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
b4b6923e.

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.

src/backend/commands/vacuum.c

@@ -493,7 +493,9 @@ vac_estimate_reltuples(Relation relation, bool is_analyze,
 
 	/*
 	 * If scanned_pages is zero but total_pages isn't, keep the existing value
-	 * of reltuples.
+	 * 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;

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;
 
 	/* measure elapsed time iff autovacuum logging requires it */
 	if (IsAutoVacuumWorkerProcess() && Log_autovacuum_min_duration >= 0)
@@ -178,6 +182,7 @@ 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;
 
@@ -207,20 +212,39 @@ 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)
@@ -240,7 +264,7 @@ 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))));
 	}
 }

src/backend/utils/cache/relcache.c

@@ -1415,8 +1415,8 @@ formrdesc(const char *relationName, Oid relationReltype,
 	/* formrdesc is used only for permanent relations */
 	relation->rd_rel->relpersistence = RELPERSISTENCE_PERMANENT;
 
-	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;