Exchange Partition Problem

Question

Hi,
I am trying to create a partition table for a non parttition table.
create table dept_test partition by list(deptno)
(partition p10 values(10),
partition p20 values(20),
partition p30 values(30),
partition p40 values(40))
as select * from dept where 1=0;
create table dept1
as
select * from dept;
alter table DEPT_TEST
exchange partition p10
with table dept1
WITHOUT VALIDATION;
BEGIN
EXEC dbms_stats.gather_table_stats('usr','DEPT_TEST','P10',33);
EXEC dbms_stats.gather_table_stats('usr','DEPT_TEST','P20',33);
EXEC dbms_stats.gather_table_stats('usr','DEPT_TEST','P30',33);
EXEC dbms_stats.gather_table_stats('usr','DEPT_TEST','P40',33);
END;
select * from dba_tab_partitions where table_name='DEPT_TEST';
SELECT * FROM DEPT_TEST WHERE DEPTNO=10;
SELECT * FROM DEPT_TEST WHERE DEPTNO=20;
In the dba_tab_partitions,row count for p10 is showing as 4. when i ran the query for the value 10 ,i got 4 row but for 20 no rows.
Can you please tell me what is the problem?
Thanks.

Answer 1

Partition exchange moved all the data to partition p10. Partitions p20, p30 and p40 are empty. Evaluating condition WHERE DEPTNO = 10 optimizer relys on the partition definition VALUES(10). It's enough to do partition pruning only to satisfy the condition. The same partition pruning takes place for the condition DEPTNO = 20, but partition p20 is empty and the query returns no rows.
SQL> set autotrace on explain
SQL> SELECT * FROM DEPT_TEST WHERE DEPTNO=10;
    DEPTNO DNAME          LOC
        10 ACCOUNTING     NEW YORK
        20 RESEARCH       DALLAS
        30 SALES          CHICAGO
        40 OPERATIONS     BOSTON
Execution Plan
Plan hash value: 2891473902
| Id  | Operation             | Name      | Rows  | Bytes | Cost (%CPU)| Time     | Pstart| Pstop |
|   0 | SELECT STATEMENT      |           |     1 |    20 |     3   (0)| 00:00:01 |       |       |
|   1 |  PARTITION LIST SINGLE|           |     1 |    20 |     3   (0)| 00:00:01 |     1 | 1 |
|   2 |   TABLE ACCESS FULL   | DEPT_TEST |     1 |    20 |     3   (0)| 00:00:01 |     1 | 1 |
SQL> SELECT * FROM DEPT_TEST WHERE DEPTNO=20;
no rows selected
Execution Plan
Plan hash value: 2891473902
| Id  | Operation             | Name      | Rows  | Bytes | Cost (%CPU)| Time     | Pstart| Pstop |
|   0 | SELECT STATEMENT      |           |     1 |    30 |     2   (0)| 00:00:01 |       |       |
|   1 |  PARTITION LIST SINGLE|           |     1 |    30 |     2   (0)| 00:00:01 |     2 | 2 |
|   2 |   TABLE ACCESS FULL   | DEPT_TEST |     1 |    30 |     2   (0)| 00:00:01 |     2 | 2 |
---------------------------------------------------------------------------------------------------Regards,
Dima
Message was edited by:
dimacit