pgsql: Reuse abbreviated keys in ordered [set] aggregates.

Reuse abbreviated keys in ordered [set] aggregates.

When processing ordered aggregates following a sort that could make use
of the abbreviated key optimization, only call the equality operator to
compare successive pairs of tuples when their abbreviated keys were not
equal.

Peter Geoghegan, reviewd by Andreas Karlsson and by me.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/f1f5ec1efafe74ca45e24e0bf3371b1d6985c8ee

Modified Files
--------------
src/backend/catalog/index.c | 2 +-
src/backend/executor/nodeAgg.c | 16 ++++++--
src/backend/executor/nodeSort.c | 2 +-
src/backend/utils/adt/orderedsetaggs.c | 33 ++++++++++-----
src/backend/utils/sort/tuplesort.c | 74 ++++++++++++++++++++++++----------
src/include/utils/tuplesort.h | 4 +-
6 files changed, 92 insertions(+), 39 deletions(-)