postgres_fdw: oddity in costing aggregate pushdown paths

From: Etsuro Fujita <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp>
To: Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: postgres_fdw: oddity in costing aggregate pushdown paths
Date: 2018-11-27 12:55:09
Message-ID: 5BFD3EAD.2060301@lab.ntt.co.jp
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Hi,

While working on [1], I noticed that since we don't set the selectivity
and cost of the local_conds (i.e., fpinfo->local_conds_sel and
fpinfo->local_conds_cost) properly in add_foreign_grouping_paths and
foreign_grouping_ok, estimate_path_cost_size produces considerably
underestimated results when use_remote_estimate. I think this would
cause problems in later planning steps. So, why not add something like
this to add_foreign_grouping_paths, as done in other functions such as
postgresGetForeignJopinPaths?

--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -5496,6 +5496,19 @@ add_foreign_grouping_paths(PlannerInfo *root,
RelOptInfo\
*input_rel,
if (!foreign_grouping_ok(root, grouped_rel, extra->havingQual))
return;

+ /*
+ * Compute the selectivity and cost of the local_conds, so we don't have
+ * to do it over again for each path. The best we can do for these
+ * conditions is to estimate selectivity on the basis of local
statistics.
+ */
+ fpinfo->local_conds_sel = clauselist_selectivity(root,
+ fpinfo->local_conds,
+ 0,
+ JOIN_INNER,
+ NULL);
+
+ cost_qual_eval(&fpinfo->local_conds_cost, fpinfo->local_conds, root);
+
/* Estimate the cost of push down */
estimate_path_cost_size(root, grouped_rel, NIL, NIL, &rows,
&width, &startup_cost, &total_cost);

Best regards,
Etsuro Fujita

[1] https://www.postgresql.org/message-id/5BBC4140.50403%40lab.ntt.co.jp

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