Re: Re: Slow query with indexed ORDER BY and LIMIT when using OR'd conditions

Поиск
Список
Период
Сортировка
От johno
Тема Re: Re: Slow query with indexed ORDER BY and LIMIT when using OR'd conditions
Дата
Msg-id CACuOPqC+AXr4zGTD+Re8Y7R9RwA1s-h2Vn361-+FNasNMHqgPA@mail.gmail.com
обсуждение исходный текст
Ответ на Re: Slow query with indexed ORDER BY and LIMIT when using OR'd conditions  (David G Johnston <david.g.johnston@gmail.com>)
Ответы Re: Slow query with indexed ORDER BY and LIMIT when using OR'd conditions  (David G Johnston <david.g.johnston@gmail.com>)
Список pgsql-performance
Дерево обсуждения
Oh, yes I do understand that if I remove the outer limit, the semantics of the query would change. However I am looking for the counterexample *with* the limit clauses. Maybe I just don't understand what relationally equivalent means, sorry about that.

BTW this is to my understanding a very similar scenario to how partitioned tables work and push down limit and where conditions. Why is this not possible in this case? 

Jano


On Mon, Jul 21, 2014 at 11:54 PM, David G Johnston <david.g.johnston@gmail.com> wrote:
johno wrote
> Thanks for the quick reply David!
>
> However I am still unsure how these two queries are not relationally
> equivalent. I am struggling to find a counterexample where the first and
> third query (in email, not in gist) would yield different results. Any
> ideas?

Remove the outer LIMIT 100 from both queries...

The first query would return a maximal number of rows that meet the OR
criteria while the second query would return at most 200 rows since both
sub-queries would still have their own independent LIMIT 100 clauses.

David J.




--
View this message in context: http://postgresql.1045698.n5.nabble.com/Slow-query-with-indexed-ORDER-BY-and-LIMIT-when-using-OR-d-conditions-tp5812282p5812289.html
Sent from the PostgreSQL - performance mailing list archive at Nabble.com.


--
Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-performance

В списке pgsql-performance по дате отправления:

Предыдущее
От: David G Johnston
Дата:
Сообщение: Re: Slow query with indexed ORDER BY and LIMIT when using OR'd conditions
Следующее
От: David G Johnston
Дата:
Сообщение: Re: Slow query with indexed ORDER BY and LIMIT when using OR'd conditions