LF The determining factor is the number of unique values. Use High_Group if the number of unique values for the column is high. Use Low_Fast if the number of unique values is low.
HNG The determining factor is whether the column is a join column, and/or whether GROUP BY may be processed on the column. If either of these is true, use High_Group, either alone or in combination with High_Non_Group. Otherwise, use High_Non_Group to save disk space.
