Want to Know the Reason

[sammy]

I recently heard from my friend that Distinct usage would reduce the performance of a query. But he could not state the reason for the same when I asked for it. Can someone let us know why does Distinct usage would reduce the performance of a query.

[Uldis]

You have to sort the result set first to find the distinct values. However, you may lose nothing if there's a sort already for other reasons. You always may look into "explain plan" to see the difference, of course.