I have a couple of questions on Compression on Exadata.
1. Is there a query/way to estimate the amount of compression that can be done?
2. How would we be able to define which compression is best done ?
The My Oracle Support Community which covers all support related questions about EXADATA is available under: https://communities.oracle.com/portal/server.pt/community/exadata_general/360
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Additional details about the availability of the Compression Advisor are available on the My Oracle Support Community for Exadata
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Incidentally I have covered your two questions with blog posts:
"Don't believe it, test it!"