Reject degenerate SPLIT PARTITION with DEFAULT partition

Reject degenerate SPLIT PARTITION with DEFAULT partition

ALTER TABLE ... SPLIT PARTITION allows a DEFAULT partition to be created
as one of the replacement partitions when the parent table does not
already have one.  However, it should not allow the degenerate case where
a non-DEFAULT partition keeps exactly the same bound as the split
partition and the command merely adds a DEFAULT partition through the
SPLIT PARTITION path.

Detect that case by comparing the bound of the split partition with the
bound of the only non-DEFAULT replacement partition, and raise an error
when they are the same.  Users should add a DEFAULT partition directly
with CREATE TABLE ... PARTITION OF ... DEFAULT or ALTER TABLE ... ATTACH
PARTITION ... DEFAULT instead.

The comparison goes through the partition operator family rather than
byte equality so that values which are binary-different but compare
equal under the partition key's comparator are treated as the same
bound.  The corresponding regression test uses a float8 LIST partition
with -0.0 and 0.0 -- they have different bit patterns but are equal
under float8 -- to verify that a datumIsEqual()-based check would let
the degenerate split through while the partsupfunc-based check
correctly rejects it.

Author: Chao Li <lic@highgo.com>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>
Discussion: https://postgr.es/m/C18878AB-DEB2-4A61-9995-A035DD644B81@gmail.com