You can prevent and remove fragmentation:
To prevent fragmentation, optimize data loads by sorting load records based on sparse dimension members. For a comprehensive discussion of optimizing data load by grouping sparse members, see Grouping Sparse Member Combinations.
To remove fragmentation, perform an export of the database, delete all data in the database with CLEARDATA, and reload the export file. See the Oracle Hyperion Enterprise Performance Management System Backup and Recovery Guide.
To remove fragmentation, force a dense restructure of the database. See Types of Database Restructuring.