parse_clause.c

any implicit casting previously applied to the targetlist item.  This is
reasonable because the implicit cast, by definition, wasn't written by the
user; so we are preserving the expected behavior that ORDER BY items match
textually equivalent tlist items.  The case never arose before because there
couldn't be any implicit casting of a top-level SELECT item before we process
ORDER BY etc.  But now it can arise in the context of aggregates containing
ORDER BY clauses, since the "targetlist" is the already-casted list of
arguments for the aggregate.  The net effect is that the datatype used for
ORDER BY/DISTINCT purposes is the aggregate's declared input type, not that
of the original input column; which is a bit debatable but not horrendous,
and to do otherwise would require major rework that doesn't seem justified.

Per bug #5564 from Daniel Grace.  Back-patch to 9.0 where aggregate ORDER BY
was implemented.