Re: Retrieve the snapshot's LSN

On Thu, Jul 16, 2015 at 6:40 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> I don't think "the snapshot's LSN" has a well-defined meaning in
> general.  The obvious meaning would be "the LSN such that all commits
> prior to that LSN are visible and all later commits are invisible",

I like this definition.

> but such an LSN need not exist.  Suppose A writes a commit record at
> LSN 0/10000, and then B writes a commit record at 0/10100, and then B
> calls ProcArrayEndTransaction().  At this point, B is visible and A is
> not visible, even though A's commit record precedes that of B.

Maybe that's what Andres referred as "doable with some finicky locking".

There is some race conditions to build a snapshot with an associated
consistent LSN. If I understand your example, A is supposed to call
ProcArrayEndTransaction() anytime soon. Could we wait/lock until it
happens?

--
Florent