postgres_fdw: Fix costing of pre-sorted foreign paths with local stats.

Commit aa09cd242 modified estimate_path_cost_size() so that it reuses
cached costs of a basic foreign path for a given foreign-base/join
relation when costing pre-sorted foreign paths for that relation, but it
incorrectly re-computed retrieved_rows, an estimated number of rows
fetched from the remote side, which is needed for costing both the basic
and pre-sorted foreign paths.  To fix, handle retrieved_rows the same way
as the cached costs: store in that relation's fpinfo the retrieved_rows
estimate computed for costing the basic foreign path, and reuse it when
costing the pre-sorted foreign paths.  Also, reuse the rows/width
estimates stored in that relation's fpinfo when costing the pre-sorted
foreign paths, to make the code consistent.

In commit ffab494a4, to extend the costing mentioned above to the
foreign-grouping case, I made a change to add_foreign_grouping_paths() to
store in a given foreign-grouped relation's RelOptInfo the rows estimate
for that relation for reuse, but this patch makes that change unnecessary
since we already store the row estimate in that relation's fpinfo, which
this patch reuses when costing a foreign path for that relation with the
sortClause ordering; remove that change.

In passing, fix thinko in commit 7012b132d: in estimate_path_cost_size(),
the width estimate for a given foreign-grouped relation to be stored in
that relation's fpinfo was reset incorrectly when costing a basic foreign
path for that relation with local stats.

Apply the patch to HEAD only to avoid destabilizing existing plan choices.

Author: Etsuro Fujita
Discussion: https://postgr.es/m/CAPmGK17jaJLPDEkgnP2VmkOg=5wT8YQ1CqssU8JRpZ_NSE+dqQ@mail.gmail.com

1 files changed, 7 insertions, 2 deletions

diff --git a/contrib/postgres_fdw/postgres_fdw.h b/contrib/postgres_fdw/postgres_fdw.h
index 30b6502bc05..35545e1831d 100644
--- a/contrib/postgres_fdw/postgres_fdw.h
+++ b/contrib/postgres_fdw/postgres_fdw.h
@@ -59,12 +59,17 @@ typedef struct PgFdwRelationInfo
 	/* Selectivity of join conditions */
 	Selectivity joinclause_sel;
 
-	/* Estimated size and cost for a scan or join. */
+	/* Estimated size and cost for a scan, join, or grouping/aggregation. */
 	double		rows;
 	int			width;
 	Cost		startup_cost;
 	Cost		total_cost;
-	/* Costs excluding costs for transferring data from the foreign server */
+	/*
+	 * Estimated number of rows fetched from the foreign server, and costs
+	 * excluding costs for transferring those rows from the foreign server.
+	 * These are only used by estimate_path_cost_size().
+	 */
+	double		retrieved_rows;
 	Cost		rel_startup_cost;
 	Cost		rel_total_cost;