pgsql: Fix executor prune failure when plan already pruned

Fix executor prune failure when plan already pruned

In a multi-layer partitioning setup, if at plan time all the
sub-partitions are pruned but the intermediate one remains, the executor
later throws a spurious error that there's nothing to prune. That is
correct, but there's no reason to throw an error. Therefore, don't.

Reported-by: Andreas Seltenreich <seltenreich(at)gmx(dot)de>
Author: David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>
Discussion: https://postgr.es/m/87in4h98i0.fsf@ansel.ydns.eu

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1eb9221585c25cad1a563bc3414f697dae3fbc8b

Modified Files
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src/backend/executor/execPartition.c | 9 +++++++--
src/test/regress/expected/partition_prune.out | 18 ++++++++++++++++++
src/test/regress/sql/partition_prune.sql | 12 ++++++++++++
3 files changed, 37 insertions(+), 2 deletions(-)