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.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/backend/utils/cache/relcache.c b/src/backend/utils/cache/relcache.c
index 9f6b12707bf..603e4c1b621 100644
--- a/src/backend/utils/cache/relcache.c
+++ b/src/backend/utils/cache/relcache.c
@@ -1414,6 +1414,7 @@ formrdesc(const char *relationName, Oid relationReltype,
 
 	relation->rd_rel->relpages = 0;
 	relation->rd_rel->reltuples = 0;
+	relation->rd_rel->relallvisible = 0;
 	relation->rd_rel->relkind = RELKIND_RELATION;
 	relation->rd_rel->relhasoids = hasoids;
 	relation->rd_rel->relnatts = (int16) natts;
@@ -2668,6 +2669,7 @@ RelationSetNewRelfilenode(Relation relation, TransactionId freezeXid)
 	{
 		classform->relpages = 0;	/* it's empty until further notice */
 		classform->reltuples = 0;
+		classform->relallvisible = 0;
 	}
 	classform->relfrozenxid = freezeXid;