Restore preprocess_groupclause()

0452b461bc made optimizer explore alternative orderings of group-by pathkeys.
It eliminated preprocess_groupclause(), which was intended to match items
between GROUP BY and ORDER BY.  Instead, get_useful_group_keys_orderings()
function generates orderings of GROUP BY elements at the time of grouping
paths generation.  The get_useful_group_keys_orderings() function takes into
account 3 orderings of GROUP BY pathkeys and clauses: original order as written
in GROUP BY, matching ORDER BY clauses as much as possible, and matching the
input path as much as possible.  Given that even before 0452b461b,
preprocess_groupclause() could change the original order of GROUP BY clauses
we don't need to consider it apart from ordering matching ORDER BY clauses.

This commit restores preprocess_groupclause() to provide an ordering of
GROUP BY elements matching ORDER BY before generation of paths.  The new
version of preprocess_groupclause() takes into account an incremental sort.
The get_useful_group_keys_orderings() function now takes into 2 orderings of
GROUP BY elements: the order generated preprocess_groupclause() and the order
matching the input path as much as possible.

Discussion: https://postgr.es/m/CAPpHfdvyWLMGwvxaf%3D7KAp-z-4mxbSH8ti2f6mNOQv5metZFzg%40mail.gmail.com
Author: Alexander Korotkov
Reviewed-by: Andrei Lepikhov, Pavel Borisov

1 files changed, 5 insertions, 1 deletions

diff --git a/src/include/nodes/pathnodes.h b/src/include/nodes/pathnodes.h
index 78489398294..2ba297c1172 100644
--- a/src/include/nodes/pathnodes.h
+++ b/src/include/nodes/pathnodes.h
@@ -426,7 +426,11 @@ struct PlannerInfo
 	 * items to be proven redundant, implying that there is only one group
 	 * containing all the query's rows.  Hence, if you want to check whether
 	 * GROUP BY was specified, test for nonempty parse->groupClause, not for
-	 * nonempty processed_groupClause.
+	 * nonempty processed_groupClause.  Optimizer chooses specific order of
+	 * group-by clauses during the upper paths generation process, attempting
+	 * to use different strategies to minimize number of sorts or engage
+	 * incremental sort.  See preprocess_groupclause() and
+	 * get_useful_group_keys_orderings() for details.
 	 *
 	 * Currently, when grouping sets are specified we do not attempt to
 	 * optimize the groupClause, so that processed_groupClause will be