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Oracle 9i 数据库WITH查询语法小议

Oracle9i新增了WITH语法功能,可以将查询中的子查询命名,放到SELECT语句的最前面。

下面看一个简单的例子:

SQL> WITH

2 SEG AS (SELECT SEGMENT_NAME, SUM(BYTES)/1024 K FROM USER_SEGMENTS GROUP BY SEGMENT_NAME),

3 OBJ AS (SELECT OBJECT_NAME, OBJECT_TYPE FROM USER_OBJECTS)

4 SELECT O.OBJECT_NAME, OBJECT_TYPE, NVL(S.K, 0) SIZE_K

5 FROM OBJ O, SEG S

6 WHERE O.OBJECT_NAME = S.SEGMENT_NAME (+)

7 ;

OBJECT_NAME OBJECT_TYPE SIZE_K

DAIJC_TEST TABLE 128

P_TEST PROCEDURE 0

      IND_DAIJC_TEST_C1 INDEX 128

通过WITH语句定义了两个子查询SEG和OBJ,在随后的SELECT语句中可以直接对预定义的子查询进行查询。从上面的例子也可以看出,使用WITH语句,将一个包含聚集、外连接等操作SQL清晰的展现出来。

WITH定义的子查询不仅可以使查询语句更加简单、清晰,而且WITH定义的子查询还具有在SELECT语句的任意层均可见的特点。

即使是在WITH的定义层中,后定义的子查询都可以使用前面已经定义好的子查询:

SQL> WITH

2 Q1 AS (SELECT 3 + 5 S FROM DUAL),

3 Q2 AS (SELECT 3 * 5 M FROM DUAL),

4 Q3 AS (SELECT S, M, S + M, S * M FROM Q1, Q2)

5 SELECT * FROM Q3;

S M S+M S*M

      8 15 23 120

利用WITH定义查询中出现多次的子查询还能带来性能提示。Oracle会对WITH进行性能优化,当需要多次访问WITH定义的子查询时,Oracle会将子查询的结果放到一个临时表中,避免同样的子查询多次执行,从而有效的减少了查询的IO数量。

看一个简单的例子,首先构造一张大表,现在要取出大表中ID最小、ID最大以及ID等于平均值的记录,看看普通写法和WITH语句的区别:

SQL> CREATE TABLE T_WITH AS SELECT ROWNUM ID, A.* FROM DBA_SOURCE A WHERE ROWNUM < 100001; 

表已创建。 

SQL> SET TIMING ON 

SQL> SET AUTOT ON 

SQL> SELECT ID, NAME FROM T_WITH

2 WHERE ID IN 

3 (

4 SELECT MAX(ID) FROM T_WITH 

5 UNION ALL

6 SELECT MIN(ID) FROM T_WITH

7 UNION ALL

8 SELECT TRUNC(AVG(ID)) FROM T_WITH

9 );

ID NAME

1 STANDARD

50000 DBMS_BACKUP_RESTORE

100000 INITJVMAUX

已用时间: 00: 00: 00.09

执行计划

Plan hash value: 647530712

-----------------------------------------------------------

| Id | Operation | Name | Rows | Bytes |

-----------------------------------------------------------

| 0 | SELECT STATEMENT | | 3 | 129 |

|* 1 | HASH JOIN | | 3 | 129 |

| 2 | VIEW | VW_NSO_1 | 3 | 39 |

| 3 | HASH UNIQUE | | 3 | 39 |

| 4 | UNION-ALL | | | |

| 5 | SORT AGGREGATE | | 1 | 13 |

| 6 | TABLE ACCESS FULL| T_WITH | 112K| 1429K|

| 7 | SORT AGGREGATE | | 1 | 13 |

| 8 | TABLE ACCESS FULL| T_WITH | 112K| 1429K|

| 9 | SORT AGGREGATE | | 1 | 13 |

| 10 | TABLE ACCESS FULL| T_WITH | 112K| 1429K|

| 11 | TABLE ACCESS FULL | T_WITH | 112K| 3299K|

-----------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

1 - access("ID"="$nso_col_1")

Note

-----

- dynamic sampling used for this statement

统计信息

----------------------------------------------------------

0 recursive calls

0 db block gets

5529 consistent gets

0 physical reads

0 redo size

543 bytes sent via SQL*Net to client

385 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

      3 rows processed

 

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