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

Question

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

Answer 1

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