pgsql: Fix asymmetry in setting EquivalenceClass.ec_sortref

Fix asymmetry in setting EquivalenceClass.ec_sortref

0452b461bc made get_eclass_for_sort_expr() always set
EquivalenceClass.ec_sortref if it's not done yet. This leads to an asymmetric
situation when whoever first looks for the EquivalenceClass sets the
ec_sortref. It is also counterintuitive that get_eclass_for_sort_expr()
performs modification of data structures.

This commit makes make_pathkeys_for_sortclauses_extended() responsible for
setting EquivalenceClass.ec_sortref. Now we set the
EquivalenceClass.ec_sortref's needed to explore alternative GROUP BY ordering
specifically during building pathkeys by the list of grouping clauses.

Discussion: https://postgr.es/m/17037754-f187-4138-8285-0e2bfebd0dea%40postgrespro.ru
Reported-by: Tom Lane
Author: Andrei Lepikhov
Reviewed-by: Alexander Korotkov, Pavel Borisov

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/199012a3d844c6283e0ab4b1139440840a91433d

Modified Files
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src/backend/optimizer/path/equivclass.c | 13 +--------
src/backend/optimizer/path/pathkeys.c | 18 ++++++++++--
src/backend/optimizer/plan/planner.c | 16 ++++++++---
src/include/optimizer/paths.h | 3 +-
src/test/regress/expected/aggregates.out | 47 ++++++++++++++++++++++++++++++++
src/test/regress/sql/aggregates.sql | 14 ++++++++++
6 files changed, 92 insertions(+), 19 deletions(-)