pgsql: Avoid changing an index's indcheckxmin horizon during REINDEX.

Avoid changing an index's indcheckxmin horizon during REINDEX.

There can never be a need to push the indcheckxmin horizon forward, since
any HOT chains that are actually broken with respect to the index must
pre-date its original creation.  So we can just avoid changing pg_index
altogether during a REINDEX operation.

This offers a cleaner solution than my previous patch for the problem
found a few days ago that we mustn't try to update pg_index while we are
reindexing it.  System catalog indexes will always be created with
indcheckxmin = false during initdb, and with this modified code we should
never try to change their pg_index entries.  This avoids special-casing
system catalogs as the former patch did, and should provide a performance
benefit for many cases where REINDEX formerly caused an index to be
considered unusable for a short time.

Back-patch to 8.3 to cover all versions containing HOT.  Note that this
patch changes the API for index_build(), but I believe it is unlikely that
any add-on code is calling that directly.

Branch
------
REL8_4_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/5a4305fdf7ecf8218051f4f2a151d57fa71b9e6b

Modified Files
--------------
src/backend/bootstrap/bootstrap.c |    2 +-
src/backend/catalog/heap.c        |    2 +-
src/backend/catalog/index.c       |   38 ++++++++++++++++++++++++++++++------
src/backend/commands/cluster.c    |    6 +++++
src/backend/commands/indexcmds.c  |    2 +-
src/include/catalog/index.h       |    3 +-
6 files changed, 42 insertions(+), 11 deletions(-)