Expert Oracle Exadata - Parent Directory

CHAPTER 4 STORAGE INDEXESPartitions –Storage Indexes work with partitioned objects. Individualstatements can benefit from partition eliminate and storage indexes during thesame execution.Sub-queries: Storage Indexes work with predicates that compare a column to avalue returned by a sub-query.Encryption: Storage Indexes work on encrypted tables.There are of course limitations. Following are some features and syntax that prevent the use ofstorage indexes:CLOBs: Storage Indexes are not created on CLOBs.!=: Storage Indexes do not work with predicates that use the != comparisonoperator.Wildcards: Storage Indexes do not work on predicates that use the % wildcard.A further limitation is that storage indexes may contain only eight columns. They are created andmaintained for eight-columns per table. This does not mean that queries with more than 8 predicatescannot make use of Storage Indexes. In such cases, the storage software can use the indexes that exist,but by default there will be a maximum of eight columns that can be indexed. It does appear that thedevelopers have parameterized this setting, so it may be possible to change this value with help fromOracle support.Finally, bear in mind that storage indexes are not persisted to disk. The storage cell must rebuildthem whenever the cellsrv program is restarted. They are generally created during the first smart scanthat references a given column after a storage server has been restarted. They can also be created when atable is created via a CREATE TABLE AS SELECT statement, or during other direct-path loads. And ofcourse, the storage cell will update storage indexes in response to changes that applications make to thedata in the tables.PerformanceStorage Indexes provide some of the most dramatic performance benefits available on the Exadataplatform. Depending on the clustering factor of a particular column (that is, how well the column’s datais sorted on disk), the results can be spectacular. Here’s a typical example showing the performance of aquery with and without the benefit of Storage Indexes:SYS@EXDB1> alter session set cell_offload_processing=false;Session altered.SYS@EXDB1> alter session set "_kcfis_storageidx_disabled"=true;Session altered.114