pgsql: Take fewer snapshots.

Take fewer snapshots.

When a PORTAL_ONE_SELECT query is executed, we can opportunistically
reuse the parse/plan shot for the execution phase.  This cuts down the
number of snapshots per simple query from 2 to 1 for the simple
protocol, and 3 to 2 for the extended protocol.  Since we are only
reusing a snapshot taken early in the processing of the same protocol
message, the change shouldn't be user-visible, except that the remote
possibility of the planning and execution snapshots being different is
eliminated.

Note that this change does not make it safe to assume that the parse/plan
snapshot will certainly be reused; that will currently only happen if
PortalStart() decides to use the PORTAL_ONE_SELECT strategy.  It might
be worth trying to provide some stronger guarantees here in the future,
but for now we don't.

Patch by me; review by Dimitri Fontaine.

Branch
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master

Details
-------
http://git.postgresql.org/pg/commitdiff/d573e239f03506920938bf0be56c868d9c3416da

Modified Files
--------------
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 +-
6 files changed, 39 insertions(+), 32 deletions(-)