Re: Hash Indexes

On Tue, Oct 18, 2016 at 8:27 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> By this problem, I mean to say deadlocks for suspended scans, that can
> happen in btree for non-Mvcc or other type of scans where we don't
> release pin during scan. In my mind, we have below options:
>
> a. problem of deadlocks for suspended scans should be tackled as a
> separate patch as it exists for other indexes (at least for some type
> of scans).
> b. Implement page-scan mode and then we won't have deadlock problem
> for MVCC scans.
> c. Let's not care for non-MVCC scans unless we have some way to hit
> those for hash indexes and proceed with Dead tuple marking idea. I
> think even if we don't care for non-MVCC scans, we might hit this
> problem (deadlocks) when the index relation is unlogged.
>
> Here, even if we want to go with (b), I think we can handle it in a
> separate patch, unless you think otherwise.

After some off-list discussion with Amit, I think I get his point
here: the deadlock hazard which is introduced by this patch already
exists for btree and has for a long time, and nobody's gotten around
to fixing it (although 2ed5b87f96d473962ec5230fd820abfeaccb2069
improved things). So it's probably OK for hash indexes to have the
same issue.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company