Expert Oracle Exadata - Parent Directory

CHAPTER 3 HYBRID COLUMNAR COMPRESSIONBlock Flagged forOLTP CompressionPointer toUpdated RowFree SpaceFree SpaceUncompressedUpdated RowFigure 3-4. Row migrationThis diagram represents a situation where the entire row has been relocated, leaving behind only apointer to the new location.Oracle Compression MechanismsOracle provides several compression mechanisms in addition to HCC. The naming is somewhatconfusing due to the common marketing terms and some changes in nomenclature along the way. Herewe’ll refer to the three main flavors of compression used by Oracle as BASIC, OLTP, and HCC.BASICThis compression method is a base feature of Oracle Database 11g Enterprise Edition. It compressesdata only on direct path loads. Modifications force the data to be stored in an uncompressed format, asdo inserts that do not use the direct path load mechanism. Rows are still stored together in the normalrow-major form. The compression unit is a single Oracle block. BASIC is the default compressionmethod, from a syntax standpoint. For example, BASIC compression will be used if you issue thefollowing command:CREATE TABLE … COMPRESS;Basic compression was introduced in Oracle Database version 9i. This form of compression was alsoreferred to as DSS Compression in the past. The syntax COMPRESS FOR DIRECT_LOAD OPERATIONS can stillbe used to enable BASIC compression, although this syntax has now been deprecated.OLTPThe OLTP compression method allows data to be compressed for all operations, not just direct pathloads. It is part of an extra-cost option called Advanced Compression and was introduced in Oracle68