Expert Oracle Exadata - Parent Directory

C H A P T E R 3Hybrid Columnar CompressionHybrid Columnar Compression (HCC) is probably one of the least understood of the features that areunique to Exadata. The feature was originally rolled out in a beta version of 11gR2 and was enabled onboth Exadata and non-Exadata platforms. The production release of 11gR2 restricted the feature toExadata platforms only. Ostensibly this decision was made because the additional processing poweravailable on the Exadata storage servers. Because the feature is restricted to Exadata, the recentdocumentation refers to it as Exadata Hybrid Columnar Compression (EHCC).This chapter will first present basic information on both of Oracle’s older row-major formatcompression methods (BASIC and OLTP). We will then explore in detail the mechanics of Oracle’s newHCC feature—how it works, including how the data is actually stored on disk, where and when thecompression and decompression occur, and when its use is appropriate. Finally we’ll provide some realworld examples of HCC usage.Oracle Storage ReviewAs you probably already know, Oracle stores data in a block structure. These blocks are typically 8K andfrom a simplistic point of view they consist of a block header, a row directory, row data and free space.The block header starts at the top of a block and works its way down, while the row data starts at thebottom and works its way up. The free space generally sits in the middle. Figure 3-1 shows arepresentation of a traditional Oracle block format.65