MYSQL 分表原理（转）

时间 2019-12-10

标签 mysql 分表原理栏目 MySQL 繁體版

原文原文链接

简介:
引用MySQL官方文档中的一段话:MERGE存储引擎,也被认识为MRG_MyISAM引擎,是一个相同的能够被看成一个来用的MyISAM表的集合."相同"意味着全部表一样的列和索引信息.你不能合并列被以不一样顺序列于其中的表,没有刚好一样列的表,或有不一样顺序索引的表.并且,任何或者全部的表能够用myisampack来压缩.

例子:
mysql> show engines;
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| Engine | Support | Comment | Transactions | XA | Savepoints |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
| MEMORY | YES | Hash based, stored in memory, useful for temporary tables | NO | NO | NO |
| MRG_MYISAM | YES | Collection of identical MyISAM tables | NO | NO | NO |
| MyISAM | YES | MyISAM storage engine | NO | NO | NO |
| BLACKHOLE | YES | /dev/null storage engine (anything you write to it disappears) | NO | NO | NO |
| CSV | YES | CSV storage engine | NO | NO | NO |
| PERFORMANCE_SCHEMA | YES | Performance Schema | NO | NO | NO |
| ARCHIVE | YES | Archive storage engine | NO | NO | NO |
| FEDERATED | NO | Federated MySQL storage engine | NULL | NULL | NULL |
| InnoDB | DEFAULT | Supports transactions, row-level locking, and foreign keys | YES | YES | YES |
+--------------------+---------+----------------------------------------------------------------+--------------+------+------------+
9 rows in set (0.00 sec)

mysql> create table test1 (id int not null auto_increment,name varchar(10) default null ,primary key (id)) engine=myisam auto_increment=1;
Query OK, 0 rows affected (0.01 sec)

mysql> create table test2 (id int not null auto_increment,name varchar(10) default null ,primary key (id)) engine=myisam auto_increment=1;
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO `test1` (`name`) VALUES('beijing1');
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO `test2` (`name`) VALUES('beijing2');
Query OK, 1 row affected (0.00 sec)

mysql> create table test (id int not null auto_increment,name varchar(10) default null ,index(id)) engine=mrg_myisam union=(test1,test2) insert_method=last auto_increment=1;
Query OK, 0 rows affected (0.03 sec)

mysql> select id,name from test;
+----+----------+
| id | name |
+----+----------+
| 1 | beijing1 |
| 1 | beijing2 |
+----+----------+
2 rows in set (0.00 sec)

mysql> INSERT INTO `test` (`name`) VALUES('beijing3');
Query OK, 1 row affected (0.00 sec)

mysql> select id,name from test;
+----+----------+
| id | name |
+----+----------+
| 1 | beijing1 |
| 1 | beijing2 |
| 2 | beijing3 |
+----+----------+
3 rows in set (0.00 sec)

mysql> select id, name from test2
-> ;
+----+----------+
| id | name |
+----+----------+
| 1 | beijing2 |
| 2 | beijing3 |
+----+----------+
2 rows in set (0.00 sec)

mysql> system ls -l /mysql/data/test
total 164
-rw-rw---- 1 mysql mysql 8586 Feb 2 16:40 test1.frm
-rw-rw---- 1 mysql mysql 20 Feb 2 16:40 test1.MYD
-rw-rw---- 1 mysql mysql 2048 Feb 2 16:40 test1.MYI
-rw-rw---- 1 mysql mysql 8586 Feb 2 16:40 test2.frm
-rw-rw---- 1 mysql mysql 40 Feb 2 16:44 test2.MYD
-rw-rw---- 1 mysql mysql 2048 Feb 2 16:44 test2.MYI
-rw-rw---- 1 mysql mysql 8586 Feb 2 16:43 test.frm
-rw-rw---- 1 mysql mysql 32 Feb 2 16:43 test.MRG

在这里须要注意创建MRG_MYISAM须要必须指定一个UNION=(list-of-tables)子句,它说明你要把哪些表看成一个表来用.另一个重要的参数INSERT_METHOD,此参数INSERT_METHOD = NO 表示该表不能作任何写入操做只做为查询使用,INSERT_METHOD = LAST表示插入到最后的一张表里面.

例子:
mysql> show create table test\G
*************************** 1. row ***************************
Table: test
Create Table: CREATE TABLE `test` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(10) DEFAULT NULL,
KEY `id` (`id`)
) ENGINE=MRG_MyISAM DEFAULT CHARSET=utf8 INSERT_METHOD=LAST UNION=(`test1`,`test2`)
1 row in set (0.00 sec)

mysql> CREATE TABLE `test` (`id` int(11) NOT NULL AUTO_INCREMENT,`name` varchar(10) DEFAULT NULL,KEY `id` (`id`))ENGINE=MRG_MyISAM DEFAULT CHARSET=utf8 INSERT_METHOD=NO UNION=(`test1`,`test2`);
Query OK, 0 rows affected (0.00 sec)

mysql> select * from test;
+----+----------+
| id | name |
+----+----------+
| 1 | beijing1 |
| 1 | beijing2 |
| 2 | beijing3 |
+----+----------+
3 rows in set (0.00 sec)

mysql> INSERT INTO `test` (`name`) VALUES('beijing4');
ERROR 1036 (HY000): Table 'test' is read only

当你须要在现有MRG_MYISAM添加新表的时候能够这样作
mysql> create table test3 (id int not null auto_increment,name varchar(10) default null ,primary key (id)) engine=myisam auto_increment=1;
Query OK, 0 rows affected (0.02 sec)

mysql> alter table test engine=mrg_myisam union=(test1,test2,test3 ) insert_method=last;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> select * from test;
+----+----------+
| id | name |
+----+----------+
| 1 | beijing1 |
| 1 | beijing2 |
| 2 | beijing3 |
+----+----------+
3 rows in set (0.00 sec)

mysql> INSERT INTO `test` (`name`) VALUES('beijing4');
Query OK, 1 row affected (0.00 sec)

mysql> select * from test;
+----+----------+
| id | name |
+----+----------+
| 1 | beijing1 |
| 1 | beijing2 |
| 2 | beijing3 |
| 3 | beijing4 |
+----+----------+
4 rows in set (0.00 sec)

mysql> select * from test3;
+----+----------+
| id | name |
+----+----------+
| 3 | beijing4 |
+----+----------+
1 row in set (0.00 sec)

OK,完成了.

对于平常来讲MRG_MYISAM分表优势主要有如下几点:
(1)对于日志类应用的表.好比,你能够把每个月的数据放进分离的表中,用myisampack压缩这些表,建立一个MERGE表来把它们看成一个表来使用很是方便快捷.
(2)在查询速度上应该更优,对于一些表能够分割大的只读表,放进不一样磁盘上的单个表中.基于这个创建一个MERGE表会比单纯一个大表速度应该会快很多.
(3)超过操做系统的文件尺寸限制,每一个MyISAM表都受制于这个限制,可是MRG_MYSIAM则不会.
缺点:
MRG_MYISAM不支持全文索引还有其它一些MyISAM功能,固然能够在底表建立,可是仍然不能利用全文索引在MRG表上搜索.创建全文会报以下错误
ERROR 1214 (HY000): The used table type doesn't support FULLTEXT indexeshtml

转自： http://zzjlzx.blog.chinaunix.net/uid-10661836-id-4095860.htmlmysql