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MySQL和Oracle中的隐式转换

作者:互联网

今天在处理一个问题的时候,需要根据其他部门提供的sql语句对一个表中的数据进行了筛查。  
语句类似下面的形式  
> SELECT MAX_LEVEL,LOGOUT_TIME,CURRENT_DATE AS NOWTIME,cn_master FROM t_test_october_back_a WHERE ID in ( 100, 200, 300, 400, 500) ;  
+-----------+---------------+------------+-----------+  
| MAX_LEVEL | LOGOUT_TIME   | NOWTIME    | ID|  
+-----------+---------------+------------+-----------+  
|         1 | 1440407918000 | 2015-08-31 | 100|  
|        100| 1441009281000 | 2015-08-31 | 200|  
|         1 | 1440408002000 | 2015-08-31 | 300|  
+-----------+---------------+------------+-----------+  
x rows in set,  65535 warnings (  10.98 sec)  
本来这一个简单查询就完成了,也得到了业务部门需要的数据情况,但是查看最后一行的内容,还是有些蹊跷。如果观察仔细,对于这种id的数据查询,走索引的话,绝对不会再10秒左右,这是第一个奇怪的地方,第二个奇怪的地方就是65535 warnings,一个简单查询怎么会有这么多的warnings  
最开始怕show warnings的时候会一下子显示出来6万多行数据,还小小担心了一下,结果输出的结果只有64行。  
>show warnings;  
+---------+------+----------------------------------------------------------------+  
| Level   | Code | Message                                                        |  
+---------+------+----------------------------------------------------------------+  
| Warning | 1292 | Truncated incorrect DOUBLE value: '3506996@abc.com'       |  
| Warning | 1292 | Truncated incorrect DOUBLE value: '28366@abc.com'    |  
| Warning | 1292 | Truncated incorrect DOUBLE value: '81700700@abc.com'     |  
| Warning | 1292 | Truncated incorrect DOUBLE value: '391112900@abc.com'     |  
| Warning | 1292 | Truncated incorrect DOUBLE value: '867964771@abc.com'    |  
...  
64 rows in set (0.00 sec)

查看建表语句,发现这个id列是varchar类型的。  
>show create table  t_test_october_back_a;  
| t_tl_october_back_a | CREATE TABLE `t_tl_october_back_a` (  
  `id` bigint(20) NOT NULL AUTO_INCREMENT,  
    `id   ` varchar(60) NOT NULL,  
  `area_server` varchar(80) NOT NULL,  
  `max_level` tinyint(4) NOT NULL,  
  `logout_time` bigint(20) NOT NULL,  
  PRIMARY KEY (`id`),  
  UNIQUE KEY `idx_test_october_back_cn_master` (`id`)  
) ENGINE=InnoDB AUTO_INCREMENT=2042252493 DEFAULT CHARSET=gbk |  

我们就顺着错误信息来看看,把警告中的部分直接作为参数,发现查询的时间极快。  
>select *from t_tl_october_back_a where cn_master ='   867964771@abc   .com';  
+-------+-------------------------+---------------------------+-----------+---------------+  
| id    | cn_master               | area_server               | max_level | logout_time   |  
+-------+-------------------------+---------------------------+-----------+---------------+  
| 18723 |   867964771@abc   .com | abcd-abcd            |       104 | 1434446979000 |  
+-------+-------------------------+---------------------------+-----------+---------------+  
1 row in set (0.00 sec)  
对此我们来通过执行计划来简单对比一下。  
如果使用数字的方式,会走全表扫描  
> explain SELECT MAX_LEVEL,LOGOUT_TIME FROM t_tl_october_back_a WHERE CN_MASTER in ( 100, 200);  
+----+-------------+---------------------+------+-------------------------------+------+---------+------+----------+-------------+  
| id | select_type | table               | type | possible_keys                 | key  | key_len | ref  | rows     | Extra       |  
+----+-------------+---------------------+------+-------------------------------+------+---------+------+----------+-------------+  
|  1 | SIMPLE      | t_test_october_back_a | ALL  | idx_test_october_back_cn_master | NULL | NULL    | NULL | 28597841 | Using where |  
+----+-------------+---------------------+------+-------------------------------+------+---------+------+----------+-------------+  
1 row in set (0.00 sec)  
而如果使用字符的形式,执行计划就会走索引,执行效果就是预期的样子。  
>explain SELECT MAX_LEVEL,LOGOUT_TIME FROM t_tl_october_back_a WHERE CN_MASTER = '100' ;  
+----+-------------+---------------------+-------+-------------------------------+-------------------------------+---------+-------+------+-------+  
| id | select_type | table               | type  | possible_keys                 | key                           | key_len | ref   | rows | Extra |  
+----+-------------+---------------------+-------+-------------------------------+-------------------------------+---------+-------+------+-------+  
|  1 | SIMPLE      | t_test_october_back_a | const | idx_test_october_back_cn_master | idx_test_october_back_cn_master | 122     | const |    1 |       |  
+----+-------------+---------------------+-------+-------------------------------+-------------------------------+---------+-------+------+-------+  
1 row in set (0.00 sec)  
通过上面的例子可以看到,在查询的时候抛出的警告,其实就是在做类型转换的时候本来输出的是数字类型,就会尝试做隐式转换,而那个65535只是一个最大限制而已,表中的数据其实已经远远超过千万。  
这个时候我们大体感受到了隐式转换在MySQL中的一些影响,我们来看看在MySQL 5.6和Oracle中的表现如何。  
###############  
MYSQL 5.6  
mysql> select version();  
+-------------------------------------------+  
| version()                                 |  
+-------------------------------------------+  
| 5.6.23-enterprise-commercial-advanced-log |  
+-------------------------------------------+  
mysql> create table test (id1 int,id2 varchar(10));  
mysql> insert into test values(1,'1');  
mysql> insert into test values(2,'2');  
mysql> insert into test values(3,'3');  
mysql> commit;  
mysql> create index idx_id1 on test(id1);  
mysql> create index idx_id2 on test(id2);  
隐式转换,有数字转换为字符的时候,直接走了索引扫描  
mysql> explain select * from test where id1='1';  
+----+-------------+-------+------+---------------+---------+---------+-------+------+-------+  
| id | select_type | table | type | possible_keys | key     | key_len | ref   | rows | Extra |  
+----+-------------+-------+------+---------------+---------+---------+-------+------+-------+  
|  1 | SIMPLE      | test  | ref  | idx_id1       | idx_id1 | 5       | const |    1 | NULL  |  
+----+-------------+-------+------+---------------+---------+---------+-------+------+-------+  
1 row in set (0.01 sec)  

隐式转换,由字符转换为数字的时候,直接走了全表扫描  
mysql> explain select *from test where id2=2;  
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+  
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |  
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+  
|  1 | SIMPLE      | test  | ALL  | idx_id2       | NULL | NULL    | NULL |    3 | Using where |  
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+  
1 row in set (0.00 sec)  


###############
Oracle 11gR2
 
sqlplus -v  
SQL*Plus: Release 10.2.0.3.0 - Production  

SQL> create table test (id1 number,id2 varchar2(10));
SQL> insert into test values(1,'1');
SQL> begin                   
  2  for i in 1..100 loop
  3  insert into test values(i,chr(39)||i||chr(39));
  4  end loop;
  5  commit;
  6  end;
  7  /
PL/SQL procedure successfully completed.
SQL> exec dbms_stats.gather_table_stats('TEST','TEST',CASCADE=>TRUE);
隐式转换,由数字转换为字符的时候,直接走了索引扫描
SQL> explain plan for select *from test where id1='2';
SQL> select *from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 3847161316
---------------------------------------------------------------------------------------
| Id  | Operation                   | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |         |     1 |     7 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST    |     1 |     7 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | IDX_ID1 |     1 |       |     1   (0)| 00:00:01 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
PLAN_TABLE_OUTPUT
   2 - access("ID1"=2)

隐式转换,由字符转换为数字的时候,直接走了全表扫描
SQL> explain plan for select *from test where id2=3;
SQL> select *from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 1357081020
--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     1 |     7 |     3   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| TEST |     1 |     7 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
   1 - filter(TO_NUMBER("ID2")=3)

可见在这个方面MySQL和Oracle中的表现是一致的,对于这种隐式转换还是要多加注意。


标签:october,back,id,test,MySQL,Oracle,NULL,隐式,select
来源: https://blog.51cto.com/lhrbest/2698486