pgsql: Split some code out from MergeAttributes()

Split some code out from MergeAttributes()

- Separate function to merge a child attribute into matching inherited
attribute: The logic to merge a child attribute into matching
inherited attribute in MergeAttribute() is only applicable to
regular inheritance child. The code is isolated and coherent enough
that it can be separated into a function of its own.

- Separate function to merge next parent attribute: Partitions inherit
from only a single parent. The logic to merge an attribute from the
next parent into the corresponding attribute inherited from previous
parents in MergeAttribute() is only applicable to regular
inheritance children. This code is isolated enough that it can be
separate into a function by itself.

These separations makes MergeAttribute() more readable by making it
easier to follow high level logic without getting entangled into
details.

Author: Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/flat/52a125e4-ff9a-95f5-9f61-b87cf447e4da(at)eisentraut(dot)org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f7cf9494bad3aef1b2ba1cd84376a1e71797ac50

Modified Files
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src/backend/commands/tablecmds.c | 536 +++++++++++++++++++++------------------
1 file changed, 296 insertions(+), 240 deletions(-)