Hi all !
I've encountered a very strange problem on a very simple query...

Here is the query :
select * from bigtable where col1='XXX'

The col1 columns is indexed (with * bitmap index because of the low cardinality of the column).

Both the table and index are freshly analyzed.

test1 :
When I try to explain plan, I realize that the CBO make an access full to bigtable table and it takes 1 minute to execute.

test2 :
When I add a INDEX hint (select /*+ INDEX (bigtable idx_col1) */ ...), the index is used and it takes less than 1 second to execute !

test3 :
When I set the optimizer_index_cost_adj to a low value than the default one (from 60 to 1), the explain plan shows that the index is well used, BUT the query takes one minute to execute ...

1) With exactly the same execution plan (test2 and 3) but different cost, we obtain different response time. How could it be possible if the plans are the same ?
2) Why the CBO does not choose the good plan itself (test2), whereas it's a very simple query ...

Thanks a lot in advance for your replies ...