Using Composite Index Performance Issue

Question

Hi,
Need help:
I have a table 'TABLEA' which has composite index on CODE and DEPT_CODE
While fetching the data from the table 'TABLEA' in the WHERE condition I am using only CODE and not DEPT_CODE.
Is the usage of the WHERE condition by using only the CODE column and not DEPT_CODE column affects the performance?
Any help will be needful for me

Answer 1

See the test case below
SQL> create table test_emp
  2  (
  3  emp_ssn number,
  4  emp_name varchar2(50),
  5  emp_state varchar2(15),
  6  emp_city varchar2(20)
  7  );
Table created
SQL> create index test_emp_idx on test_emp(emp_ssn,emp_name);
Index created
SQL> insert into test_emp values (123456789,'Robben','New York','Buffalo');
1 row inserted
SQL> insert into test_emp values (223456789,'Jack','Florida','Miami');
1 row inserted
SQL> insert into test_emp values (323456789,'Peter','Texas','Dallas');
1 row inserted
SQL> insert into test_emp values (423456789,'Johny','Georgia','Atlanta');
1 row inserted
SQL> insert into test_emp values (523456789,'Carmella','California','San Diego');
1 row inserted
SQL> commit;
Commit complete
SQL> explain plan for select /*+ index(test_emp test_emp_idx) */ * from test_emp where emp_ssn = 323456789;
Explained
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
Plan hash value: 2345760695
| Id  | Operation                   | Name         | Rows  | Bytes | Cost (%CPU)
|   0 | SELECT STATEMENT            |              |     1 |    61 |   163   (1)
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST_EMP     |     1 |    61 |   163   (1)
|*  2 |   INDEX RANGE SCAN          | TEST_EMP_IDX |     1 |       |     2   (0)
Predicate Information (identified by operation id):
   2 - access("EMP_SSN"=323456789)
Note
   - dynamic sampling used for this statement
18 rows selected
SQL> explain plan for select /*+ INDEX_SS(test_emp test_emp_idx) */ * from test_emp where emp_name = 'Robben';
Explained
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
Plan hash value: 85087452
| Id  | Operation                   | Name         | Rows  | Bytes | Cost (%CPU)
|   0 | SELECT STATEMENT            |              |     1 |    61 |    15   (0)
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST_EMP     |     1 |    61 |    15   (0)
|*  2 |   INDEX SKIP SCAN           | TEST_EMP_IDX |     1 |       |    11   (0)
Predicate Information (identified by operation id):
   2 - access("EMP_NAME"='Robben')
       filter("EMP_NAME"='Robben')
Note
   - dynamic sampling used for this statement
19 rows selectedThanks,
Andy