pgsql: Fix get_useful_pathkeys_for_relation for volatile expressions

Fix get_useful_pathkeys_for_relation for volatile expressions

When considering Incremental Sort below a Gather Merge, we need to be
a bit more careful when matching pathkeys to EC members. It's not enough
to find a member whose Vars are all in the current relation's target;
volatile expressions in particular need to be contained in the target,
otherwise it's too early to use the pathkey.

Reported-by: Jaime Casanova
Author: James Coleman
Reviewed-by: Tomas Vondra
Backpatch-through: 13, where the incremental sort code was added
Discussion: https://postgr.es/m/CAJGNTeNaxpXgBVcRhJX%2B2vSbq%2BF2kJqGBcvompmpvXb7pq%2BoFA%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ebb7ae839d033d0f279670e249f54646a08b8c48

Modified Files
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src/backend/optimizer/path/allpaths.c | 13 ++--
src/backend/optimizer/path/equivclass.c | 70 ++++++++++++++++++
src/include/optimizer/paths.h | 1 +
src/test/regress/expected/incremental_sort.out | 98 ++++++++++++++++++++++++++
src/test/regress/sql/incremental_sort.sql | 31 ++++++++
5 files changed, 207 insertions(+), 6 deletions(-)