| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Jesper Pedersen <jesper(dot)pedersen(at)redhat(dot)com>, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Mithun Cy <mithun(dot)cy(at)enterprisedb(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Hash Indexes |
| Date: | 2016-10-18 17:46:35 |
| Message-ID: | 20161018174635.qgndqdbz5piiwm72@alap3.anarazel.de |
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| Lists: | pgsql-hackers |
On 2016-10-18 13:38:14 -0400, Tom Lane wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> > On Tue, Oct 18, 2016 at 5:37 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >> I have implemented this idea and it works for MVCC scans. However, I
> >> think this might not work for non-MVCC scans. Consider a case where
> >> in Process-1, hash scan has returned one row and before it could check
> >> it's validity in heap, vacuum marks that tuple as dead and removed the
> >> entry from heap and some new tuple has been placed at that offset in
> >> heap.
>
> > Oops, that's bad.
>
> Do we care? Under what circumstances would a hash index be used for a
> non-MVCC scan?
Uniqueness checks, are the most important one that comes to mind.
Andres
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