Expert Oracle Exadata - Parent Directory

CHAPTER 16 UNLEARNING SOME THINGS WE THOUGHT WE KNEWFurthermore, “offloadable” functions can also impose large performance penalties. Here’s a verysimple example showing the negative effect of an offloadable function in a WHERE clause:SYS@SANDBOX1> select count(*) from kso.skew3 where col2='KSO';COUNT(*)----------9Elapsed: 00:00:00.13SYS@SANDBOX1> select count(*) from kso.skew3 where upper(col2)='KSO';COUNT(*)----------9Elapsed: 00:00:13.16SYS@SANDBOX1> @fsx4Enter value for sql_text: select count(*) from kso.skew3 where%KSO%Enter value for sql_id:SQL_ID CHILD OFFLOAD IO_SAVED_% AVG_ETIME SQL_TEXT--------------------------------------------------------------------------------------0u8gnxmm7cbdk 0 Yes 100.00 .12 select count(*) from kso.skew3 where col37t7fzkajz2fc 0 Yes 99.99 13.16 select count(*) from kso.skew3 where uppElapsed: 00:00:00.08So UPPER is an offloadable function. Yet when we used it in the WHERE clause of this particularstatement, the result was a large degradation in performance. If you’ve been following along on-screen,you may already have a pretty good idea why.SYS@SANDBOX1> select name, value from v$mystat s, v$statname n2 where n.statistic# = s.statistic#3 and name like '%storage%';NAMEVALUE--------------------------------------------------------------------------------cell physical IO bytes saved by storage index 0SYS@SANDBOX1> set timing onSYS@SANDBOX1> select count(*) from kso.skew3 where upper(col2)='KSO';517