On 11 October 2012 19:41, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Simon Riggs <simon@2ndQuadrant.com> writes:
>> On 11 October 2012 18:22, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> 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.)
>
>> So where's the race?
>
> Same example as before, except that the exclusive-lock-holding
> transaction commits (and releases its lock) between the time that the
> other transaction takes its parse/plan snapshot and the time that it
> takes AccessShare lock on the table.
A cache invalidation could also set the need-second-snapshot flag.
-- Simon Riggs http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services
