How to assure index is being used for Query

Question

Hi All,
Indexed are best way to speedup the data retrieval process selectively.
however i wanted to how to trace that weather index is being used in a particular query.
1. Table tab1 has composite index on a and b.
2. Select a,b,c from tab1 where a>500 ;
Will this query use index create from 1 ?
Conceptually it should not because where as part contains only field A where as index is composit on field A and B.
However , i wanted to know to trace that a perticual query is using index or not ?
This question was asked to me in an interview.
Pls le me know at [email protected]
Thanks in Advance
Alok

Answer 1

Hi,
Use explain plan to know if index is use.
As you can see in the following example, even if index is composed and I use only a part in query, index can be used :
SQL> create table test (col1 number, col2 number, col3 varchar2(10));
Table created.
SQL> insert into test select object_id, data_object_id, substr(object_name,1,10) from dba_objects;
70211 rows created.
SQL> create index idx on test(col1,col2);
Index created.
SQL> explain plan for
  2  select col1,col2,col3 from test where col1 > 500;
Explained.
SQL> @$ORACLE_HOME/rdbms/admin/utlxpls
PLAN_TABLE_OUTPUT
| Id  | Operation                   |  Name       | Rows  | Bytes | Cost  |
|   0 | SELECT STATEMENT            |             |       |       |       |
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST        |       |       |       |
|*  2 |   INDEX RANGE SCAN          | IDX         |       |       |       |
Predicate Information (identified by operation id):
PLAN_TABLE_OUTPUT
   2 - access("TEST"."COL1">500)
       filter("TEST"."COL1">500)
Note: rule based optimization
16 rows selected.
--If stats are collected, index is no more used
SQL> analyze table test compute statistics;
SQL> explain plan for
  2  select col1,col2,col3 from test where col1 > 500;
Explained.
SQL> @$ORACLE_HOME/rdbms/admin/utlxpls
PLAN_TABLE_OUTPUT
| Id  | Operation            |  Name       | Rows  | Bytes | Cost  |
|   0 | SELECT STATEMENT     |             | 69808 |  1227K|    16 |
|*  1 |  TABLE ACCESS FULL   | TEST        | 69808 |  1227K|    16 |
Predicate Information (identified by operation id):
PLAN_TABLE_OUTPUT
   1 - filter("TEST"."COL1">500)
Note: cpu costing is off
14 rows selected.
--But if you use an equlity in your query instead of >, index is still used
SQL> explain plan for
  2  select col1,col2,col3 from test where col1 = 500;
Explained.
SQL> @$ORACLE_HOME/rdbms/admin/utlxpls
PLAN_TABLE_OUTPUT
| Id  | Operation                   |  Name       | Rows  | Bytes | Cost  |
|   0 | SELECT STATEMENT            |             |     1 |    18 |     3 |
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST        |     1 |    18 |     3 |
|*  2 |   INDEX RANGE SCAN          | IDX         |     1 |       |     2 |
Predicate Information (identified by operation id):
PLAN_TABLE_OUTPUT
   2 - access("TEST"."COL1"=500)
Note: cpu costing is off
15 rows selected.
SQL> Nicolas.