pgsql: Fix pruning of DEFAULT partition in RANGE partitioned tables

Fix pruning of DEFAULT partition in RANGE partitioned tables

Some code added in 489247b0e tried to prune the DEFAULT partition when
the next partition had a MINVALUE clause and likewise when the final
partition to scan had a MAXVALUE clause for the final partition key in
the pruning step. This code was incorrect as it could prune the default
partition incorrectly in cases such as:

p: PARTITION BY RANGE (a, b)
p1: FOR VALUES FROM (13, 0) TO (19, MAXVALUE)
pd: DEFAULT

SELECT * FROM t WHERE a = 32 AND b >= -7;

Here the pruning step for a = 32 and b >= -7 would see that only the
default partition needs to be scan, but it would then see that the
partition prior to the default had a MAXVALUE bound then prune away the
default thinking that it needn't be scanned. This could result in
incorrect results.

Fix this by moving the code that looks for the MAXVALUE bound into the
code handling BTEqualStrategyNumber so that when we're pruning with a
prefix of the partition keys, we check if the bound for the offset we've
calculated lands on a partition where the next partition key is bounded
with MAXVALUE. If so we don't include the default partition.

This leaves only the case of the first partition key. We handle that by
modifying the existing code that was checking the last key covered by
the given values.

Author: Ewan Young <kdbase(dot)hack(at)gmail(dot)com>
Reviewed-by: Tender Wang <tndrwang(at)gmail(dot)com>
Reviewed-by: David Rowley <dgrowleyml(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAON2xHO=sqdqp=z8zWkybnWp0AuvefnAi2ez2vOYWrhXB6hHWQ@mail.gmail.com
Backpatch-through: 14

Branch
------
REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/63d9e26c27b9965a78f0d59512a80d84935a7c4b

Modified Files
--------------
src/backend/partitioning/partprune.c | 62 ++++++++++--------
src/test/regress/expected/partition_prune.out | 94 +++++++++++++++++++++++++++
src/test/regress/sql/partition_prune.sql | 47 ++++++++++++++
3 files changed, 177 insertions(+), 26 deletions(-)