3.9.4 System Index Compression
When you create a database, you can specify that Oracle Rdb compress the system indexes. To do so, use the SYSTEM INDEX COMPRESSION IS ENABLED clause of the CREATE DATABASE statement. To enable system index compression for an existing database, you must export and import the database and specify the SYSTEM INDEX COMPRESSION IS ENABLED clause with the IMPORT statement.
For system indexes, Oracle Rdb uses run-length compression, which compresses a sequence of space characters from text data types and binary zeros (also called ASCII NUL) from nontext data types. It compresses any sequences of two or more spaces for text data types or two or more binary zeros for nontext data types. See Section 18.104.22.168 for more information on run-length compression.
Compressing system indexes results in reduced storage and improved I/O. Unless your applications often perform data definition concurrently, you should use compressed system indexes.