcount_pages == 0 && new_rel_pages > 0) { new_rel_pages = vacrelstats->old_rel_pages; new_rel_tuples = vacrelstats->old_rel_tuples; @@ -423,6 +428,7 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats, nblocks = RelationGetNumberOfBlocks(onerel); vacrelstats->rel_pages = nblocks; vacrelstats->scanned_pages = 0; + vacrelstats->tupcount_pages = 0; vacrelstats->nonempty_pages = 0; vacrelstats->latestRemovedXid = InvalidTransactionId; @@ -613,6 +619,7 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats, } vacrelstats->scanned_pages++; + vacrelstats->tupcount_pages++; page = BufferGetPage(buf); @@ -999,7 +1006,7 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats, /* now we can compute the new value for pg_class.reltuples */ vacrelstats->new_rel_tuples = vac_estimate_reltuples(onerel, false, nblocks, - vacrelstats->scanned_pages, + vacrelstats->tupcount_pages, num_tuples); /* @@ -1264,7 +1271,7 @@ lazy_cleanup_index(Relation indrel, ivinfo.index = indrel; ivinfo.analyze_only = false; - ivinfo.estimated_count = (vacrelstats->scanned_pages < vacrelstats->rel_pages); + ivinfo.estimated_count = (vacrelstats->tupcount_pages < vacrelstats->rel_pages); ivinfo.message_level = elevel; ivinfo.num_heap_tuples = vacrelstats->new_rel_tuples; ivinfo.strategy = vac_strategy; -- 2.40.0