Re: synchronized snapshots

On Oct19, 2011, at 19:49 , Kevin Grittner wrote:
> Robert Haas <robertmhaas@gmail.com> wrote:
>> Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> Florian Pflug <fgp@phlo.org> writes:
> 
>>>> This allows a deferrable snapshot to be used on a second
>>>> connection (by e.g. pg_dump), and still be marked as DEFERRABLE.
>>>> If we throw an error unconditionally, the second connection has
>>>> to import the snapshot without marking it DEFERRABLE, which I
>>>> think has consequences for performance.
>>> 
>>> No, I don't believe that either.  AIUI the performance benefit
>>> comes if the snapshot is recognized as safe.  DEFERRABLE only
>>> means to keep retrying until you get a safe one.
> 
> Right, there are other circumstances in which a READ ONLY
> transaction's snapshot may be recognized as safe, and it can opt out
> of all the additional SSI logic.  As you say, DEFERRABLE means we
> *wait* for that.

Oh, cool. I thought the opt-out only works for explicitly DEFERRABLE
transactions.

best regards,
Florian Pflug