You can create join indexes that limit the number of rows in the index to only those that are accessed when, for example, a frequently run query references only a small, well known subset of the rows of a large base table. By using a constant expression to filter the rows included in the join index, you can create what is known as a sparse index.
Any join index, whether simple or aggregate, multitable or single-table, can be sparse.
To create a sparse index, use the WHERE clause in the CREATE JOIN INDEX statement.