On Wed, Aug 10, 2016 at 5:54 PM, Alexander Korotkov
<a.korot...@postgrespro.ru> wrote:
> Oh, I found that I underestimated complexity of async commit...  :)

Indeed. I think Tom's attitude was right even if the specific
conclusion was wrong. While I don't think removing async commit is
viable I think it would be a laudable goal if we can remove some of
the complication in it. I normally describe async commit as "just like
a normal commit but don't block on the commit" but it's actually a bit
more complicated.

AIUI the additional complexity is that while async commits are visible
to everyone immediately other non-async transactions can be committed
but then be in limbo for a while before they are visible to others. So
other sessions will see the async commit "jump ahead" of any non-async
transactions even if those other transactions were committed first.
Any standbys will see the non-async transaction in the logs before the
async transaction and in a crash it's possible to lose the async
transaction even though it was visible but not the sync transaction
that wasn't.

Complexity like this makes it hard to implement other features such as
CSNs. IIRC this already bit hot standby as well. I think it would be a
big improvement if we had a clear, well defined commit order that was
easy to explain and easy to reason about when new changes are being
made.


-- 
greg


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