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Commit5bba65d

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Fix a missed case in code for "moving average" estimate of reltuples.
It is possible for VACUUM to scan no pages at all, if the visibility mapshows that all pages are all-visible. In this situation VACUUM has no newinformation to report about the relation's tuple density, so it wasn'tchanging pg_class.reltuples ... but it updated pg_class.relpages anyway.That's wrong in general, since there is no evidence to justify changing thedensity ratio reltuples/relpages, but it's particularly bad if the previousstate was relpages=reltuples=0, which means "unknown tuple density".We just replaced "unknown" with "zero". ANALYZE would eventually recoverfrom this, but it could take a lot of repetitions of ANALYZE to do so ifthe relation size is much larger than the maximum number of pages ANALYZEwill scan, because of the moving-average behavior introduced by commitb4b6923.The only known situation where we could have relpages=reltuples=0 and yetthe visibility map asserts everything's visible is immediately followinga pg_upgrade. It might be advisable for pg_upgrade to try to preserve therelpages/reltuples statistics; but in any case this code is wrong on itsown terms, so fix it. Per report from Sergey Koposov.Back-patch to 8.4, where the visibility map was introduced, same as theprevious change.
1 parentb83bb97 commit5bba65d

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3 files changed

+37
-11
lines changed

3 files changed

+37
-11
lines changed

‎src/backend/commands/vacuum.c‎

Lines changed: 3 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -493,7 +493,9 @@ vac_estimate_reltuples(Relation relation, bool is_analyze,
493493

494494
/*
495495
* If scanned_pages is zero but total_pages isn't, keep the existing value
496-
* of reltuples.
496+
* of reltuples. (Note: callers should avoid updating the pg_class
497+
* statistics in this situation, since no new information has been
498+
* provided.)
497499
*/
498500
if (scanned_pages==0)
499501
returnold_rel_tuples;

‎src/backend/commands/vacuumlazy.c‎

Lines changed: 32 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -84,6 +84,7 @@ typedef struct LVRelStats
8484
/* hasindex = true means two-pass strategy; false means one-pass */
8585
boolhasindex;
8686
/* Overall statistics about rel */
87+
BlockNumberold_rel_pages;/* previous value of pg_class.relpages */
8788
BlockNumberrel_pages;/* total number of pages */
8889
BlockNumberscanned_pages;/* number of pages we examined */
8990
doublescanned_tuples;/* counts only tuples on scanned pages */
@@ -154,6 +155,9 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
154155
TimestampTzstarttime=0;
155156
boolscan_all;
156157
TransactionIdfreezeTableLimit;
158+
BlockNumbernew_rel_pages;
159+
doublenew_rel_tuples;
160+
TransactionIdnew_frozen_xid;
157161

158162
/* measure elapsed time iff autovacuum logging requires it */
159163
if (IsAutoVacuumWorkerProcess()&&Log_autovacuum_min_duration >=0)
@@ -178,6 +182,7 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
178182

179183
vacrelstats= (LVRelStats*)palloc0(sizeof(LVRelStats));
180184

185+
vacrelstats->old_rel_pages=onerel->rd_rel->relpages;
181186
vacrelstats->old_rel_tuples=onerel->rd_rel->reltuples;
182187
vacrelstats->num_index_scans=0;
183188

@@ -207,20 +212,39 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
207212
FreeSpaceMapVacuum(onerel);
208213

209214
/*
210-
* Update statistics in pg_class. But don't change relfrozenxid if we
211-
* skipped any pages.
215+
* Update statistics in pg_class.
216+
*
217+
* A corner case here is that if we scanned no pages at all because every
218+
* page is all-visible, we should not update relpages/reltuples, because
219+
* we have no new information to contribute. In particular this keeps
220+
* us from replacing relpages=reltuples=0 (which means "unknown tuple
221+
* density") with nonzero relpages and reltuples=0 (which means "zero
222+
* tuple density") unless there's some actual evidence for the latter.
223+
*
224+
* Also, don't change relfrozenxid if we skipped any pages, since then
225+
* we don't know for certain that all tuples have a newer xmin.
212226
*/
227+
new_rel_pages=vacrelstats->rel_pages;
228+
new_rel_tuples=vacrelstats->new_rel_tuples;
229+
if (vacrelstats->scanned_pages==0&&new_rel_pages>0)
230+
{
231+
new_rel_pages=vacrelstats->old_rel_pages;
232+
new_rel_tuples=vacrelstats->old_rel_tuples;
233+
}
234+
235+
new_frozen_xid=FreezeLimit;
236+
if (vacrelstats->scanned_pages<vacrelstats->rel_pages)
237+
new_frozen_xid=InvalidTransactionId;
238+
213239
vac_update_relstats(onerel,
214-
vacrelstats->rel_pages,vacrelstats->new_rel_tuples,
240+
new_rel_pages,new_rel_tuples,
215241
vacrelstats->hasindex,
216-
(vacrelstats->scanned_pages<vacrelstats->rel_pages) ?
217-
InvalidTransactionId :
218-
FreezeLimit);
242+
new_frozen_xid);
219243

220244
/* report results to the stats collector, too */
221245
pgstat_report_vacuum(RelationGetRelid(onerel),
222246
onerel->rd_rel->relisshared,
223-
vacrelstats->new_rel_tuples);
247+
new_rel_tuples);
224248

225249
/* and log the action if appropriate */
226250
if (IsAutoVacuumWorkerProcess()&&Log_autovacuum_min_duration >=0)
@@ -240,7 +264,7 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
240264
vacrelstats->pages_removed,
241265
vacrelstats->rel_pages,
242266
vacrelstats->tuples_deleted,
243-
vacrelstats->new_rel_tuples,
267+
new_rel_tuples,
244268
pg_rusage_show(&ru0))));
245269
}
246270
}

‎src/backend/utils/cache/relcache.c‎

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -1415,8 +1415,8 @@ formrdesc(const char *relationName, Oid relationReltype,
14151415
/* formrdesc is used only for permanent relations */
14161416
relation->rd_rel->relpersistence=RELPERSISTENCE_PERMANENT;
14171417

1418-
relation->rd_rel->relpages=1;
1419-
relation->rd_rel->reltuples=1;
1418+
relation->rd_rel->relpages=0;
1419+
relation->rd_rel->reltuples=0;
14201420
relation->rd_rel->relkind=RELKIND_RELATION;
14211421
relation->rd_rel->relhasoids=hasoids;
14221422
relation->rd_rel->relnatts= (int16)natts;

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