pgsql: Fix bug in cbc127917 to handle nested Append correctly

Fix bug in cbc127917 to handle nested Append correctly

A non-leaf partition with a subplan that is an Append node was
omitted from PlannedStmt.unprunableRelids because it was mistakenly
included in PlannerGlobal.prunableRelids due to the way
PartitionedRelPruneInfo.leafpart_rti_map[] is constructed. This
happened when a non-leaf partition used an unflattened Append or
MergeAppend. As a result, ExecGetRangeTableRelation() reported an
error when called from CreatePartitionPruneState() to process the
partition's own PartitionPruneInfo, since it was treated as prunable
when it should not have been.

Reported-by: Alexander Lakhin <exclusion(at)gmail(dot)com> (via sqlsmith)
Diagnosed-by: Tender Wang <tndrwang(at)gmail(dot)com>
Reviewed-by: Tender Wang <tndrwang(at)gmail(dot)com>
Discussion: https://postgr.es/m/74839af6-aadc-4f60-ae77-ae65f94bf607@gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/cbb9086c9ef64b020cb9036f50afc14644bbf734

Modified Files
--------------
src/backend/executor/execPartition.c | 13 +++++---
src/backend/partitioning/partprune.c | 9 ++++-
src/test/regress/expected/partition_prune.out | 48 +++++++++++++++++++++++++++
src/test/regress/sql/partition_prune.sql | 12 +++++++
4 files changed, 77 insertions(+), 5 deletions(-)