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? James
