Renato,
>

>The TOP N clause I think it will find ALL the result and then return only the first N, so if the query can return 250000 rows it will take time.
>If user make a mistake using the interface I cannot let the app work for minutes without a result.
>

I have the same need, but I live the result. I used TOP 20000 on a query which potentially could pull 1 million records, and it took three seconds.