pgsql: Don't search for volatile expr in find_em_expr_usable_for_sortin

Don't search for volatile expr in find_em_expr_usable_for_sorting_rel

While prepare_sort_from_pathkeys has to be concerned about matching up
a volatile expression to the proper tlist entry, we don't need to do
that in find_em_expr_usable_for_sorting_rel becausee such a sort will
have to be postponed anyway.

Author: James Coleman
Reviewed-by: Tomas Vondra
Backpatch-through: 13
Discussion: https://postgr.es/m/CAAaqYe8cK3g5CfLC4w7bs%3DhC0mSksZC%3DH5M8LSchj5e5OxpTAg%40mail.gmail.com

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/aa97890b6ec2ad07700c6e4825022ae3979ece7f

Modified Files
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src/backend/optimizer/path/equivclass.c | 28 +++++-----------------------
1 file changed, 5 insertions(+), 23 deletions(-)