"It means,after delta merge, data in main memory and disk are compressed one with the same compression techniques."
No, that doesn't follow from anything I wrote at all.
Typically the actual compression of the data structures on memory and on disk are very different.
But data that is stored in the main store gets persisted in the main store persistence while the delta store is stored differently.
After all, it really doesn't matter that much how the data is stored to disk.
The primary storage is memory for SAP HANA, so whatever ends up on disk is equivalent to a database backup.
- Lars