My query is taking too long to finish for finding the pair of rows where the difference of columns value is maximum group by another column

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醉话见心 2021-01-26 19:10

Say, I\'ve a table like this:

I want to find the pair of Centers whose Performance difference is highest for each session, like this:

I have th

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  • 2021-01-26 19:33

    Grouping by session and taking the group's min and max performance seems logical. The actual centers unfortunately need a subquery/join here.

    select g.session as Session,
        (select min(center) from mytable
         where session = g.session and performance = g.maxim) as Center1,
        (select min(center) from mytable
         where session = g.session and performance = g.minim) as Center2,
        g.maxim - g.minim as Performance
    from (select 
            t1.session,
            min(t1.performance) as minim,
            max(t1.performance) as maxim
        from mytable t1
        group by t1.session)
        as g
    

    Ensure an index on session and performance.

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  • 2021-01-26 19:42

    Use row_number():

    select session, center1, center2, performance
    from (select t1.center as center1, t2.center as center2,
                 (t1.performance - t2.performance) as performance,
                 row_number() over (partition by t1.session order by (t1.performance - t2.performance) desc) as seqnum
          from mytable t1 join
               mytable t2
               on t1.session = t2.session
    where seqnum = 1;
    

    Or for better performance. The maximum difference is the maximum minus the minimum. You want the centers, here is a method without subqueries:

    select session,
           max(case when seqnum_desc = 1 then center end) as center1,
           max(case when seqnum_asc = 1 then center end) as center2,
           max(performance) - min(performance)
    from (select t.*,
                 row_number() over (partition by session order by performance) as seqnum_asc,
                 row_number() over (partition by session order by performance desc) as seqnum_desc
          from mytable t
    where 1 in (seqnum_asc, seqnum_desc)
    group by session
    
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  • 2021-01-26 19:43
    select 
            t1.session,
            t1.center center1,
            t2.center center2,
            t1.performance - t2.performance performance
        from mytable t1
        inner join mytable t2 
           on t1.session = t2.session
        WHERE t1.performance = (SELECT MAX(performance) 
                                FROM mytable t3 WHERE t3.session = t1.session)
          AND t2.performance = (SELECT MIN(performance) 
                                FROM mytable t3 WHERE t3.session = t2.session)
    
         // Im thinking this will solve the border case when performance is a tie 
         // and difference 0 will return 2 rows
    
         AND (CASE WHEN t1.performance = t2.performance 
                   THEN CASE WHEN t1.center < t2.center
                             THEN 1
                             ELSE 0
                        END
                   ELSE 1
              END) = 1
    

    As long as you have an index on performance and session should be fine.

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  • 2021-01-26 19:49
    select distinct(session) * from (
    select  t1.session, t1.center, t2.center, 
    (case when t1.performance > t2.performance then (t1.performance-t2.performance) else (t2.performance-t1.performance))as performance_diff 
    from mytable t1, mytable t2 
    where t1.session=t2.session and t1.center!=t2.center) as T1 order by session,performance_diff desc limit 1;
    
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