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
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