Revert ill-considered change of index-size fudge factor.

tglsfdc · tglsfdc · commit 3e9960e9d935 · 2013-01-11T13:08:19.000-05:00
This partially reverts commit 21a39de, restoring the pre-9.2 cost estimates for index usage. That change introduced much too large a bias against larger indexes, as per reports from Jeff Janes and others. The whole thing needs a rewrite, which I've done in HEAD, but the safest thing to do in 9.2 is just to undo this multiplier change.
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
@@ -6130,11 +6130,11 @@ genericcostestimate(PlannerInfo *root,
 	 *
 	 * 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
-	 * spc_random_page_cost per 10000 index pages, which should be small
+	 * spc_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 * spc_random_page_cost / 10000.0;
+	*indexTotalCost += index->pages * spc_random_page_cost / 100000.0;
 
 	/*
 	 * CPU cost: any complex expressions in the indexquals will need to be
