>>Your order by here is strange. I would probably re-write the whole query as
>>

>>select * from (select T.Numero, T.Title, T.Url, T.AddDate, row_number() 
>>over (partition by R.NoThread order by R.NoThread) as Row from Thread T LEFT JOIN Reply R on T.NoThread = R.NoThread) X 
>>where Row <=10

>
>I do not understand why it would be strange. The 10 records I need to select from Reply are subject to a specific order. To me, this is the clean way to do it.

I don't see an ORDER BY field in your example. ORDER BY 1 is no order - do you want them to be in the clustered index order?