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 map
shows that all pages are all-visible.  In this situation VACUUM has no new
information to report about the relation's tuple density, so it wasn't
changing pg_class.reltuples ... but it updated pg_class.relpages anyway.
That's wrong in general, since there is no evidence to justify changing the
density ratio reltuples/relpages, but it's particularly bad if the previous
state was relpages=reltuples=0, which means "unknown tuple density".
We just replaced "unknown" with "zero".  ANALYZE would eventually recover
from this, but it could take a lot of repetitions of ANALYZE to do so if
the relation size is much larger than the maximum number of pages ANALYZE
will scan, because of the moving-average behavior introduced by commit
b4b6923e03f4d29636a94f6f4cc2f5cf6298b8c8.

The only known situation where we could have relpages=reltuples=0 and yet
the visibility map asserts everything's visible is immediately following
a pg_upgrade.  It might be advisable for pg_upgrade to try to preserve the
relpages/reltuples statistics; but in any case this code is wrong on its
own terms, so fix it.  Per report from Sergey Koposov.

Back-patch to 8.4, where the visibility map was introduced, same as the
previous change.

diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index 5decc5cac670b4b6a59ce35fccbdaa8905a5c064..606e782ba98c6202480697e50874f518a29b1b2b 100644 (file)
--- a/src/backend/commands/vacuum.c
+++ b/src/backend/commands/vacuum.c
@@ -485,8 +485,10 @@ vac_estimate_reltuples(Relation relation, bool is_analyze,
        return scanned_tuples;
 
    /*
-    * If scanned_pages is zero but total_pages isn't, keep the existing
-    * value of reltuples.
+    * If scanned_pages is zero but total_pages isn't, keep the existing value
+    * of reltuples.  (Note: callers should avoid updating the pg_class
+    * statistics in this situation, since no new information has been
+    * provided.)
     */
    if (scanned_pages == 0)
        return old_rel_tuples;

diff --git a/src/backend/commands/vacuumlazy.c b/src/backend/commands/vacuumlazy.c
index 23c4d5a00b9652063b1878a9605811163bb97c70..89de6693ae2bc4a2eed02041d4a9e2401ff08f9e 100644 (file)
--- a/src/backend/commands/vacuumlazy.c
+++ b/src/backend/commands/vacuumlazy.c
@@ -84,6 +84,7 @@ typedef struct LVRelStats
    /* hasindex = true means two-pass strategy; false means one-pass */
    bool        hasindex;
    /* Overall statistics about rel */
+   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 */
@@ -154,6 +155,9 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
    TimestampTz starttime = 0;
    bool        scan_all;
    TransactionId freezeTableLimit;
+   BlockNumber new_rel_pages;
+   double      new_rel_tuples;
+   TransactionId new_frozen_xid;
 
    pg_rusage_init(&ru0);
 
@@ -176,6 +180,7 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
 
    vacrelstats = (LVRelStats *) palloc0(sizeof(LVRelStats));
 
+   vacrelstats->old_rel_pages = onerel->rd_rel->relpages;
    vacrelstats->old_rel_tuples = onerel->rd_rel->reltuples;
    vacrelstats->num_index_scans = 0;
 
@@ -205,20 +210,39 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
    FreeSpaceMapVacuum(onerel);
 
    /*
-    * Update statistics in pg_class.  But don't change relfrozenxid if we
-    * skipped any pages.
+    * Update statistics in pg_class.
+    *
+    * A corner case here is that if we scanned no pages at all because every
+    * page is all-visible, we should not update relpages/reltuples, because
+    * we have no new information to contribute.  In particular this keeps
+    * us from replacing relpages=reltuples=0 (which means "unknown tuple
+    * density") with nonzero relpages and reltuples=0 (which means "zero
+    * tuple density") unless there's some actual evidence for the latter.
+    *
+    * Also, don't change relfrozenxid if we skipped any pages, since then
+    * we don't know for certain that all tuples have a newer xmin.
     */
+   new_rel_pages = vacrelstats->rel_pages;
+   new_rel_tuples = vacrelstats->new_rel_tuples;
+   if (vacrelstats->scanned_pages == 0 && new_rel_pages > 0)
+   {
+       new_rel_pages = vacrelstats->old_rel_pages;
+       new_rel_tuples = vacrelstats->old_rel_tuples;
+   }
+
+   new_frozen_xid = FreezeLimit;
+   if (vacrelstats->scanned_pages < vacrelstats->rel_pages)
+       new_frozen_xid = InvalidTransactionId;
+
    vac_update_relstats(onerel,
-                       vacrelstats->rel_pages, vacrelstats->new_rel_tuples,
+                       new_rel_pages, new_rel_tuples,
                        vacrelstats->hasindex,
-                       (vacrelstats->scanned_pages < vacrelstats->rel_pages) ?
-                       InvalidTransactionId :
-                       FreezeLimit);
+                       new_frozen_xid);
 
    /* report results to the stats collector, too */
    pgstat_report_vacuum(RelationGetRelid(onerel),
                         onerel->rd_rel->relisshared,
-                        vacrelstats->new_rel_tuples);
+                        new_rel_tuples);
 
    /* and log the action if appropriate */
    if (IsAutoVacuumWorkerProcess() && Log_autovacuum_min_duration >= 0)
@@ -238,7 +262,7 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
                            vacrelstats->pages_removed,
                            vacrelstats->rel_pages,
                            vacrelstats->tuples_deleted,
-                           vacrelstats->new_rel_tuples,
+                           new_rel_tuples,
                            pg_rusage_show(&ru0))));
    }
 }

diff --git a/src/backend/utils/cache/relcache.c b/src/backend/utils/cache/relcache.c
index d72c64f737bc97cf39ae5cd641b23f33ea1af041..5e6158db25bb0cdb6917066a118c5c8c79d5c3de 100644 (file)
--- a/src/backend/utils/cache/relcache.c
+++ b/src/backend/utils/cache/relcache.c
@@ -1454,8 +1454,8 @@ formrdesc(const char *relationName, Oid relationReltype,
     */
    relation->rd_rel->relistemp = false;
 
-   relation->rd_rel->relpages = 1;
-   relation->rd_rel->reltuples = 1;
+   relation->rd_rel->relpages = 0;
+   relation->rd_rel->reltuples = 0;
    relation->rd_rel->relkind = RELKIND_RELATION;
    relation->rd_rel->relhasoids = hasoids;
    relation->rd_rel->relnatts = (int16) natts;