OR is taking much more time than UNION

Question

hi gems..
i have written a query using UNION clause and it took 12 seconds to give result.
then i wrote the same query using OR operator and then it took 78 seconds to give the resultset.
The tables which are referred by this qurey have no indexes.
the cost plans for the query with OR is also much more lesser than that with UNION.
please suggest why OR is taking more time.
thanks in advance

Answer 1

Here's a ridiculously simple example.  (these tables don't even have any rows in them)
If you had separate indexes on col1 and col2, the optimizer might use indexes in the union but not in the or statement:
Which is faster will depend on the usual list of things.
Of course, the union also requires a sort operation.
SQL> create table table1
  2  (col1 number, col2 number, col3 number, col4 number);
Table created.
SQL> create index t1_idx1 on table1(col1);
Index created.
SQL> create index t1_idx2 on table1(col2);
Index created.
SQL> explain plan for
  2  select col1, col2, col3, col4
  3  from table1
  4  where col1> = 123
  5  or col2 <= 456;
Explained.
SQL> @xp
| Id  | Operation         | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT  |        |     1 |    52 |     2   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| TABLE1 |     1 |    52 |     2   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   1 - filter("COL1">=123 OR "COL2"<=456)
SQL> explain plan for
  2  select col1, col2, col3, col4
  3  from table1
  4  where col1 >= 123
  5  union
  6  select col1, col2, col3, col4
  7  from table1
  8  where col2 <= 456;
Explained.
SQL> @xp
| Id  | Operation                     | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT              |         |     2 |   104 |     4  (75)| 00:00:01 |
|   1 |  SORT UNIQUE                  |         |     2 |   104 |     4  (75)| 00:00:01 |
|   2 |   UNION-ALL                   |         |       |       |            |          |
|   3 |    TABLE ACCESS BY INDEX ROWID| TABLE1  |     1 |    52 |     1   (0)| 00:00:01 |
|*  4 |     INDEX RANGE SCAN          | T1_IDX1 |     1 |       |     1   (0)| 00:00:01 |
|   5 |    TABLE ACCESS BY INDEX ROWID| TABLE1  |     1 |    52 |     1   (0)| 00:00:01 |
|*  6 |     INDEX RANGE SCAN          | T1_IDX2 |     1 |       |     1   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   4 - access("COL1">=123)
   6 - access("COL2"<=456)