pgsql: Add support for index-only scans in GiST.

Add support for index-only scans in GiST.

This adds a new GiST opclass method, 'fetch', which is used to reconstruct
the original Datum from the value stored in the index. Also, the 'canreturn'
index AM interface function gains a new 'attno' argument. That makes it
possible to use index-only scans on a multi-column index where some of the
opclasses support index-only scans but some do not.

This patch adds support in the box and point opclasses. Other opclasses
can added later as follow-on patches (btree_gist would be particularly
interesting).

Anastasia Lubennikova, with additional fixes and modifications by me.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/d04c8ed9044eccebce043143a930617e3998c005

Modified Files
--------------
doc/src/sgml/catalogs.sgml                 |    4 +-
doc/src/sgml/gist.sgml                     |   73 +++++++++++++-
doc/src/sgml/indexam.sgml                  |   15 +--
src/backend/access/gist/gist.c             |    8 ++
src/backend/access/gist/gistget.c          |   66 ++++++++++++-
src/backend/access/gist/gistproc.c         |   37 +++++++
src/backend/access/gist/gistscan.c         |   18 ++++
src/backend/access/gist/gistutil.c         |   64 +++++++++++-
src/backend/access/index/indexam.c         |   12 ++-
src/backend/access/spgist/spgscan.c        |    1 +
src/backend/optimizer/path/indxpath.c      |   22 +++--
src/backend/optimizer/util/plancat.c       |    3 +-
src/include/access/genam.h                 |    2 +-
src/include/access/gist.h                  |    3 +-
src/include/access/gist_private.h          |    9 +-
src/include/catalog/catversion.h           |    2 +-
src/include/catalog/pg_am.h                |    2 +-
src/include/catalog/pg_amproc.h            |    2 +
src/include/catalog/pg_proc.h              |   10 +-
src/include/nodes/relation.h               |    3 +-
src/include/utils/geo_decls.h              |    3 +
src/test/regress/expected/create_index.out |   70 ++++++-------
src/test/regress/expected/gist.out         |  146 ++++++++++++++++++++++++++++
src/test/regress/sql/gist.sql              |   73 ++++++++++++++
24 files changed, 575 insertions(+), 73 deletions(-)