Hi
No RMAN by default doesnt compress backup sets you need to tell it explicitly using the
Code:
configure command or using backup up as compressed
Compression ratio varies you need to test it in your specific case
(in my case the compression was 63% to 80% of the original size in some cases but without parallelism ) (it was also 5 times slower in 10gr2 hehe)
Compression would be CPU intensive because the compression algorithm would come into play and has to execute it to make your backups smaller
It would not do additional compression .
RMAN in 11i i have heard is better when it comes to compression .
regards
Hrishy