select_common_typmod

While working on another patch, I figured adding a
select_common_typmod() to go along with select_common_type() and
select_common_collation() would be handy. Typmods were previously
combined using hand-coded logic in several places, and not at all in
other places. The logic in select_common_typmod() isn't very exciting,
but it makes the code more compact and readable in a few locations, and
in the future we can perhaps do more complicated things if desired.

There might have been a tiny bug in transformValuesClause() because
while consolidating the typmods it does not take into account whether
the types are actually the same (as more correctly done in
transformSetOperationTree() and buildMergedJoinVar()).

--
Peter Eisentraut http://www.2ndQuadrant.com/
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