Re: Postgresql 8.1.4 - performance issues for select on

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От Jeff Davis
Тема Re: Postgresql 8.1.4 - performance issues for select on
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Msg-id 1161210735.31645.277.camel@dogma.v10.wvs
обсуждение исходный текст
Ответ на Re: Postgresql 8.1.4 - performance issues for select on  ("Jim C. Nasby")
Ответы Re: Postgresql 8.1.4 - performance issues for select on  ("Jim C. Nasby")
Re: Postgresql 8.1.4 - performance issues for select on  (Tom Lane)
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Postgresql 8.1.4 - performance issues for select on view using max  (Ioana Danes, )
 Re: Postgresql 8.1.4 - performance issues for select on view using max  (Dimitri Fontaine, )
  Re: Postgresql 8.1.4 - performance issues for select on view using max  (Ioana Danes, )
   Re: Postgresql 8.1.4 - performance issues for select on view using max  (Ioana Danes, )
    Re: Postgresql 8.1.4 - performance issues for select on view using max  (Dimitri Fontaine, )
     Re: Postgresql 8.1.4 - performance issues for select on  (Jeff Davis, )
      Re: Postgresql 8.1.4 - performance issues for select on  ("Jim C. Nasby", )
       Re: Postgresql 8.1.4 - performance issues for select on  (Jeff Davis, )
        Re: Postgresql 8.1.4 - performance issues for select on  ("Jim C. Nasby", )
         Re: Postgresql 8.1.4 - performance issues for select on  (Jeff Davis, )
         Re: Postgresql 8.1.4 - performance issues for select on  (Ioana Danes, )
        Re: Postgresql 8.1.4 - performance issues for select on  (Tom Lane, )
         Re: Postgresql 8.1.4 - performance issues for select on  (Ioana Danes, )
      Re: Postgresql 8.1.4 - performance issues for select on view using max  (Ioana Danes, )
 Re: Postgresql 8.1.4 - performance issues for select on view using max  ("Joshua Marsh", )
 Re: Postgresql 8.1.4 - performance issues for select on  (Jeff Davis, )
On Wed, 2006-10-18 at 17:10 -0500, Jim C. Nasby wrote:
> Sorry, don't have the earlier part of this thread, but what about...
>
> SELECT greatest(max(a), max(b)) ...
>
> ?

To fill you in, we're trying to get the max of a union (a view across
two physical tables).

It can be done if you're creative with the query; I suggested a query
that selected the max of the max()es of the individual tables. Your
query could work too. However, the trick would be getting postgresql to
recognize that it can transform "SELECT max(x) FROM foo" into that,
where foo is a view of a union.

If PostgreSQL could sort the result of a union by merging the results of
two index scans, I think the problem would be solved. Is there something
preventing this, or is it just something that needs to be added to the
planner?

Regards,
    Jeff Davis


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