pgsql: Rearrange the implementation of index-only scans.

Rearrange the implementation of index-only scans.

This commit changes index-only scans so that data is read directly from the
index tuple without first generating a faux heap tuple.  The only immediate
benefit is that indexes on system columns (such as OID) can be used in
index-only scans, but this is necessary infrastructure if we are ever to
support index-only scans on expression indexes.  The executor is now ready
for that, though the planner still needs substantial work to recognize
the possibility.

To do this, Vars in index-only plan nodes have to refer to index columns
not heap columns.  I introduced a new special varno, INDEX_VAR, to mark
such Vars to avoid confusion.  (In passing, this commit renames the two
existing special varnos to OUTER_VAR and INNER_VAR.)  This allows
ruleutils.c to handle them with logic similar to what we use for subplan
reference Vars.

Since index-only scans are now fundamentally different from regular
indexscans so far as their expression subtrees are concerned, I also chose
to change them to have their own plan node type (and hence, their own
executor source file).

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/a0185461dd94c8d31d8d55a7f2839b0d2f172ab9

Modified Files
--------------
src/backend/commands/explain.c             |  119 ++++--
src/backend/commands/trigger.c             |    8 +-
src/backend/executor/Makefile              |    3 +-
src/backend/executor/execAmi.c             |   21 +-
src/backend/executor/execCurrent.c         |    1 +
src/backend/executor/execProcnode.c        |   14 +
src/backend/executor/execQual.c            |   24 +-
src/backend/executor/execScan.c            |    9 +-
src/backend/executor/execUtils.c           |   12 +-
src/backend/executor/nodeAgg.c             |    4 +-
src/backend/executor/nodeBitmapIndexscan.c |    1 -
src/backend/executor/nodeHash.c            |    4 +-
src/backend/executor/nodeIndexonlyscan.c   |  542 ++++++++++++++++++++++++++++
src/backend/executor/nodeIndexscan.c       |  142 +-------
src/backend/executor/nodeNestloop.c        |    4 +-
src/backend/nodes/copyfuncs.c              |   29 ++-
src/backend/nodes/outfuncs.c               |   22 +-
src/backend/nodes/print.c                  |    8 +-
src/backend/optimizer/path/indxpath.c      |   23 +-
src/backend/optimizer/path/pathkeys.c      |   68 +---
src/backend/optimizer/plan/createplan.c    |  138 +++++--
src/backend/optimizer/plan/setrefs.c       |  138 ++++++--
src/backend/optimizer/plan/subselect.c     |   12 +
src/backend/optimizer/util/pathnode.c      |    3 +-
src/backend/optimizer/util/plancat.c       |   71 ++++
src/backend/utils/adt/ruleutils.c          |  159 ++++++---
src/backend/utils/adt/tid.c                |    2 +-
src/include/executor/nodeIndexonlyscan.h   |   26 ++
src/include/executor/nodeIndexscan.h       |    7 +-
src/include/nodes/execnodes.h              |   36 ++-
src/include/nodes/nodes.h                  |    2 +
src/include/nodes/plannodes.h              |   38 ++-
src/include/nodes/primnodes.h              |   16 +-
src/include/nodes/relation.h               |   27 +-
34 files changed, 1313 insertions(+), 420 deletions(-)