Add a fudge factor to genericcostestimate() to prevent the planner from

tglsfdc · tglsfdc · commit 8dcaea7be0a5 · 2006-07-24T01:19:48.000Z
thinking that indexes of different sizes are equally attractive.  Per
gripe from Jim Nasby.  (I remain unconvinced that there's such a problem
in existing releases, but CVS HEAD definitely has got a problem because
of its new count-only-leaf-pages approach to indexscan costing.)
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
@@ -15,7 +15,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/adt/selfuncs.c,v 1.209 2006/07/14 14:52:24 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/adt/selfuncs.c,v 1.210 2006/07/24 01:19:48 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -4657,6 +4657,24 @@ genericcostestimate(PlannerInfo *root,
 		*indexTotalCost = numIndexPages * random_page_cost;
 	}
 
+	/*
+	 * A difficulty with the leaf-pages-only cost approach is that for
+	 * small selectivities (eg, single index tuple fetched) all indexes
+	 * will look equally attractive because we will estimate exactly 1
+	 * leaf page to be fetched.  All else being equal, we should prefer
+	 * physically smaller indexes over larger ones.  (An index might be
+	 * smaller because it is partial or because it contains fewer columns;
+	 * presumably the other columns in the larger index aren't useful to
+	 * the query, or the larger index would have better selectivity.)
+	 *
+	 * We can deal with this by adding a very small "fudge factor" that
+	 * depends on the index size.  The fudge factor used here is one
+	 * random_page_cost per 100000 index pages, which should be small
+	 * enough to not alter index-vs-seqscan decisions, but will prevent
+	 * indexes of different sizes from looking exactly equally attractive.
+	 */
+	*indexTotalCost += index->pages * random_page_cost / 100000.0;
+
 	/*
 	 * CPU cost: any complex expressions in the indexquals will need to be
 	 * evaluated once at the start of the scan to reduce them to runtime keys
