Re: [HACKERS] No: implied sort with group by
От | darrenk@insightdist.com (Darren King) |
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Тема | Re: [HACKERS] No: implied sort with group by |
Дата | |
Msg-id | 9801271343.AA65898@ceodev обсуждение исходный текст |
Список | pgsql-hackers |
> > > Does the SQL standard say anything about an implied sort when > > grouping or is it up to the user to include an ORDER BY clause? > > darrenk > > Without order by the result set is never guaranteed to be ordered in a > specific way (standard speak). The order is dependent on the chosen > query path, which changes from query to query. > > Andreas Postgres should then do an internal sort before grouping. In the second of your examples, I take the above to mean that either row could be returned first. In order to get that result set though, the data needs to be sorted before getting to the group by node in the executor. The order of that internal sort is purely arbitrary, it just has to be done. This is what I think is missing or broken right now. > > select * from t1; > a b c > 1 x > 2 x > 3 z > 2 x > > 4 row(s) retrieved. > > select b,c,sum(a) from t1 group by b,c; > b c (sum) > > x 5 > z 3 > > 2 row(s) retrieved. darrenk
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