pgsql: Introduce custom path and scan providers.

Introduce custom path and scan providers.

This allows extension modules to define their own methods for
scanning a relation, and get the core code to use them.  It's
unclear as yet how much use this capability will find, but we
won't find out if we never commit it.

KaiGai Kohei, reviewed at various times and in various levels
of detail by Shigeru Hanada, Tom Lane, Andres Freund, Álvaro
Herrera, and myself.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/0b03e5951bf0a1a8868db13f02049cf686a82165

Modified Files
--------------
src/backend/commands/explain.c          |   25 ++++++
src/backend/executor/Makefile           |    2 +-
src/backend/executor/execAmi.c          |   38 +++++++++-
src/backend/executor/execProcnode.c     |   14 ++++
src/backend/executor/nodeCustom.c       |  126 +++++++++++++++++++++++++++++++
src/backend/nodes/copyfuncs.c           |   26 +++++++
src/backend/nodes/outfuncs.c            |   29 +++++++
src/backend/optimizer/path/allpaths.c   |    3 +
src/backend/optimizer/path/costsize.c   |    2 +-
src/backend/optimizer/plan/createplan.c |   60 ++++++++++++++-
src/backend/optimizer/plan/setrefs.c    |   26 ++++++-
src/backend/optimizer/plan/subselect.c  |   21 ++++++
src/backend/optimizer/util/pathnode.c   |   47 ++++++++++++
src/backend/utils/adt/ruleutils.c       |   70 +++++++++++++++++
src/include/executor/executor.h         |    3 +-
src/include/executor/nodeCustom.h       |   30 ++++++++
src/include/nodes/execnodes.h           |   40 ++++++++++
src/include/nodes/nodes.h               |    3 +
src/include/nodes/plannodes.h           |   29 +++++++
src/include/nodes/relation.h            |   42 +++++++++++
src/include/optimizer/pathnode.h        |    9 +++
src/include/optimizer/planmain.h        |    3 +
22 files changed, 638 insertions(+), 10 deletions(-)