pgsql: Add combine functions for various floating-point aggregates.

Add combine functions for various floating-point aggregates.

This allows parallel aggregation to use them. It may seem surprising
that we use float8_combine for both float4_accum and float8_accum
transition functions, but that's because those functions differ only
in the type of the non-transition-state argument.

Haribabu Kommi, reviewed by David Rowley and Tomas Vondra

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/af025eed536d3842d085ed9e4f9107eb976575cc

Modified Files
--------------
doc/src/sgml/func.sgml | 40 +++++++-------
src/backend/utils/adt/float.c | 107 +++++++++++++++++++++++++++++++++++++
src/include/catalog/pg_aggregate.h | 52 +++++++++---------
src/include/catalog/pg_proc.h | 4 ++
src/include/utils/builtins.h | 2 +
5 files changed, 159 insertions(+), 46 deletions(-)