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Oracle has introduced several types of compression. The 11g data compression is threshold-based and allows more rows per data block.

Historical External Compression:

The blocks are compressed outbound and uncompressed before presenting to the database in historical external compression. It is far simpler because all index objects are treated the same way, whereas with the 11g table compression, a data block may contain both compressed and uncompressed row data.

Oracle Data Buffers:

Within the data buffers, the fully uncompressed version of the data remains, even though the information remains compressed on the disk data blocks themselves.  This leads to a discrepancy between the size of information on the data blocks and the size of the information within the data buffers.  Oracle data compression allows more rows to fit on a data block of a given size, but there is still no impact on the data base management system from the point of view of the SGA.

Because the decompression routine is called upon block fetch, the Oracle data buffers remain largely unchanged while the data blocks themselves tend to have a lot more data on them. 

Oracle 11g new inline data compression utility:

Oracle 11g new inline data compression utility promises these benefits:

• - Depending on the nature of your data, Oracle compression will result in huge savings on disk space. 
• - Because compressed tables reside on fewer disk blocks, shops that might not otherwise be able to afford solid-state flash disk can now enjoy I/O speeds up to 300x faster than platter disk.  
• - Because tables will reside on less data blocks, full table scans and index range scans can retrieve the rows with less disk I/O.
• - Because the data blocks are compressed/decompressed only within Oracle, the external network packets will be significantly smaller.

Oracle 11g compression overhead:

Oracle 11g data compression offers huge benefits, but the exact overhead costs remain unknown. 

The compress/decompress operations are computationally intensive but super small.  This CPU overhead might be significantly measurable, but we can assume that the overhead will be the same (or smaller) than data compression in legacy databases.  In a perfect implementation, incoming data would only be decompressed once and the uncompressed copy of the disk block would reside in RAM, thereby minimizing changes to the Oracle kernel code.

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