pgsql: Support index-only scans using the visibility map to avoid heap

Support index-only scans using the visibility map to avoid heap fetches.

When a btree index contains all columns required by the query, and the
visibility map shows that all tuples on a target heap page are
visible-to-all, we don't need to fetch that heap page. This patch depends
on the previous patches that made the visibility map reliable.

There's a fair amount left to do here, notably trying to figure out a less
chintzy way of estimating the cost of an index-only scan, but the core
functionality seems ready to commit.

Robert Haas and Ibrar Ahmed, with some previous work by Heikki Linnakangas.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/a2822fb9337a21f98ac4ce850bb4145acf47ca27

Modified Files
--------------
doc/src/sgml/catalogs.sgml | 7 +
doc/src/sgml/config.sgml | 19 ++-
doc/src/sgml/indexam.sgml | 32 ++++-
doc/src/sgml/ref/postgres-ref.sgml | 7 +-
src/backend/access/index/genam.c | 4 +
src/backend/access/index/indexam.c | 236 ++++++++++++++++---------
src/backend/access/nbtree/nbtree.c | 92 ++++++++++
src/backend/commands/explain.c | 5 +-
src/backend/executor/nodeIndexscan.c | 123 ++++++++++++-
src/backend/nodes/copyfuncs.c | 1 +
src/backend/nodes/outfuncs.c | 2 +
src/backend/optimizer/path/costsize.c | 21 +++
src/backend/optimizer/path/indxpath.c | 89 +++++++++
src/backend/optimizer/plan/createplan.c | 9 +-
src/backend/optimizer/plan/planner.c | 2 +-
src/backend/optimizer/util/pathnode.c | 8 +-
src/backend/optimizer/util/plancat.c | 1 +
src/backend/optimizer/util/var.c | 43 +++--
src/backend/tcop/postgres.c | 5 +-
src/backend/utils/cache/relcache.c | 4 +-
src/backend/utils/misc/guc.c | 9 +
src/backend/utils/misc/postgresql.conf.sample | 1 +
src/include/access/genam.h | 3 +
src/include/access/relscan.h | 5 +
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_am.h | 52 +++---
src/include/nodes/execnodes.h | 2 +
src/include/nodes/plannodes.h | 5 +
src/include/nodes/relation.h | 6 +
src/include/optimizer/cost.h | 4 +-
src/include/optimizer/pathnode.h | 1 +
src/include/optimizer/var.h | 2 +-
src/test/regress/expected/aggregates.out | 78 ++++----
src/test/regress/expected/rangefuncs.out | 27 ++--
34 files changed, 704 insertions(+), 203 deletions(-)