Why is this query not using my index ?

Question

hey
i have the following situation
(i'm sorry but i can't add the acctual script or output)
create table x as
select      level high_card ,
     mod(level,1000) medium_card ,
     mod(level,10) low_card ,
     '***' padding
from     dual
connect by level < 100001;
create index x_med_low_ix on x(low_card,medium_card);
create index x_high_ix on x(high_card);
exec dbms_stats.gather_table_stats(user,'x',cascade=>true,method_opt=>'for all indexed columns size auto');
i'm running the following query
select      *
from      x
where      low_card = 70
and     medium_card = 70
and     high_card = 70;
i'm expecting a range scan on the x_high_ix index. the optimizer expects only one row. the stats on the high_card columns shows that (num_rows * density = 1).
when i run the query the optimizer uses the x_med_low_ix and does 14 cr.
when i force the use of x_high_ix the cr goes down to 4.
i don't want to declare x_high_ix as unique.
so, why isn't he using my index ?

Answer 1

It's using that index for me:
SQL> create table x as
  2  select level high_card ,
  3  mod(level,1000) medium_card ,
  4  mod(level,10) low_card ,
  5  '***' padding
  6  from dual
  7  connect by level < 100001;
Table created.
SQL>
SQL> create index x_med_low_ix on x(low_card,medium_card);
Index created.
SQL>
SQL> create index x_high_ix on x(high_card);
Index created.
SQL>
SQL> exec dbms_stats.gather_table_stats(user,'x',cascade=>true,method_opt=>'for all indexed columns
size auto');
PL/SQL procedure successfully completed.
SQL> ed
Wrote file afiedt.buf
  1   select *
  2   from x
  3   where low_card = 70
  4   and medium_card = 70
  5*  and high_card = 70
SQL>
SQL> /
no rows selected
Execution Plan
Plan hash value: 775193209
| Id  | Operation                   | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT            |           |     1 |    15 |     2   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS BY INDEX ROWID| X         |     1 |    15 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | X_HIGH_IX |     1 |       |     1   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   1 - filter("LOW_CARD"=70 AND "MEDIUM_CARD"=70)
   2 - access("HIGH_CARD"=70)
Statistics
          0  recursive calls
          0  db block gets
          3  consistent gets
          0  physical reads
          0  redo size
        454  bytes sent via SQL*Net to client
        370  bytes received via SQL*Net from client
          1  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          0  rows processed
SQL> set autot off
SQL> select * from v$version;
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Prod
PL/SQL Release 10.2.0.1.0 - Production
CORE    10.2.0.1.0      Production
TNS for 32-bit Windows: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - Productionwhat is your version?
By the way, when i force optimizer to use index X_MED_LOW_IX, it does 2 CR for me.