pgsql: Improve make_subplanTargetList to avoid including Vars unnecessa

Improve make_subplanTargetList to avoid including Vars unnecessarily.

If a Var was used only in a GROUP BY expression, the previous
implementation would include the Var by itself (as well as the expression)
in the generated targetlist.  This wouldn't affect the efficiency of the
scan/join part of the plan at all, but it could result in passing
unnecessarily-wide rows through sorting and grouping steps.  It turns out
to take only a little more code, and not noticeably more time, to generate
a tlist without such redundancy, so let's do that.  Per a recent gripe from
HarmeekSingh Bedi.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/1bc16a946008a7cbb33a9a06a7c6765a807d7f59

Modified Files
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src/backend/optimizer/plan/planner.c |  162 +++++++++++++++++++++++-----------
1 files changed, 110 insertions(+), 52 deletions(-)