Fix planner error with multiple copies of an AlternativeSubPlan

Enterprise / PostgreSQL - Tom Lane [sss.pgh.pa.us] - 14 September 2021 19:11 UTC

It's possible for us to copy an AlternativeSubPlan expression node into multiple places, for example the scan quals of several partition children. Then it's possible that we choose a different one of the alternatives as optimal in each place. Commit 41efb8340 failed to consider this scenario, so its attempt to remove "unused" subplans could remove subplans that were still used elsewhere.

Fix by delaying the removal logic until we've examined all the AlternativeSubPlans in a given query level. (This does assume that AlternativeSubPlans couldn't get copied to other query levels, but for the foreseeable future that's fine; cf qual_is_pushdown_safe.)

Per report from Rajkumar Raghuwanshi. Back-patch to v14 where the faulty logic came in.

Discussion: https://postgr.es/m/CAKcux6==O3NNZC3bZ2prRYv3cjm3_Zw1GfzmOjEVqYN4jub2+Q@mail.gmail.com

e8638d78a2 Fix planner error with multiple copies of an AlternativeSubPlan.
src/backend/optimizer/plan/setrefs.c | 71 ++++++++++++++++++++++-----------
src/include/nodes/pathnodes.h | 4 ++
src/test/regress/expected/subselect.out | 40 +++++++++++++++++++
src/test/regress/sql/subselect.sql | 12 ++++++
4 files changed, 103 insertions(+), 24 deletions(-)

Upstream: git.postgresql.org


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