pgsql: Revert patch for taking fewer snapshots.

Revert patch for taking fewer snapshots.

This reverts commit d573e239f03506920938bf0be56c868d9c3416da, "Take fewer
snapshots". While that seemed like a good idea at the time, it caused
execution to use a snapshot that had been acquired before locking any of
the tables mentioned in the query. This created user-visible anomalies
that were not present in any prior release of Postgres, as reported by
Tomas Vondra. While this whole area could do with a redesign (since there
are related cases that have anomalies anyway), it doesn't seem likely that
any future patch would be reasonably back-patchable; and we don't want 9.2
to exhibit a behavior that's subtly unlike either past or future releases.
Hence, revert to prior code while we rethink the problem.

Branch
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/786afc1ce53126feecf4d02e96e7455669ccbf5a

Modified Files
--------------
doc/src/sgml/release-9.2.sgml | 7 -------
src/backend/commands/portalcmds.c | 2 +-
src/backend/commands/prepare.c | 2 +-
src/backend/executor/spi.c | 18 +++++++++++-------
src/backend/tcop/postgres.c | 32 +++++++++++---------------------
src/backend/tcop/pquery.c | 15 +++++++--------
src/include/tcop/pquery.h | 2 +-
7 files changed, 32 insertions(+), 46 deletions(-)