The recommended use for an LF index is as follows:
A column has fewer than 1,000 unique values.
A column has fewer than 1,000 unique values and is used in a join predicate.
Never use an LF index for a column with 10,000 or more unique values. If the table has fewer than 25,000 rows, use an HG index, as fewer disk I/O operations are required for the same operation.