HG indexes and multicolumn HG indexes offer different functionality that must be considered when choosing which to use.
The column will be used in a join predicate
A column has more than 1000 unique values
Use multicolumn HG indexes to enhance the performance of ORDER BY queries with reference to multiple columns. This change is transparent to users, but improves query performance.
Foreign key columns require their own, individual HG index. However, if a join index exists, the same column cannot have both an explicitly created HG index and a foreign key constraint.