Re: select with max functions

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