Re: btree vacuum and suspended scans can deadlock

On Fri, Oct 14, 2016 at 3:14 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Thu, Oct 13, 2016 at 6:33 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>> If we agree that above is a problematic case, then some of the options
>>> to solve it could be (a) Vacuum should not wait for a cleanup lock and
>>> instead just give up and start again which I think is a bad idea (b)
>>> don't allow to take lock of higher granularity after the scan is
>>> suspended, not sure if that is feasible (c) document the above danger,
>>> this sounds okay on the ground that nobody has reported the problem
>>> till now
>
>> I don't think any of these sound particularly good.
>
> Note that it's a mistake to imagine that this is specific to indexes;
> the same type of deadlock risk exists just when considering heap cleanup.
> We've ameliorated the heap case quite a bit by recognizing situations
> where it's okay to skip a page and move on, but it's not gone.
> Unfortunately indexes don't get to decide that deletion is optional.
>
> I was about to suggest that maybe we didn't need cleanup locks in btree
> indexes anymore now that SnapshotNow is gone,
>

Wouldn't it still be a problem for SnapshotAny?

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com