Re: pg_verify_checksums failure with hash indexes

On Sat, Sep 1, 2018 at 10:28 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>
> On Sat, Sep 1, 2018 at 8:22 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> > On Thu, Aug 30, 2018 at 7:27 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > I wouldn't bother bumping HASH_VERSION. First, the fix needs to be
> > back-patched, and you certainly can't back-patch a HASH_VERSION bump.
> > Second, you should just pick a formula that gives the same answer as
> > now for the cases where the overrun doesn't occur, and some other
> > sufficiently-value for the cases where an overrun currently does
> > occur. If you do that, you're not changing the behavior in any case
> > that currently works, so there's really no reason for a version bump.
> > It just becomes a bug fix at that point.
> >

makes sense.

>
> I think if we compute with below formula which I suggested upthread
>
> #define HASH_MAX_BITMAPS Min(BLCKSZ / 8, 1024)
>
> then for BLCKSZ 8K and bigger, it will remain the same value where it
> does not overrun. And, for the small BLCKSZ, I think it will give
> sufficient space for the hash map. If the BLCKSZ is 1K then the sizeof
> (HashMetaPageData) + sizeof (HashPageOpaque) = 968 which is very close
> to the BLCKSZ.
>

Yeah, so at 1K, the value of HASH_MAX_BITMAPS will be 128 as per above
formula which is what it was its value prior to the commit 620b49a1.
I think it will be better if you add a comment in your patch
indicating the importance/advantage of such a formula.

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