Resolution
In general, encrypted data is not compressible. Oracle speaks to the compression limitations of TDE in the following documents. TDE data will be compressed prior to encryption only if TDE tablespace encryption is used, but not if TDE column encryption is used.
From Oracle's "Transparent Data Encryption (TDE) Frequently Asked Questions":
Does compression work with TDE?
Customers using TDE tablespace encryption get the full benefit of compression (standard and Advanced Compression, as well as Exadata Hybrid Columnar Compression (EHCC)) because compression is applied before the data blocks are encrypted. Customers using TDE column encryption will get the full benefit of compression only on table columns that are not encrypted. Individual table columns that are encrypted using TDE column encryption will have a much lower level of compression because the encryption takes place in the SQL layer before the advanced compression process.
And in
"Oracle Database – Compression Best Practices"
Compression DOs
- Works best with tablespace encryption
- With tablespace encryption, table and index compression is done before encryption
- With column encryption, compression is done after encryption -- compression will have minimal effectiveness on encrypted columns
