Expert Oracle Exadata - Parent Directory

CHAPTER 2 OFFLOADING / SMART SCANSQL_ID CHILD OFFLOAD IO_SAVED_% AVG_ETIME SQL_TEXT------------- ------ ------- ---------- ------------- --------------------6u7v77c2f8x5r 0 Yes 59.93 45.15 select /* multi cold43dr7hvmw3yb 0 Yes 71.85 31.94 select /* single col2 rows selected.Note that the extra column resulted in a great deal of extra time required to complete the query andthat the columns in V$SQL verified the increased volume of data that had to be transferred. We’ve alsoshown the output of a modified version of the fsx.sql script, which we’ll discuss in more detail later inthis chapter. For now, please just accept that it shows us whether a statement was offloaded or not.Predicate FilteringThe second of the big three Smart Scan optimizations is Predicate Filtering. This term refers to Exadata’sability to return only rows of interest to the database tier. Since iDB includes the predicate informationin its requests, this is accomplished by performing the standard filtering operations at the storage cellsbefore returning the data. On databases using non-Exadata storage, filtering is done on the databaseservers. This generally means that a large number of records that will eventually be discarded will bereturned to the database tier. Filtering these rows at the storage layer can provide a very significantdecrease in the volume of data that must be transferred to the database tier. While this optimization alsoresults in some savings in CPU usage on the database servers, the biggest advantage is generally thereduction in data transfer.Here is an example:SYS@SANDBOX> alter session set cell_offload_processing=false;Session altered.Elapsed: 00:00:00.01SYS@SANDBOX> select count(pk_col) from kso.skew3;COUNT(PK_COL)-------------384000036Elapsed: 00:00:48.45SYS@SANDBOX> alter session set cell_offload_processing=true;Session altered.Elapsed: 00:00:00.01SYS@SANDBOX> select count(pk_col) from kso.skew3;COUNT(PK_COL)-------------384000036Elapsed: 00:00:26.61SYS@SANDBOX> -- disable storage indexesSYS@SANDBOX> alter system set "_kcfis_storageidx_disabled"=true;32