Re: sqlsmith crash incremental sort
От | Tomas Vondra |
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Тема | Re: sqlsmith crash incremental sort |
Дата | |
Msg-id | 20200419234729.tglhf2iqlpkkpk3m@development обсуждение исходный текст |
Ответ на | Re: sqlsmith crash incremental sort (James Coleman <jtc331@gmail.com>) |
Ответы |
Re: sqlsmith crash incremental sort
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Список | pgsql-hackers |
On Sat, Apr 18, 2020 at 02:23:25PM -0400, James Coleman wrote: >On Thu, Apr 16, 2020 at 9:26 PM Tom Lane <tgl@sss.pgh.pa.us> wrote: >> >> Tomas Vondra <tomas.vondra@2ndquadrant.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? > If we had a better intuition what a better value is, maybe. But I don't think we have that at all, so I'd just use the existing one. regards -- Tomas Vondra http://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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