Fix brain fade in cost estimation for index-only scans.

visibility_fraction should not be applied to regular indexscans.
Noted by Cédric Villemain.

src/backend/optimizer/path/costsize.c

@@ -264,6 +264,7 @@ cost_index(IndexPath *path, PlannerInfo *root,
 
 	if (!enable_indexscan)
 		startup_cost += disable_cost;
+	/* we don't need to check enable_indexonlyscan; indxpath.c does that */
 
 	/*
 	 * Call index-access-method-specific code to estimate the processing cost
@@ -345,6 +346,7 @@ cost_index(IndexPath *path, PlannerInfo *root,
 											(double) index->pages,
 											root);
 
+		if (indexonly)
 			pages_fetched = ceil(pages_fetched * visibility_fraction);
 
 		max_IO_cost = (pages_fetched * spc_random_page_cost) / num_scans;
@@ -366,6 +368,7 @@ cost_index(IndexPath *path, PlannerInfo *root,
 											(double) index->pages,
 											root);
 
+		if (indexonly)
 			pages_fetched = ceil(pages_fetched * visibility_fraction);
 
 		min_IO_cost = (pages_fetched * spc_random_page_cost) / num_scans;
@@ -381,6 +384,7 @@ cost_index(IndexPath *path, PlannerInfo *root,
 											(double) index->pages,
 											root);
 
+		if (indexonly)
 			pages_fetched = ceil(pages_fetched * visibility_fraction);
 
 		/* max_IO_cost is for the perfectly uncorrelated case (csquared=0) */
@@ -389,6 +393,7 @@ cost_index(IndexPath *path, PlannerInfo *root,
 		/* min_IO_cost is for the perfectly correlated case (csquared=1) */
 		pages_fetched = ceil(indexSelectivity * (double) baserel->pages);
 
+		if (indexonly)
 			pages_fetched = ceil(pages_fetched * visibility_fraction);
 
 		min_IO_cost = spc_random_page_cost;