Re: select with max functions

On 03/10/17 04:29, Tom Lane wrote:
> Mariel Cherkassky <mariel(dot)cherkassky(at)gmail(dot)com> writes:
>> explain analyze SELECT Ma.User_Id,
>> COUNT(*) COUNT
>> FROM Manuim Ma
>> WHERE Ma.Bb_Open_Date =
>> (SELECT Bb_Open_Date
>> FROM Manuim Man
>> WHERE Man.User_Id = Ma.User_Id order
>> by bb_open_date desc limit 1
>> )
>> GROUP BY Ma.User_Id
>> HAVING COUNT(*) > 1;
> The core problem with this query is that the sub-select has to be done
> over again for each row of the outer table, since it's a correlated
> sub-select (ie, it refers to Ma.User_Id from the outer table). Replacing
> a max() call with handmade logic doesn't do anything to help that.
> I'd try refactoring it so that you calculate the max Bb_Open_Date just
> once for each user id, perhaps along the lines of
>
> SELECT Ma.User_Id,
> COUNT(*) COUNT
> FROM Manuim Ma,
> (SELECT User_Id, max(Bb_Open_Date) as max
> FROM Manuim Man
> GROUP BY User_Id) ss
> WHERE Ma.User_Id = ss.User_Id AND
> Ma.Bb_Open_Date = ss.max
> GROUP BY Ma.User_Id
> HAVING COUNT(*) > 1;
>
> This is still not going to be instantaneous, but it might be better.
>
> It's possible that an index on (User_Id, Bb_Open_Date) would help,
> but I'm not sure.
>
> regards, tom lane
>
>

Further ideas based on Tom's rewrite: If that MAX is still expensive it
might be worth breaking

SELECT User_Id, max(Bb_Open_Date) as max
FROM Manuim Man
GROUP BY User_Id

out into a VIEW, and considering making it MATERIALIZED, or creating an
equivalent  trigger based summary table (there are examples in the docs
of how to do this).

Cheers

Mark