SQL> set linesize 200
SQL> set pagesize 200
SQL> set autot trace
SQL> select distinct department_name
from hr.departments dept, hr.employees emp
where dept.department_id = emp.department_id; 2 3
11 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 93782236
-----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 27 | 513 | 4 (25)| 00:00:01 |
| 1 | HASH UNIQUE | | 27 | 513 | 4 (25)| 00:00:01 |
| 2 | NESTED LOOPS | | 106 | 2014 | 3 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL| DEPARTMENTS | 27 | 432 | 3 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | EMP_DEPARTMENT_IX | 4 | 12 | 0 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("DEPT"."DEPARTMENT_ID"="EMP"."DEPARTMENT_ID")
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
10 consistent gets
0 physical reads
0 redo size
622 bytes sent via SQL*Net to client
419 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
11 rows processed
走的是hash unique：
Oracle10g在distinct操作时作了算法改进，使用Hash Unique 代理了以前的Sort Unique.该行为由隐藏参数”
_gby_hash_aggregation_enabled”决定，optimizer_features_enable设置为10.2.0.1时默认为TRUE.
SQL> set linesize 200
SQL> set pagesize 200
SQL> ALTER SESSION SET "_gby_hash_aggregation_enabled" = FALSE;
Session altered.
SQL> set autot trace
SQL> select distinct department_name
from hr.departments dept, hr.employees emp
where dept.department_id = emp.department_id; 2 3
11 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 1155849093
-----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 27 | 513 | 4 (25)| 00:00:01 |
| 1 | SORT UNIQUE | | 27 | 513 | 4 (25)| 00:00:01 |
| 2 | NESTED LOOPS | | 106 | 2014 | 3 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL| DEPARTMENTS | 27 | 432 | 3 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | EMP_DEPARTMENT_IX | 4 | 12 | 0 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("DEPT"."DEPARTMENT_ID"="EMP"."DEPARTMENT_ID")
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
10 consistent gets
0 physical reads
0 redo size
622 bytes sent via SQL*Net to client
419 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
11 rows processed