HCC compression benefit

Problem statement: Lets assume you have a very big partitioned table of size more than 20 TB which has a local partitioned index of same size. Even though purge routine from application is in place but due to some regulatory compliance you need to maintain certain number of years of old data. Application hardly makes any read/write data which is older than 24 months. Database size has been growing and you are about to reach 85% warning  limit of storage. There is no option to expand storage as per Oracle database appliance aka ODA configuration

Current environment: ODA X8-2 with GI version as 19.18