pgsql: Perform one only projection to compute agg arguments.

Perform one only projection to compute agg arguments.

Previously we did a ExecProject() for each individual aggregate
argument. That turned out to be a performance bottleneck in queries with
multiple aggregates.

Doing all the argument computations in one ExecProject() is quite a bit
cheaper because ExecProject's fastpath can do the work at once in a
relatively tight loop, and because it can get all the required columns
with a single slot_getsomeattr and save some other redundant setup
costs.

Author: Andres Freund
Reviewed-By: Heikki Linnakangas
Discussion: /messages/by-id/20161103110721.h5i5t5saxfk5eeik@alap3.anarazel.de

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/8ed3f11bb045ad7a3607690be668dbd5b3cc31d7

Modified Files
--------------
src/backend/executor/nodeAgg.c | 167 ++++++++++++++++++++++++++++-------------
src/include/nodes/execnodes.h | 4 +
2 files changed, 119 insertions(+), 52 deletions(-)

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