Re: Hash Indexes

On Sun, Sep 18, 2016 at 11:44 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
wrote:

> On Mon, Sep 19, 2016 at 11:20 AM, Mark Kirkwood
> <mark(dot)kirkwood(at)catalyst(dot)net(dot)nz> wrote:
> > On 17/09/16 06:38, Andres Freund wrote:
> >
> > While I see the point of what you are saying here, I recall all previous
> > discussions about has indexes tended to go a bit like this:
> >
> > - until WAL logging of hash indexes is written it is not worthwhile
> trying
> > to make improvements to them
> > - WAL logging will be a lot of work, patches 1st please
> >
> > Now someone has done that work, and we seem to be objecting that because
> > they are not improved then the patches are (maybe) not worthwhile.
> >
>

+1

>
> I think saying hash indexes are not improved after proposed set of
> patches is an understatement. The read performance has improved by
> more than 80% as compare to HEAD [1] (refer data in Mithun's mail).
> Also, tests by Mithun and Jesper has indicated that in multiple
> workloads, they are better than BTREE by 30~60% (in fact Jesper
> mentioned that he is seeing 40~60% benefit on production database,
> Jesper correct me if I am wrong.). I agree that when index column is
> updated they are much worse than btree as of now,

Has anyone tested that with the relcache patch applied? I would expect
that to improve things by a lot (compared to hash-HEAD, not necessarily
compared to btree-HEAD), but if I am following the emails correctly, that
has not been done.

> but no work has been
> done improve it and I am sure that it can be improved for those cases
> as well.
>
> In general, I thought the tests done till now are sufficient to prove
> the importance of work, but if still Andres and others have doubt and
> they want to test some specific cases, then sure we can do more
> performance benchmarking.
>

I think that a precursor to WAL is enough to justify it even if the
verified performance improvements were not impressive. But they are pretty
impressive, at least for some situations.

Cheers,

Jeff