MYSQL 分表原理（轉）

時間 2019-12-10

標籤 mysql 分表原理欄目 MySQL 简体版

原文原文鏈接

簡介:
引用MySQL官方文檔中的一段話:MERGE存儲引擎,也被認識爲MRG_MyISAM引擎,是一個相同的能夠被看成一個來用的MyISAM表的集合."相同"意味着全部表一樣的列和索引信息.你不能合併列被以不一樣順序列於其中的表,沒有剛好一樣列的表,或有不一樣順序索引的表.並且,任何或者全部的表能夠用myisampack來壓縮.

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

在這裏須要注意創建MRG_MYISAM須要必須指定一個UNION=(list-of-tables)子句,它說明你要把哪些表看成一個表來用.另一個重要的參數INSERT_METHOD,此參數INSERT_METHOD = NO 表示該表不能作任何寫入操做只做爲查詢使用,INSERT_METHOD = LAST表示插入到最後的一張表裏面.

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

當你須要在現有MRG_MYISAM添加新表的時候能夠這樣作
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.

對於平常來講MRG_MYISAM分表優勢主要有如下幾點:
(1)對於日誌類應用的表.好比,你能夠把每個月的數據放進分離的表中,用myisampack壓縮這些表,建立一個MERGE表來把它們看成一個表來使用很是方便快捷.
(2)在查詢速度上應該更優,對於一些表能夠分割大的只讀表,放進不一樣磁盤上的單個表中.基於這個創建一個MERGE表會比單純一個大表速度應該會快很多.
(3)超過操做系統的文件尺寸限制,每一個MyISAM表都受制於這個限制,可是MRG_MYSIAM則不會.
缺點:
MRG_MYISAM不支持全文索引還有其它一些MyISAM功能,固然能夠在底表建立,可是仍然不能利用全文索引在MRG表上搜索.創建全文會報以下錯誤
ERROR 1214 (HY000): The used table type doesn't support FULLTEXT indexeshtml

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

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