From: | James Coleman <jtc331(at)gmail(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Justin Pryzby <pryzby(at)telsasoft(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: sqlsmith crash incremental sort |
Date: | 2020-04-18 18:23:25 |
Message-ID: | CAAaqYe-SZUZRCYKZBUvrWDv=TTS6p4Xb8wO6cLXDsJhJ2D6K1Q@mail.gmail.com |
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On Thu, Apr 16, 2020 at 9:26 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> writes:
> > I think we have essentially three options:
> > 1) assuming there's just a single group
> > 2) assuming each row is a separate group
> > 3) something in between
> > If (1) and (2) are worst/best-case scenarios, maybe we should pick
> > something in between. We have DEFAULT_NUM_DISTINCT (200) which
> > essentially says "we don't know what the number of groups is" so maybe
> > we should use that.
>
> I wouldn't recommend picking either the best or worst cases.
>
> Possibly DEFAULT_NUM_DISTINCT is a sane choice, though it's fair to
> wonder if it's quite applicable to the case where we already know
> we've grouped by some columns.
Do you think defining a new default, say,
DEFAULT_NUM_DISTINCT_PRESORTED is preferred then? And choose some
value like "1/2 of the normal DEFAULT_NUM_DISTINCT groups" or some
such?
James
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