pgsql: Fix case of window function + aggregate + GROUP BY expression.

Fix case of window function + aggregate + GROUP BY expression.

In commit 1bc16a946008a7cbb33a9a06a7c6765a807d7f59 I added a minor
optimization to drop the component variables of a GROUP BY expression from
the target list computed at the aggregation level of a query, if those Vars
weren't referenced elsewhere in the tlist. However, I overlooked that the
window-function planning code would deconstruct such expressions and thus
need to have access to their component variables. Fix it to not do that.

While at it, I removed the distinction between volatile and nonvolatile
window partition/order expressions: the code now computes all of them
at the aggregation level. This saves a relatively expensive check for
volatility, and it's unclear that the resulting plan isn't better anyway.

Per bug #7535 from Louis-David Mitterrand. Back-patch to 9.2.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/a20993608a65b9896b4c05cb7061bc60d6f1840a

Modified Files
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src/backend/optimizer/plan/planner.c | 163 +++++++++++++++++++++++++---------
src/test/regress/expected/window.out | 12 +++
src/test/regress/sql/window.sql | 6 ++
3 files changed, 137 insertions(+), 44 deletions(-)