Re: cheaper snapshots redux

On Mon, Aug 22, 2011 at 10:25 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> I've been giving this quite a bit more thought, and have decided to
> abandon the scheme described above, at least for now.

I liked your goal of O(1) snapshots and think you should go for that.

I didn't realise you were still working on this, and had some thoughts
at the weekend which I recorded just now. Different tack entirely.

> Heikki has made the suggestion a few times (and a few other people
> have since made somewhat similar suggestions in different words) of
> keeping an-up-to-date snapshot in shared memory such that transactions
> that need a snapshot can simply copy it.  I've since noted that in Hot
> Standby mode, that's more or less what the KnownAssignedXids stuff
> already does.  I objected that, first, the overhead of updating the
> snapshot for every commit would be too great, and second, it didn't
> seem to do a whole lot to reduce the size of the critical section, and
> therefore probably wouldn't improve performance that much.  But I'm
> coming around to the view that these might be solvable problems rather
> than reasons to give up on the idea altogether.

Sounds easy enough to just link up KnownAssignedXids and see...

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