Postpone calculating total_table_pages until after pruning/exclusion.

The planner doesn't make any use of root->total_table_pages until it
estimates costs of indexscans, so we don't need to compute it as
early as that's currently done.  By doing the calculation between
set_base_rel_sizes and set_base_rel_pathlists, we can omit relations
that get removed from the query by partition pruning or constraint
exclusion, which seems like a more accurate basis for costing.

(Historical note: I think at the time this code was written, there
was not a separation between the "set sizes" and "set pathlists"
steps, so that this approach would have been impossible at the time.
But now that we have that separation, this is clearly the better way
to do things.)

David Rowley, reviewed by Edmund Horner

Discussion: https://postgr.es/m/CAKJS1f-NG1mRM0VOtkAG7=ZLQWihoqees9R4ki3CKBB0-fRfCA@mail.gmail.com

1 files changed, 0 insertions, 30 deletions

diff --git a/src/backend/optimizer/plan/planmain.c b/src/backend/optimizer/plan/planmain.c
index b05adc70c4a..9b6cc9e10f0 100644
--- a/src/backend/optimizer/plan/planmain.c
+++ b/src/backend/optimizer/plan/planmain.c
@@ -57,8 +57,6 @@ query_planner(PlannerInfo *root, List *tlist,
 	Query	   *parse = root->parse;
 	List	   *joinlist;
 	RelOptInfo *final_rel;
-	Index		rti;
-	double		total_pages;
 
 	/*
 	 * If the query has an empty join tree, then it's something easy like
@@ -232,34 +230,6 @@ query_planner(PlannerInfo *root, List *tlist,
 	extract_restriction_or_clauses(root);
 
 	/*
-	 * We should now have size estimates for every actual table involved in
-	 * the query, and we also know which if any have been deleted from the
-	 * query by join removal; so we can compute total_table_pages.
-	 *
-	 * Note that appendrels are not double-counted here, even though we don't
-	 * bother to distinguish RelOptInfos for appendrel parents, because the
-	 * parents will still have size zero.
-	 *
-	 * XXX if a table is self-joined, we will count it once per appearance,
-	 * which perhaps is the wrong thing ... but that's not completely clear,
-	 * and detecting self-joins here is difficult, so ignore it for now.
-	 */
-	total_pages = 0;
-	for (rti = 1; rti < root->simple_rel_array_size; rti++)
-	{
-		RelOptInfo *brel = root->simple_rel_array[rti];
-
-		if (brel == NULL)
-			continue;
-
-		Assert(brel->relid == rti); /* sanity check on array */
-
-		if (IS_SIMPLE_REL(brel))
-			total_pages += (double) brel->pages;
-	}
-	root->total_table_pages = total_pages;
-
-	/*
 	 * Ready to do the primary planning.
 	 */
 	final_rel = make_one_rel(root, joinlist);