Expert Oracle Exadata - Parent Directory

CHAPTER 3 HYBRID COLUMNAR COMPRESSIONOWNER SEGMENT_NAME PART_NAME TOTALSIZE_MEGS COMPRESS_FOR-------------------- -------------------- -------------------- -------------- ------------KSO CLASS_SALES_P P1 24.0P224.0 QUERY HIGHP324.0 ARCHIVE LOW******************** ******************** --------------sum 72.0SummaryIntroduced in Oracle 11g R2, Hybrid Columnar Compression provides exceptional compressioncapabilities that are far beyond anything available in prior releases. This is thanks in large part to theadoption of industry-standard compression algorithms and an increase in the size of the compressionunit from a single database block (typically 8K) to a larger unit of 32K or 64K. The feature is onlyappropriate for data that is no longer being modified, though, because of locking issues and the fact thatupdated rows are moved into a much less compressed format (OLTP compression format). For thisreason, HCC should only be used with data that is no longer being modified (or only occasionallymodified). Since compression can be defined at the partition level, it is common to see tables that have amixture of compressed and uncompressed partitions. This technique can in many cases replace ILMapproaches that require moving data to alternate storage media and then purging it from the database.104