pgsql: Show sort ordering options in EXPLAIN output.

Show sort ordering options in EXPLAIN output.

Up to now, EXPLAIN has contented itself with printing the sort expressions
in a Sort or Merge Append plan node. This patch improves that by
annotating the sort keys with COLLATE, DESC, USING, and/or NULLS FIRST/LAST
whenever nondefault sort ordering options are used. The output is now a
reasonably close approximation of an ORDER BY clause equivalent to the
plan's ordering.

Marius Timmer, Lukas Kreft, and Arne Scheffer; reviewed by Mike Blackwell.
Some additional hacking by me.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/20af53d7191f84d0f5b86da4362e481b7e85d52a

Modified Files
--------------
src/backend/commands/explain.c | 90 +++++++++++++++++++++++++++++-
src/test/regress/expected/aggregates.out | 6 +-
src/test/regress/expected/collate.out | 19 +++++++
src/test/regress/expected/equivclass.out | 4 +-
src/test/regress/sql/collate.sql | 9 +++
5 files changed, 120 insertions(+), 8 deletions(-)