pgsql: In array_agg(), don't create a new context for every group.

In array_agg(), don't create a new context for every group.

Previously, each new array created a new memory context that started
out at 8kB. This is incredibly wasteful when there are lots of small
groups of just a few elements each.

Change initArrayResult() and friends to accept a "subcontext" argument
to indicate whether the caller wants the ArrayBuildState allocated in
a new subcontext or not. If not, it can no longer be released
separately from the rest of the memory context.

Fixes bug report by Frank van Vugt on 2013-10-19.

Tomas Vondra. Reviewed by Ali Akbar, Tom Lane, and me.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/b419865a814abbca12bdd6eef6a3d5ed67f432e1

Modified Files
--------------
src/backend/executor/nodeSubplan.c      |    4 +-
src/backend/utils/adt/array_userfuncs.c |   14 +++-
src/backend/utils/adt/arrayfuncs.c      |  106 +++++++++++++++++++++----------
src/backend/utils/adt/xml.c             |    2 +-
src/include/utils/array.h               |    8 ++-
src/pl/plperl/plperl.c                  |    2 +-
6 files changed, 92 insertions(+), 44 deletions(-)