how to replace distinct with something faster in mysql query?

Question

I have a query like this -

SELECT COUNT(DISTINCT c.y_no) 
FROM xyz c 
WHERE c.xdate>='2013-09-01';

Because of DISTINCT, my query taking too much time. This query scanning around 150 million records. I already have index on xdate. Please suggest me something faster.

Thank you.

Answer 1

Well, a compound index on xdate, y_nro will help too much