pgsql: Fix run-time partition pruning for appends with multiplesource

Fix run-time partition pruning for appends with multiple source rels.

The previous coding here supposed that if run-time partitioning applied to
a particular Append/MergeAppend plan, then all child plans of that node
must be members of a single partitioning hierarchy.  This is totally wrong,
since an Append could be formed from a UNION ALL: we could have multiple
hierarchies sharing the same Append, or child plans that aren't part of any
hierarchy.

To fix, restructure the related plan-time and execution-time data
structures so that we can have a separate list or array for each
partitioning hierarchy.  Also track subplans that are not part of any
hierarchy, and make sure they don't get pruned.

Per reports from Phil Florent and others.  Back-patch to v11, since
the bug originated there.

David Rowley, with a lot of cosmetic adjustments by me; thanks also
to Amit Langote for review.

Discussion: https://postgr.es/m/HE1PR03MB17068BB27404C90B5B788BCABA7B0@HE1PR03MB1706.eurprd03.prod.outlook.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/1c2cb2744bf3d8ad751cd5cf3b347f10f48492b3

Modified Files
--------------
src/backend/executor/execPartition.c          | 399 +++++++++++++++-----------
src/backend/executor/nodeAppend.c             |   4 +-
src/backend/executor/nodeMergeAppend.c        |   4 +-
src/backend/nodes/copyfuncs.c                 |  18 +-
src/backend/nodes/outfuncs.c                  |  18 +-
src/backend/nodes/readfuncs.c                 |  17 +-
src/backend/optimizer/path/allpaths.c         |  27 +-
src/backend/optimizer/plan/createplan.c       |  66 +++--
src/backend/optimizer/plan/planner.c          |   1 +
src/backend/partitioning/partprune.c          | 211 +++++++++++---
src/include/executor/execPartition.h          |  68 +++--
src/include/nodes/nodes.h                     |   1 +
src/include/nodes/plannodes.h                 |  54 ++--
src/include/partitioning/partprune.h          |   6 +-
src/test/regress/expected/partition_prune.out | 173 +++++++++++
src/test/regress/sql/partition_prune.sql      |  62 ++++
16 files changed, 842 insertions(+), 287 deletions(-)