| 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: | 2021-10-14 15:08:55 |
| Message-ID: | CAH2-Wz=a-hnwPJQeO0wWq4cbECpk+ta=J7PWwL9oeuBjTPqxDg@mail.gmail.com |
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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.
--
Peter Geoghegan
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