On Sat, Apr 18, 2020 at 02:23:25PM -0400, James Coleman wrote:
On Thu, Apr 16, 2020 at 9:26 PM Tom Lane <t...@sss.pgh.pa.us> wrote:
Tomas Vondra <tomas.von...@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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