Re: Cache Hash Index meta page.

On Fri, Jul 22, 2016 at 3:02 AM, Mithun Cy <mithun(dot)cy(at)enterprisedb(dot)com> wrote:
> I have created a patch to cache the meta page of Hash index in
> backend-private memory. This is to save reading the meta page buffer every
> time when we want to find the bucket page. In “_hash_first” call, we try to
> read meta page buffer twice just to make sure bucket is not split after we
> found bucket page. With this patch meta page buffer read is not done, if the
> bucket is not split after caching the meta page.
>
> Idea is to cache the Meta page data in rd_amcache and store maxbucket number
> in hasho_prevblkno of bucket primary page (which will always be NULL other
> wise, so reusing it here for this cause!!!).

If it is otherwise unused, shouldn't we rename the field to reflect
what it is now used for?

What happens on a system which has gone through pg_upgrade? Are we
sure that those on-disk representations will always have
InvalidBlockNumber in that fields? If not, then it seems we can't
support pg_upgrade at all. If so, I don't see a provision for
properly dealing with pages which still have InvalidBlockNumber in
them. Unless I am missing something, the code below will always think
it found the right bucket in such cases, won't it?

if (opaque->hasho_prevblkno <= metap->hashm_maxbucket)

Cheers,

Jeff