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);
 		}
 	}