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MYSQL的MERGE存储引擎

 

MYSQL的引擎不是一般的多,这次说到的是MERGE,这个引擎有很多特殊的地方:

MERGE引擎类型允许你把许多结构相同的表合并为一个表。然后,你可以执行查询,从多个表返回的结果就像从一个表返回的结果一样。每一个合并的表必须有同样的表定义。

 

MERGE存储引擎在下面这种使用场合会最为有用,如果需要把日志纪录不停的录入MySQL数据库,并且每天、每周或者每个月都创建一个单一的表,而且要制作来自多个表的合计查询,MERGE表这时会非常有效。然而,这项功能有局限性。你只能合并MyISAM表而且必须严格遵守相同的表定义的限制。虽然这看起来好像是一个大问题,但是,如果你使用另外一种表类型(例如InnoDB),这种合并可能就不需要了。

 

其创建方法如下:  

 

他将多个表在逻辑上当作一个表来查询。他建立后有两个文件, 

.frm 表结构定义 

.mrg union表的名字清单

 

两个基本表: 

CREATE TABLE TEST_MERGE_1( 

ID INT(5) NOT NULL, 

VALUE VARCHAR(100) NOT NULL, 

PRIMARY KEY(ID) 

) ENGINE=MyISAM DEFAULT CHARSET=utf8 

CREATE TABLE TEST_MERGE_2( 

ID INT(5) NOT NULL, 

VALUE VARCHAR(100) NOT NULL, 

PRIMARY KEY(ID) 

) ENGINE=MyISAM DEFAULT CHARSET=utf8

 

MERGE表的建立: 

CREATE TABLE TEST_MERGE( 

ID INT(5) NOT NULL, 

VALUE VARCHAR(100) NOT NULL, 

PRIMARY KEY(ID) 

) ENGINE=MRG_MyISAM DEFAULT CHARSET=utf8 INSERT_METHOD=LAST UNION=(TEST_MERGE_1,TEST_MERGE_2) ;

 

1. 此表类似于SQL中的union机制。 

2. 此表结构必须与基本表完全一致,包括列名、顺序。UNION表必须同属一个DATABASE。 

3. 基本表类型必须是MyISAM。 

4. 可以通过修改.mrg文件来修改MERGE表,每个基本表的名字占一行。注意:修改后要通过FLUSH TABLES刷新表缓存。  

 

5. 对基本表的更改可以直接反映在此表上。 

6. INSERT_METHOD的取值可以是: 0 不允许插入 FIRST 插入到UNION中的第一个表 LAST 插入到UNION中的最后一个表。(4.0之后可用) 

7. 定义在它上面的约束没有任何作用,约束是由基本表控制的,例如两个基本表中存在着同样的一个Key值,那么在MERGE表中会有两个一样的Key值。

 

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