The recommendation is to always have a DATE, TIME, or DTTM index on a column of DATE, TIME, DATETIME, or TIMESTAMP data type, if the column is referenced in the WHERE clause, in ON conditions, or in the GROUP BY clause. In addition, the HG or LF index may also be appropriate for a DATE, TIME, DATETIME, or TIMESTAMP column, especially if you are evaluating equality predicates against the column. A LF index is also recommended, if you frequently use the column in the GROUP BY clause and there are less than 1000 distinct values (i.e., less than three years of dates).
