| From: | Peter Geoghegan <pg(at)bowt(dot)ie> |
|---|---|
| To: | Simon Riggs <simon(dot)riggs(at)enterprisedb(dot)com> |
| Cc: | Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>, Dilip Kumar <dilipbalaut(at)gmail(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Sadhuprasad Patro <b(dot)sadhu(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, PostgreSQL Developers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: Next Steps with Hash Indexes |
| Date: | 2021-10-13 19:15:47 |
| Message-ID: | CAH2-Wz=xfxkcf+1nU7KfE9_-JTub40MHQFA8kZO7bu+PtXWESw@mail.gmail.com |
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On Wed, Oct 13, 2021 at 3:44 AM Simon Riggs
<simon(dot)riggs(at)enterprisedb(dot)com> wrote:
> > IMO it'd be nice to show some numbers to support the claims that storing
> > the extra hashes and/or 8B hashes is not worth it ...
>
> Using an 8-byte hash is possible, but only becomes effective when
> 4-byte hash collisions get hard to manage. 8-byte hash also makes the
> index 20% bigger, so it is not a good default.
Are you sure? I know that nbtree index tuples for a single-column int8
index are exactly the same size as those from a single column int4
index, due to alignment overhead at the tuple level. So my guess is
that hash index tuples (which use the same basic IndexTuple
representation) work in the same way.
--
Peter Geoghegan
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