hint之qb_name

http://www.thinkindata.com/?p=34

该hint用于子查询(query_block)   
很多的情况下，如果子查询共用相同的别名（alias）， 可以通过设定不同的qb_name分别标识子查询。

实例如下

SQL*Plus: Release 10.2.0.1.0 – Production on Sat Dec 19 22:16:13 2009

。。。。。

43@dyl@TEST>EXPLAIN PLAN SET statement_id = ‘qb_name’
 FOR
     SELECT /*+ QB_NAME(outer) */
            e.ename
     ,      e.sal
     FROM (
            SELECT /*+ QB_NAME(inline_view) */
                   *
            FROM   emp e
            WHERE e.sal > 300
            AND    e.deptno IN ( SELECT /*+ QB_NAME(dept_subquery) */ d.deptno
                                 FROM   dept d
                                 WHERE d.dname IN (‘SALES’,'ACCOUNTING’) )
           ) e;

Explained.

Elapsed: 00:00:00.17

使用10g下的DBMS_XPLAN.DISPLAY，注意必须使用ALL参数，才能看得到自定义的query_block name

43@dyl@TEST>SELECT plan_table_output
 FROM   TABLE(DBMS_XPLAN.DISPLAY(‘PLAN_TABLE’,'qb_name’,’ALL‘));

PLAN_TABLE_OUTPUT
——————————————————————————————————————————————————
Plan hash value: 351108634

—————————————————————————————-
| Id | Operation                    | Name    | Rows | Bytes | Cost (%CPU)| Time     |
—————————————————————————————-
|   0 | SELECT STATEMENT             |         |     9 |   234 |     4   (0)| 00:00:01 |
|   1 | NESTED LOOPS                |         |     9 |   234 |     4   (0)| 00:00:01 |
|* 2 |   TABLE ACCESS FULL          | EMP     |    14 |   182 |     3   (0)| 00:00:01 |
|* 3 |   TABLE ACCESS BY INDEX ROWID| DEPT    |     1 |    13 |     1   (0)| 00:00:01 |
|* 4 |    INDEX UNIQUE SCAN         | PK_DEPT |     1 |       |     0   (0)| 00:00:01 |
—————————————————————————————-

Query Block Name / Object Alias (identified by operation id):
————————————————————-

1 – SEL$B9DAFA34
2 – SEL$B9DAFA34 / E@INLINE_VIEW
3 – SEL$B9DAFA34 / D@DEPT_SUBQUERY
4 – SEL$B9DAFA34 / D@DEPT_SUBQUERY

Predicate Information (identified by operation id):
—————————————————

2 – filter(“E”.”SAL”>300)
3 – filter(“D”.”DNAME”=’ACCOUNTING’ OR “D”.”DNAME”=’SALES’)
4 – access(“E”.”DEPTNO”=”D”.”DEPTNO”)

Column Projection Information (identified by operation id):
———————————————————–

1 – (#keys=0) “E”.”ENAME”[VARCHAR2,10], “E”.”SAL”[NUMBER,22]
2 – “E”.”ENAME”[VARCHAR2,10], “E”.”SAL”[NUMBER,22], “E”.”DEPTNO”[NUMBER,22]
4 – “D”.ROWID[ROWID,10]

33 rows selected.

Elapsed: 00:00:00.05
在定义了qb_name之后最大的好处就是再设定其他hint时，可以指定qb_name，

语法相对更清晰一些，在非常复杂的select语句时不易加错hint。

43@dyl@TEST>EXPLAIN PLAN SET statement_id = ‘qb_hints’
 FOR
     SELECT /*+ QB_NAME(outer) FULL(@dept_subquery d) */
            e.ename
     ,      e.sal
     FROM (
            SELECT /*+ QB_NAME(inline_view) */
                   *
            FROM   emp e
            WHERE e.sal > 300
            AND    e.deptno IN ( SELECT /*+ QB_NAME(dept_subquery) */ d.deptno
                                 FROM   dept d
                                 WHERE d.dname IN (‘SALES’,'ACCOUNTING’) )
           ) e;
Explained.

Elapsed: 00:00:00.01
43@dyl@TEST>SELECT plan_table_output
 FROM   TABLE(DBMS_XPLAN.DISPLAY(‘PLAN_TABLE’,'qb_hints’,'ALL’));

PLAN_TABLE_OUTPUT
——————————————————————————————————————————————————
Plan hash value: 615168685

—————————————————————————
| Id | Operation          | Name | Rows | Bytes | Cost (%CPU)| Time     |
—————————————————————————
|   0 | SELECT STATEMENT   |      |     9 |   234 |     7 (15)| 00:00:01 |
|* 1 | HASH JOIN         |      |     9 |   234 |     7 (15)| 00:00:01 |
|* 2 |   TABLE ACCESS FULL| DEPT |     2 |    26 |     3   (0)| 00:00:01 |
|* 3 |   TABLE ACCESS FULL| EMP |    14 |   182 |     3   (0)| 00:00:01 |
—————————————————————————

Query Block Name / Object Alias (identified by operation id):
————————————————————-

1 – SEL$B9DAFA34
2 – SEL$B9DAFA34 / D@DEPT_SUBQUERY
3 – SEL$B9DAFA34 / E@INLINE_VIEW

Predicate Information (identified by operation id):
—————————————————

1 – access(“E”.”DEPTNO”=”D”.”DEPTNO”)
2 – filter(“D”.”DNAME”=’ACCOUNTING’ OR “D”.”DNAME”=’SALES’)
3 – filter(“E”.”SAL”>300)

Column Projection Information (identified by operation id):
———————————————————–

1 – (#keys=1) “E”.”ENAME”[VARCHAR2,10], “E”.”SAL”[NUMBER,22]
2 – “D”.”DEPTNO”[NUMBER,22]
3 – “E”.”ENAME”[VARCHAR2,10], “E”.”SAL”[NUMBER,22],
“E”.”DEPTNO”[NUMBER,22]

32 rows selected.