On 11 October 2012 18:22, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> I suggested a way to automatically trigger a second snapshot. I think
>> that would be acceptable to backpatch.
>
> If it worked, I might be amenable to that, but it doesn't. You can't
> trigger taking a new snapshot off whether we waited for a lock; that
> still has race conditions, just ones that are not so trivial to
> demonstrate manually. (The other transaction might have committed
> microseconds before you reach the point of waiting for the lock.)
> It would have to be a rule like "take a new snapshot if we acquired
> any new lock since the previous snapshot". While that would work,
> we'd end up with no performance gain worth mentioning, since there
> would almost always be some lock acquisitions during parsing.
So where's the race?
AFAICS it either waits or it doesn't - the code isn't vague on that
point. If we wait we set the flag.
The point is that lock waits are pretty rare since most locks are
compatible, so triggering a second snap if we waited is not any kind
of problem, even if we waited for a very short time.
-- Simon Riggs http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services