pgsql: Permit super-MaxAllocSize allocations with MemoryContextAllocHug

Permit super-MaxAllocSize allocations with MemoryContextAllocHuge().

The MaxAllocSize guard is convenient for most callers, because it
reduces the need for careful attention to overflow, data type selection,
and the SET_VARSIZE() limit. A handful of callers are happy to navigate
those hazards in exchange for the ability to allocate a larger chunk.
Introduce MemoryContextAllocHuge() and repalloc_huge(). Use this in
tuplesort.c and tuplestore.c, enabling internal sorts of up to INT_MAX
tuples, a factor-of-48 increase. In particular, B-tree index builds can
now benefit from much-larger maintenance_work_mem settings.

Reviewed by Stephen Frost, Simon Riggs and Jeff Janes.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/263865a48973767ce8ed7b7788059a38a24a9f37

Modified Files
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src/backend/utils/mmgr/aset.c | 5 ++
src/backend/utils/mmgr/mcxt.c | 74 +++++++++++++++++++++++++---
src/backend/utils/sort/tuplesort.c | 91 +++++++++++++++++++----------------
src/backend/utils/sort/tuplestore.c | 77 +++++++++++++++--------------
src/include/utils/memutils.h | 28 ++++++-----
src/include/utils/palloc.h | 4 ++
src/include/utils/tuplesort.h | 2 +-
7 files changed, 183 insertions(+), 98 deletions(-)