Re: determine snapshot after obtaining locks for first statement

"Kevin Grittner" <Kevin.Grittner@wicourts.gov> writes:
> The Cahill thesis mentions an interesting optimization -- they defer
> determination of the snapshot until after any locks required for the
> first statement have been acquired.  Where the first statement was,
> for example, an UPDATE, this reduced re-reads or rollbacks in the
> face of concurrent modifications.
> Does PostgreSQL currently do this?

Yes --- it's not an "optimization", it's necessary for basic
functionality to work correctly.  See for example the last para at
http://www.postgresql.org/docs/8.4/static/applevel-consistency.html
        regards, tom lane