Please explain plan with 'BITMAP CONVERSION TO ROWIDS'

Question

Hi,
in my 9.2.0.8 I've got plan like :
Plan
SELECT STATEMENT  CHOOSECost: 26,104                           
     7 TABLE ACCESS BY INDEX ROWID UMOWY Cost: 26,105  Bytes: 41  Cardinality: 1                      
          6 BITMAP CONVERSION TO ROWIDS                 
               5 BITMAP AND            
                    2 BITMAP CONVERSION FROM ROWIDS       
                         1 INDEX RANGE SCAN UMW_PRD_KPD_KOD Cost: 406  Cardinality: 111,930 
                    4 BITMAP CONVERSION FROM ROWIDS       
                         3 INDEX RANGE SCAN UMW_PRD_KPR_KOD Cost: 13,191  Cardinality: 111,930  as far as I know Oracle is trying to combine two indexes , so if I create multicolumn index the plan should be better right ?
Generally all bitmap conversions related to b-tree indexes are trying to combine multiple indexes to deal with or/ index combine operations right ?
And finally what about AND_EQUAL hint is that kind of alternative for that bitmap conversion steps ?
Regards
Greg

Answer 1

as far as I know Oracle is trying to combine two indexes , so if I create multicolumn index the plan should be better right ?Only you can really tell - but if this is supposed to be a "precision" query the optimizer thinks you don't have a good index into the target data. Don't forget to consider the benefits of compressed indexes if you do follow this route.
Generally all bitmap conversions related to b-tree indexes are trying to combine multiple indexes to deal with or/ index combine operations right ?Bitmap conversions when there are no real bitmap indexes involved are always about combining multiple b-tree index range scans to minimise the number of reads from the table.
And finally what about AND_EQUAL hint is that kind of alternative for that bitmap conversion steps ?AND_EQUAL was an older mechanism for combining index range scans to minimise visits to the table - it was restricted to a maximum of 5 indexes per table - the indexes had to be single column, non-unique, and the predicates had to be equality. The access method is deprecated in 10g. (See the following note, and the comments in particular, for more details: http://jonathanlewis.wordpress.com/2009/05/08/ioug-day-4/ )
Regards
Jonathan Lewis