The following is a comparison of other indexes.
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.