Order of indexed columns in the index

Question

Hi Gurus,
I have a small problem in front of me and I wud like to take some inputs
from you Gurus.Here is question
1. Is order of the index is important in CBO?
say
1.I have a concatenated index on col1,col2
2.In the sql statement's used in my project I have statements "where
col1=1 and col2 = 1" as well as
"where col2=1 and col1=1" and my opinion is that CBO will use the Index
in both the scenarios( I have cheked RBO for the same and RBO is able to
use the index) whereas some senior persons out here are telling
otherwise..... that it will not use the index for "where col2=1 and
Col1=1)
pls clarify
Cheers
Sriram Kumar

Answer 1

When I attended an Oracle 8i SQL Statement Tuning Workshop, I believe I understood things as in the response above. However, the class was applicable to Oracle 8.1.5 and I am now using Oracle 8.1.7, so some things may have changed. I have included a test of various scenarios below. I would have expected all except the last one to use the index, because col2 is not the leading edge of the composite index on col1, col2. Much to my surprise it also used the index there.
SQL> CREATE TABLE test_table AS
  2  SELECT empno col1, mgr col2 FROM emp
  3  /
Table created.
SQL> UPDATE test_table
  2  SET    col1 = 1, col2 = 1
  3  WHERE  col1 = 7782
  4  AND    col2 = 7839
  5  /
1 row updated.
SQL> CREATE INDEX test_idx
  2  ON test_table (col1, col2)
  3  /
Index created.
SQL> ANALYZE TABLE test_table
  2  COMPUTE STATISTICS
  3  FOR TABLE
  4  FOR ALL INDEXES
  5  FOR ALL INDEXED COLUMNS
  6  /
Table analyzed.
SQL> SET AUTOTRACE ON EXPLAIN
SQL> SELECT * FROM test_table
  2  WHERE col1 = 1 AND col2 = 1
  3  /
      COL1       COL2                                      
         1          1                                      
1 row selected.
Execution Plan
   0                                                       
SELECT STATEMENT Optimizer=CHOOSE (Cost=1 Card=1 Bytes=6)  
   1    0                                                  
  INDEX (RANGE SCAN) OF 'TEST_IDX' (NON-UNIQUE) (Cost=1 Card
=1 Bytes=6)                                                
SQL> SELECT * FROM test_table
  2  WHERE col2 = 1 AND col1 = 1
  3  /
      COL1       COL2                                      
         1          1                                      
1 row selected.
Execution Plan
   0                                                       
SELECT STATEMENT Optimizer=CHOOSE (Cost=1 Card=1 Bytes=6)  
   1    0                                                  
  INDEX (RANGE SCAN) OF 'TEST_IDX' (NON-UNIQUE) (Cost=1 Card
=1 Bytes=6)                                                
SQL> SELECT * FROM test_table
  2  WHERE col1 = 1
  3  /
      COL1       COL2                                      
         1          1                                      
1 row selected.
Execution Plan
   0                                                       
SELECT STATEMENT Optimizer=CHOOSE (Cost=1 Card=1 Bytes=6)  
   1    0                                                  
  INDEX (RANGE SCAN) OF 'TEST_IDX' (NON-UNIQUE) (Cost=1 Card
=1 Bytes=6)                                                
SQL> SELECT * FROM test_table
  2  WHERE col2 = 1
  3  /
      COL1       COL2                                      
         1          1                                      
1 row selected.
Execution Plan
   0                                                       
SELECT STATEMENT Optimizer=CHOOSE (Cost=1 Card=1 Bytes=6)  
   1    0                                                  
  INDEX (FULL SCAN) OF 'TEST_IDX' (NON-UNIQUE) (Cost=1 Card=
1 Bytes=6)