Re: Next Steps with Hash Indexes

On Sun, Oct 17, 2021 at 4:30 PM Simon Riggs
<simon(dot)riggs(at)enterprisedb(dot)com> wrote:
>
> On Thu, 14 Oct 2021 at 16:09, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> >
> > On Thu, Oct 14, 2021 at 12:48 AM Simon Riggs
> > <simon(dot)riggs(at)enterprisedb(dot)com> wrote:
> > > The hash index tuples are 20-bytes each. If that were rounded up to
> > > 8-byte alignment, then that would be 24 bytes.
> > >
> > > Using pageinspect, the max(live_items) on any data page (bucket or
> > > overflow) is 407 items, so they can't be 24 bytes long.
> >
> > That's the same as an nbtree page, which confirms my suspicion. The 20
> > bytes consists of a 16 byte tuple, plus a 4 byte line pointer. The
> > tuple-level alignment overhead gets you from 12 bytes to 16 bytes with
> > a single int4 column. So the padding is there for the taking.
>
> Thank you for nudging me to review the tuple length.
>
> Since hash indexes never store Nulls, and the hash is always fixed
> length, ISTM that we can compress the hash index entries down to
> ItemPointerData (6 bytes) plus any hashes.
>

Nice observation but we use INDEX_AM_RESERVED_BIT (as
INDEX_MOVED_BY_SPLIT_MASK) for hash indexes, so we need to take care
of that in some way.

--
With Regards,
Amit Kapila.