By using a data compression algorithm specifically designed for relational data, Oracle is able to compress data much more effectively than standard compression techniques. More significantly, unlike other compression techniques, Oracle incurs virtually no performance penalty for SQL queries accessing compressed tables.
Compression reduces the amount of data being stored, reduces memory usage (more data per memory block), and increases query performance.
Although Oracle Database supports compression for all DML operations, it is still more efficient to modify data in a noncompressed table.
You can specify table compression by using the COMPRESS clause of the CREATE TABLE statement or you can enable compression for an existing table by using ALTER TABLE statement as shown below.
alter table <tablename> move compress;