Indexes can be declared as either clustered or unclustered. Only one index on a table can be clustered. If you determine that an index should be clustered, you do not need to drop and recreate the index: the clustering characteristic of an index can be removed or added by issuing an ALTER INDEX statement. Clustered indexes may assist performance by allowing the query optimizer to make more accurate decisions about the cost of index scans.
SQL Anywhere implements B-link indexes.