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Commit047f205

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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 parent2de0fde commit047f205

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3 files changed

+38
-12
lines changed

3 files changed

+38
-12
lines changed

‎src/backend/commands/vacuum.c

Lines changed: 4 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -485,8 +485,10 @@ vac_estimate_reltuples(Relation relation, bool is_analyze,
485485
returnscanned_tuples;
486486

487487
/*
488-
* If scanned_pages is zero but total_pages isn't, keep the existing
489-
* value of reltuples.
488+
* If scanned_pages is zero but total_pages isn't, keep the existing value
489+
* of reltuples. (Note: callers should avoid updating the pg_class
490+
* statistics in this situation, since no new information has been
491+
* provided.)
490492
*/
491493
if (scanned_pages==0)
492494
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
pg_rusage_init(&ru0);
159163

@@ -176,6 +180,7 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
176180

177181
vacrelstats= (LVRelStats*)palloc0(sizeof(LVRelStats));
178182

183+
vacrelstats->old_rel_pages=onerel->rd_rel->relpages;
179184
vacrelstats->old_rel_tuples=onerel->rd_rel->reltuples;
180185
vacrelstats->num_index_scans=0;
181186

@@ -205,20 +210,39 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
205210
FreeSpaceMapVacuum(onerel);
206211

207212
/*
208-
* Update statistics in pg_class. But don't change relfrozenxid if we
209-
* skipped any pages.
213+
* Update statistics in pg_class.
214+
*
215+
* A corner case here is that if we scanned no pages at all because every
216+
* page is all-visible, we should not update relpages/reltuples, because
217+
* we have no new information to contribute. In particular this keeps
218+
* us from replacing relpages=reltuples=0 (which means "unknown tuple
219+
* density") with nonzero relpages and reltuples=0 (which means "zero
220+
* tuple density") unless there's some actual evidence for the latter.
221+
*
222+
* Also, don't change relfrozenxid if we skipped any pages, since then
223+
* we don't know for certain that all tuples have a newer xmin.
210224
*/
225+
new_rel_pages=vacrelstats->rel_pages;
226+
new_rel_tuples=vacrelstats->new_rel_tuples;
227+
if (vacrelstats->scanned_pages==0&&new_rel_pages>0)
228+
{
229+
new_rel_pages=vacrelstats->old_rel_pages;
230+
new_rel_tuples=vacrelstats->old_rel_tuples;
231+
}
232+
233+
new_frozen_xid=FreezeLimit;
234+
if (vacrelstats->scanned_pages<vacrelstats->rel_pages)
235+
new_frozen_xid=InvalidTransactionId;
236+
211237
vac_update_relstats(onerel,
212-
vacrelstats->rel_pages,vacrelstats->new_rel_tuples,
238+
new_rel_pages,new_rel_tuples,
213239
vacrelstats->hasindex,
214-
(vacrelstats->scanned_pages<vacrelstats->rel_pages) ?
215-
InvalidTransactionId :
216-
FreezeLimit);
240+
new_frozen_xid);
217241

218242
/* report results to the stats collector, too */
219243
pgstat_report_vacuum(RelationGetRelid(onerel),
220244
onerel->rd_rel->relisshared,
221-
vacrelstats->new_rel_tuples);
245+
new_rel_tuples);
222246

223247
/* and log the action if appropriate */
224248
if (IsAutoVacuumWorkerProcess()&&Log_autovacuum_min_duration >=0)
@@ -238,7 +262,7 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
238262
vacrelstats->pages_removed,
239263
vacrelstats->rel_pages,
240264
vacrelstats->tuples_deleted,
241-
vacrelstats->new_rel_tuples,
265+
new_rel_tuples,
242266
pg_rusage_show(&ru0))));
243267
}
244268
}

‎src/backend/utils/cache/relcache.c

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -1454,8 +1454,8 @@ formrdesc(const char *relationName, Oid relationReltype,
14541454
*/
14551455
relation->rd_rel->relistemp= false;
14561456

1457-
relation->rd_rel->relpages=1;
1458-
relation->rd_rel->reltuples=1;
1457+
relation->rd_rel->relpages=0;
1458+
relation->rd_rel->reltuples=0;
14591459
relation->rd_rel->relkind=RELKIND_RELATION;
14601460
relation->rd_rel->relhasoids=hasoids;
14611461
relation->rd_rel->relnatts= (int16)natts;

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