Real,

>SELECT * FROM invoice ORDER BY CLT_CODE,DATE && works perfect

If it works perfect, why not use that?

>SELECT * FROM invoice ORDER BY DATE,CLT_CODE && problem
>gives the same result as ORDER BY DATE
>
>I also tried a separate index and DTOS(DATE)+CLT_CODE

Either of those two will give you all clients for a date, in client order, then all clients for the next date.

Use the first example.