pgsql: Save a few cycles in advance_transition_function().

Save a few cycles in advance_transition_function().

Keep a pre-initialized FunctionCallInfoData in AggStatePerAggData, and
re-use that at each row instead of doing InitFunctionCallInfoData each
time. This saves only half a dozen assignments and maybe some stack
manipulation, and yet that seems to be good for a percent or two of the
overall query run time for simple aggregates such as count(*). The cost
is that the FunctionCallInfoData (which is about a kilobyte, on 64-bit
machines) stays allocated for the duration of the query instead of being
short-lived stack data. But we're already paying an equivalent space cost
for each regular FuncExpr or OpExpr node, so I don't feel bad about paying
it for aggregate functions. The code seems a little cleaner this way too,
since the number of things passed to advance_transition_function decreases.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/e6336b8b5772b9856d65ef967e0b9f748f0f7b0b

Modified Files
--------------
src/backend/executor/nodeAgg.c | 65 +++++++++++++++++++++++-----------------
1 file changed, 38 insertions(+), 27 deletions(-)

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