select distinct on a lot of data

I need to select distinct value from a Date Filed named DATA1.
The field DATA1 is indexed.
i have tested a query with Select Distinct or Group by, but the result in other case, are very slow!

I use Access, the conn is a tipical ADO conn, and the records are approx 500.000-

have a good suggestion to solve the time delay for sleect distinct...?

GROUP BY should be faster but with 500K records in Access I doubt you will something better.