pgsql: Speed up SUM calculation in numeric aggregates.

Speed up SUM calculation in numeric aggregates.

This introduces a numeric sum accumulator, which performs better than
repeatedly calling add_var(). The performance comes from using wider digits
and delaying carry propagation, tallying positive and negative values
separately, and avoiding a round of palloc/pfree on every value. This
speeds up SUM(), as well as other standard aggregates like AVG() and
STDDEV() that also calculate a sum internally.

Reviewed-by: Andrey Borodin
Discussion: 

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/9cca11c915e458323d0e746c68203f2c11da0302

Modified Files
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src/backend/utils/adt/numeric.c       | 601 ++++++++++++++++++++++++++++------
src/test/regress/expected/numeric.out |  16 +
src/test/regress/sql/numeric.sql      |   8 +
3 files changed, 522 insertions(+), 103 deletions(-)