I have a scenario where data size is ~3M rows. My query is 

select C1, C2, C3 from T1 group by C1,C2,C3

This returns result in 30 Sec ( number of groups around 1 M )

While, if I modify the query to

 select C1, C2, C3 from T1 group by C1,C2,C3 Order by C1, C2

on the same dataset, it takes ~ 1 hour to get result. The platforms I tried is both widows 64 bit and Linux 64 bit, with latest monet installed. Both the machines have 4 cores. And the behavior is same on both the machines.

I am really surprised to see overhead of second query. One more observation is that this query occupies only single core, while monet is free to utilize 4 cores.

Regards,

Manish