oracle index in the underlying sql query - oracle

Question

I have a table with 2 columns
item_name varchar2
brand varchar2
both of them have bitmap index
let's say I create a view for a specific brand and rename the column item_name ,something like that
create view my_brand as
select item_name as item from table x where brand='x'
We cannot create an index on a normal view but what is Oracle doing when issuing the underlying query of that view? Is the index of the item_name column being used if we write select item from my_brand where item='item1'?
thanks

Answer 1

The answer will be “it depends”. The index access path is certainly an option open to the optimizer; but remember that the optimizer makes a cost based decision. So essentially it will evaluate the cost of all the available plans and choose the one with the lowest cost.
Here is an example:
create table tab1 ( item_name varchar2(15), brand varchar2(15) );
insert into tab1
select 'Name '||to_char( rownum), 'Brand '||to_char(mod(rownum,10))
from dual
connect by rownum < 1000000
commit;
exec dbms_stats.gather_table_stats( user, 'TAB1' );
create bitmap index bm1 on tab1 ( item_name );
create bitmap index bm2 on tab1 ( brand );
create or replace view my_brand
as select item_name as item from tab1 where brand = 'Brand 1';
explain plan for
select item from my_brand where item = 'Name 1001'
select * from table( dbms_xplan.display )
--------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 20 | 3 (0)| 00:00:01 |
|* 1 | TABLE ACCESS BY INDEX ROWID BATCHED| TAB1 | 1 | 20 | 3 (0)| 00:00:01 |
| 2 | BITMAP CONVERSION TO ROWIDS | | | | | |
|* 3 | BITMAP INDEX SINGLE VALUE | BM1 | | | | |
--------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("BRAND"='Brand 1')
3 - access("ITEM_NAME"='Name 1001')

Answer 2

We can create an index on a normal view
No, you can't
SQL> create table as_idx_view (item_name varchar2(10), brand varchar2(10));
Table created.
SQL> create view as_view as select item_name item from as_idx_view where brand = 'X';
View created.
SQL> create index as_idx_view_idx1 on as_view (item);
create index as_idx_view_idx1 on as_view (item)
*
ERROR at line 1:
ORA-01702: a view is not appropriate here