Measure the number of all-visible pages for use in index-only scan costing.

Add a column pg_class.relallvisible to remember the number of pages that
were all-visible according to the visibility map as of the last VACUUM
(or ANALYZE, or some other operations that update pg_class.relpages).
Use relallvisible/relpages, instead of an arbitrary constant, to estimate
how many heap page fetches can be avoided during an index-only scan.

This is pretty primitive and will no doubt see refinements once we've
acquired more field experience with the index-only scan mechanism, but
it's way better than using a constant.

Note: I had to adjust an underspecified query in the window.sql regression
test, because it was changing answers when the plan changed to use an
index-only scan.  Some of the adjacent tests perhaps should be adjusted
as well, but I didn't do that here.

src/backend/commands/analyze.c

@@ -19,6 +19,7 @@
 #include "access/transam.h"
 #include "access/tupconvert.h"
 #include "access/tuptoaster.h"
+#include "access/visibilitymap.h"
 #include "access/xact.h"
 #include "catalog/index.h"
 #include "catalog/indexing.h"
@@ -534,7 +535,10 @@ do_analyze_rel(Relation onerel, VacuumStmt *vacstmt, bool inh)
 	if (!inh)
 		vac_update_relstats(onerel,
 							RelationGetNumberOfBlocks(onerel),
-							totalrows, hasindex, InvalidTransactionId);
+							totalrows,
+							visibilitymap_count(onerel),
+							hasindex,
+							InvalidTransactionId);
 
 	/*
 	 * Same for indexes. Vacuum always scans all indexes, so if we're part of
@@ -551,7 +555,10 @@ do_analyze_rel(Relation onerel, VacuumStmt *vacstmt, bool inh)
 			totalindexrows = ceil(thisdata->tupleFract * totalrows);
 			vac_update_relstats(Irel[ind],
 								RelationGetNumberOfBlocks(Irel[ind]),
-								totalindexrows, false, InvalidTransactionId);
+								totalindexrows,
+								0,
+								false,
+								InvalidTransactionId);
 		}
 	}
 

src/backend/commands/cluster.c

@@ -1205,6 +1205,7 @@ swap_relation_files(Oid r1, Oid r2, bool target_is_pg_class,
 	{
 		int4		swap_pages;
 		float4		swap_tuples;
+		int4		swap_allvisible;
 
 		swap_pages = relform1->relpages;
 		relform1->relpages = relform2->relpages;
@@ -1213,6 +1214,10 @@ swap_relation_files(Oid r1, Oid r2, bool target_is_pg_class,
 		swap_tuples = relform1->reltuples;
 		relform1->reltuples = relform2->reltuples;
 		relform2->reltuples = swap_tuples;
+
+		swap_allvisible = relform1->relallvisible;
+		relform1->relallvisible = relform2->relallvisible;
+		relform2->relallvisible = swap_allvisible;
 	}
 
 	/*

src/backend/commands/vacuum.c

@@ -569,6 +569,7 @@ vac_estimate_reltuples(Relation relation, bool is_analyze,
 void
 vac_update_relstats(Relation relation,
 					BlockNumber num_pages, double num_tuples,
+					BlockNumber num_all_visible_pages,
 					bool hasindex, TransactionId frozenxid)
 {
 	Oid			relid = RelationGetRelid(relation);
@@ -599,6 +600,11 @@ vac_update_relstats(Relation relation,
 		pgcform->reltuples = (float4) num_tuples;
 		dirty = true;
 	}
+	if (pgcform->relallvisible != (int32) num_all_visible_pages)
+	{
+		pgcform->relallvisible = (int32) num_all_visible_pages;
+		dirty = true;
+	}
 	if (pgcform->relhasindex != hasindex)
 	{
 		pgcform->relhasindex = hasindex;

src/backend/commands/vacuumlazy.c

@@ -158,6 +158,7 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
 	TransactionId freezeTableLimit;
 	BlockNumber new_rel_pages;
 	double		new_rel_tuples;
+	BlockNumber new_rel_allvisible;
 	TransactionId new_frozen_xid;
 
 	/* measure elapsed time iff autovacuum logging requires it */
@@ -222,6 +223,10 @@ 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.
 	 *
+	 * 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.
+	 *
 	 * 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.
 	 */
@@ -233,12 +238,18 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
 		new_rel_tuples = vacrelstats->old_rel_tuples;
 	}
 
+	new_rel_allvisible = visibilitymap_count(onerel);
+	if (new_rel_allvisible > new_rel_pages)
+		new_rel_allvisible = new_rel_pages;
+
 	new_frozen_xid = FreezeLimit;
 	if (vacrelstats->scanned_pages < vacrelstats->rel_pages)
 		new_frozen_xid = InvalidTransactionId;
 
 	vac_update_relstats(onerel,
-						new_rel_pages, new_rel_tuples,
+						new_rel_pages,
+						new_rel_tuples,
+						new_rel_allvisible,
 						vacrelstats->hasindex,
 						new_frozen_xid);
 
@@ -1063,8 +1074,11 @@ lazy_cleanup_index(Relation indrel,
 	 */
 	if (!stats->estimated_count)
 		vac_update_relstats(indrel,
-							stats->num_pages, stats->num_index_tuples,
-							false, InvalidTransactionId);
+							stats->num_pages,
+							stats->num_index_tuples,
+							0,
+							false,
+							InvalidTransactionId);
 
 	ereport(elevel,
 			(errmsg("index \"%s\" now contains %.0f row versions in %u pages",