| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Simon Riggs <simon(at)2ndQuadrant(dot)com> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Tomas Vondra <tv(at)fuzzy(dot)cz>, Thom Brown <thom(at)linux(dot)com>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: change in LOCK behavior |
| Date: | 2012-10-11 18:41:58 |
| Message-ID: | 24391.1349980918@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> On 11 October 2012 18:22, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)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.
regards, tom lane
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