is index required on distinct(count(ColumnName)) with index on where clause?
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I have a table like this

    Subscribers
      Id (PK, int)
      OwnedBy (FK, int)
      EmailAddress (varchar 50)


If I query like this:

    SELECT COUNT(DISTINCT(EmailAddress))
    FROM Subscribers
    WHERE Subscribers.OwnedBy = 67;

There is an index on the `OwnedBy` column. Assuming the table is very large, should I also index `EmailAddress`? I am not concerned about write performance degrading.

