Subject: Re: select with max functions
To: pgsql-performance@postgresql.org
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From: Andreas Kretschmer <andreas@a-kretschmer.de>
Message-ID: <0970df65-602e-724d-2c39-e4695ae48bb6@a-kretschmer.de>
Date: Sun, 1 Oct 2017 20:48:45 +0200
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Am 01.10.2017 um 14:41 schrieb Mariel Cherkassky:
> Hi,
> I need to use the max function in my query. I had very bad performance 
> when I used the max  :
>
>                SELECT Ma.User_Id,
>                       COUNT(*) COUNT
>                FROM   Manuim Ma
>                WHERE  Ma.Bb_Open_Date  =
>                                   (SELECT max(Bb_Open_Date)
>                                    FROM  Manuim Man
>                                    WHERE Man.User_Id = Ma.User_Id
>                                   )
>                GROUP  BY Ma.User_Id
>                HAVING COUNT(*) > 1;
>
>
> Any idea how can I continue from here ? Thanks , Mariel.


Maybe you can rewrite it, for instance to

select distinct on (user_id, bb_open_date) user_id, bb_open_date, 
count(1) from Manuim group by 1,2 having count(1) > 1;

maybe much cheaper, but untested! If not, please share more details, at 
least table-definition.

Regards, Andreas

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