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一个守望数据库的老菜鸟

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一个守望数据库的老菜鸟

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分类: Oracle

博客文章除注明转载外,均为原创。转载请注明出处。
本文链接地址:http://blog.chinaunix.net/uid-31396856-id-5780032.html


我们有时候会遇到,SQL传递参数的数据类型与表索引字段类型不一致,而无法走索引
SQL> desc test1
 Name                                                                                Null?    Type
 ----------------------------------------------------------------------------------- -------- --------------------------------------------------------
 OWNER                                                                                        VARCHAR2(30)
 OBJECT_NAME                                                                                  VARCHAR2(128)
 SUBOBJECT_NAME                                                                               VARCHAR2(30)
 OBJECT_ID                                                                                    NUMBER
 DATA_OBJECT_ID                                                                               NUMBER
 OBJECT_TYPE                                                                                  VARCHAR2(19)
 CREATED                                                                                      DATE
 LAST_DDL_TIME                                                                                DATE
 TIMESTAMP                                                                                    VARCHAR2(19)
 STATUS                                                                                       VARCHAR2(7)
 TEMPORARY                                                                                    VARCHAR2(1)
 GENERATED                                                                                    VARCHAR2(1)
 SECONDARY                                                                                    VARCHAR2(1)

SQL> desc test2
 Name                                                                                Null?    Type
 ----------------------------------------------------------------------------------- -------- --------------------------------------------------------
 OWNER                                                                                        VARCHAR2(30)
 OBJECT_NAME                                                                                  VARCHAR2(128)
 SUBOBJECT_NAME                                                                               VARCHAR2(30)
 OBJECT_ID                                                                                    VARCHAR2(100)
 DATA_OBJECT_ID                                                                               NUMBER
 OBJECT_TYPE                                                                                  VARCHAR2(19)
 CREATED                                                                                      DATE
 LAST_DDL_TIME                                                                                DATE
 TIMESTAMP                                                                                    VARCHAR2(19)
 STATUS                                                                                       VARCHAR2(7)
 TEMPORARY                                                                                    VARCHAR2(1)
 GENERATED                                                                                    VARCHAR2(1)
 SECONDARY                                                                                    VARCHAR2(1)
 OBJECT_ID2                                                                                   NUMBER

以上两张测试表,test1和test2。

其中test1的object_id为NUMBER,test2表的object_id是VARCHAR2,和test1表的object_id数据类型不一致。

object_id2是NUMBER,和test1表的object_id数据类型一致

对test2表的object_id和object_id2两个字段都创建了索引


SQL> CREATE INDEX ind_test2_id1 ON test2(object_id);                            


Index created.

SQL> CREATE INDEX ind_test2_id2 ON test2(object_id2); 

Index created.

我们先来看看TEST2的object_id和TEST1中的object_id数据类型一致的情况

SQL> 
  2  UPDATE test1 t  
  3  SET    a.object_type =  
  4         (SELECT i.object_type FROM test2 i WHERE i.object_id2 = t.object_id);

Explained.

看看执行计划
----------------------------------------------------------------------------------------------
| Id  | Operation                    | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------------
|   0 | UPDATE STATEMENT             |               | 53732 |  1259K|   154   (2)| 00:00:02 |
|   1 |  UPDATE                      | TEST1         |       |       |            |          |
|   2 |   TABLE ACCESS FULL          | TEST1         | 53732 |  1259K|   154   (2)| 00:00:02 |
|   3 |   TABLE ACCESS BY INDEX ROWID| TEST2         |   489 | 11736 |     6   (0)| 00:00:01 |
|*  4 |    INDEX RANGE SCAN          | IND_TEST2_ID2 |   196 |       |     1   (0)| 00:00:01 |
----------------------------------------------------------------------------------------------

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

   4 - access("I"."OBJECT_ID2"=:B1)

Note
-----
   - dynamic sampling used for this statement

20 rows selected.

执行计划中ID 4,IND_TEST2_ID2被正常使用

我们继续TEST2的object_id和TEST1中的object_id数据类型是不一致的情况.
传递参数时,类型与数据库表的字段类型不同(表字段类型为字符型),这样就会产生数据类型转换,ORACLE无法使用到索引,走了TABLE ACCESS FULL,导致 buffer get过多,成本高,执行时间长。
SQL> 
  2  UPDATE test1 t  
SET    a.object_type =  
  3    4         (SELECT i.object_type FROM test2 i WHERE i.object_id = t.object_id);  

Explained.

看看执行计划

----------------------------------------------------------------------------
| Id  | Operation          | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | UPDATE STATEMENT   |       | 53732 |  1259K|   154   (2)| 00:00:02 |
|   1 |  UPDATE            | TEST1 |       |       |            |          |
|   2 |   TABLE ACCESS FULL| TEST1 | 53732 |  1259K|   154   (2)| 00:00:02 |
|*  3 |   TABLE ACCESS FULL| TEST2 |   489 | 30807 |   168   (2)| 00:00:03 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------

   3 - filter(TO_NUMBER("I"."OBJECT_ID")=:B1)

Note
-----
   - dynamic sampling used for this statement

19 rows selected.

谓词信息可以看到object_id列进行to_number转换,但是执行计划却没有选择object_id字段的索引

解决办法1:创建进行数据类型转换的函数索引

SQL> CREATE INDEX ind_test2_id1x ON test2(TO_NUMBER(OBJECT_ID));

Index created.


执行sql:UPDATE test1 t    SET    a.object_type =    (SELECT i.object_type FROM test2 i WHERE i.object_id =t.object_id);  


执行计划为

-----------------------------------------------------------------------------------------------
| Id  | Operation                    | Name           | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------
|   0 | UPDATE STATEMENT             |                | 53732 |  1259K|   154   (2)| 00:00:02 |
|   1 |  UPDATE                      | TEST1          |       |       |            |          |
|   2 |   TABLE ACCESS FULL          | TEST1          | 53732 |  1259K|   154   (2)| 00:00:02 |
|   3 |   TABLE ACCESS BY INDEX ROWID| TEST2          |   489 | 30807 |     6   (0)| 00:00:01 |
|*  4 |    INDEX RANGE SCAN          | IND_TEST2_ID1X |   196 |       |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------

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

   4 - access(TO_NUMBER("OBJECT_ID")=:B1)

Note
-----
   - dynamic sampling used for this statement

20 rows selected.

解决办法2
修改sql语句:

    UPDATE test1 t  
SET a.object_type =  
       (SELECT i.object_type FROM test2 i WHERE TO_NUMBER(i.object_id) = t.object_id); 

执行计划:
-----------------------------------------------------------------------------------------------
| Id  | Operation                    | Name           | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------
|   0 | UPDATE STATEMENT             |                | 53732 |  1259K|   154   (2)| 00:00:02 |
|   1 |  UPDATE                      | TEST1          |       |       |            |          |
|   2 |   TABLE ACCESS FULL          | TEST1          | 53732 |  1259K|   154   (2)| 00:00:02 |
|   3 |   TABLE ACCESS BY INDEX ROWID| TEST2          |   489 | 30807 |     6   (0)| 00:00:01 |
|*  4 |    INDEX RANGE SCAN          | IND_TEST2_ID1X |   196 |       |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------

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

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

   4 - access(TO_NUMBER("OBJECT_ID")=:B1)

Note
-----
   - dynamic sampling used for this statement

20 rows selected.

另外,一些sql,在变量传递的时候,可以考虑修改传递的数据类型为所需要的类型.
--The End
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