mysql存储引擎二 MEMORY MERGE BerkeleyDB存储引擎
MEMORY存储引擎通过采用内存中的内容来创建表。每个Memory表实际上和一个磁盘文件关联起来,文件名采用”表名.frm”的格式。Memory类型的表访问速度极快,因为数据源来自内存,所以数据库关闭时,内存中的数据就会发生丢失。默认使用Hash索引。
<code class="hljs asciidoc">mysql> create table memory_table( id int primary key, name varchar(20) )engine=memory; Query OK, 0 rows affected (0.02 sec) mysql> insert into memory_table(id,name) values(2,'frank'); Query OK, 1 row affected (0.00 sec) mysql> select * from memory_table; +----+-----------+ | id | name | +----+-----------+ | 1 | frankstar | | 2 | frank | +----+-----------+ 2 rows in set (0.00 sec) mysql> show table status like 'memory_table' \G; *************************** 1. row *************************** Name: memory_table Engine: MEMORY Version: 10 Row_format: Fixed Rows: 2 Avg_row_length: 66 Data_length: 127008 Max_data_length: 12582900 Index_length: 126992 Data_free: 0 Auto_increment: NULL Create_time: 2016-05-09 22:23:47 Update_time: NULL Check_time: NULL Collation: utf8_bin Checksum: NULL Create_options: Comment: 1 row in set (0.00 sec) ERROR: No query specified mysql> show index from memory_table \G; *************************** 1. row *************************** Table: memory_table Non_unique: 0 Key_name: PRIMARY Seq_in_index: 1 Column_name: id Collation: NULL Cardinality: 2 Sub_part: NULL Packed: NULL Null: Index_type: HASH Comment: Index_comment: 1 row in set (0.00 sec) ERROR: No query specified </code>
memory表的内存储存在内存中,如果表的数据很大,那么服务器将会自动将其转换为磁盘表,阀值由temp_table_size系统变量来确定。每个memory表的容量由max_heap_table_size变量的值控制。默认16MB。<br /> 主要用于数据内容变化不频繁的代码表及访问速度要求较高、数据量不大的场合,同时需要考虑更新操作数据不回写入到磁盘文件中。
MERGE
它实际上是一组myisam表的组合,将一组结构相同的MyISAM表组合在一起,MERGE表本身没有数据,对于该类型表的插入操作,是通过INSERT_METHOD定义完成的,取值为LAST或者为FIRST,FIRST意味着数据增加到组合表中的第一个myisam表中,同理LAST意味着添加到最后一个表中。所以MERGE表的文件有2个,一个是.frm文件,用于存放数据,还有一个MRG文件,用于存放MERGE表的名称,包括其组成表。
如下:
<code class="hljs asciidoc"><code class="hljs haml">mysql> create table myisam_table1( -> id int primary key, -> data datetime -> )engine=myisam; Query OK, 0 rows affected (0.02 sec) create table myisam_table2( id int primary key, data datetime )engine=myisam; Query OK, 0 rows affected (0.01 sec) mysql> create table table1_merge_table2( -> id int primary key, -> data datetime -> )engine=merge union=(myisam_table1,myisam_table2) insert_method=first; Query OK, 0 rows affected (0.01 sec)</code></code>
<code class="hljs haml">向2个字表分别添加数据,如下:
<code class="hljs asciidoc"><code class="hljs haml"><code class="hljs cs">mysql> insert into myisam_table1 values(1,'2016-5-7'); Query OK, 1 row affected (0.00 sec) mysql> insert into myisam_table1 values(2,'2016-5-6'); Query OK, 1 row affected (0.00 sec) mysql> insert into myisam_table2 values(1,'2016-5-7'); Query OK, 1 row affected (0.00 sec) mysql> insert into myisam_table2 values(2,'2016-5-6'); Query OK, 1 row affected (0.00 sec) </code></code></code>
<code class="hljs haml"><code class="hljs cs">查询merge表,如下:
<code class="hljs asciidoc"><code class="hljs haml"><code class="hljs cs"><code class="hljs asciidoc">mysql> select * from table1_merge_table2; +----+---------------------+ | id | data | +----+---------------------+ | 1 | 2016-05-07 00:00:00 | | 2 | 2016-05-06 00:00:00 | | 1 | 2016-05-07 00:00:00 | | 2 | 2016-05-06 00:00:00 | +----+---------------------+ 4 rows in set (0.01 sec)</code></code></code></code>
<code class="hljs haml"><code class="hljs cs"><code class="hljs asciidoc">向merge表中添加一条数据,如下:
<code class="hljs asciidoc"><code class="hljs haml"><code class="hljs cs"><code class="hljs asciidoc"><code class="hljs asciidoc">mysql> insert into table1_merge_table2 values(3,'2016-5-8'); Query OK, 1 row affected (0.00 sec) mysql> select * from table1_merge_table2; +----+---------------------+ | id | data | +----+---------------------+ | 1 | 2016-05-07 00:00:00 | | 2 | 2016-05-06 00:00:00 | | 3 | 2016-05-08 00:00:00 | | 1 | 2016-05-07 00:00:00 | | 2 | 2016-05-06 00:00:00 | +----+---------------------+ 5 rows in set (0.00 sec) mysql> select * from myisam_table1; +----+---------------------+ | id | data | +----+---------------------+ | 1 | 2016-05-07 00:00:00 | | 2 | 2016-05-06 00:00:00 | | 3 | 2016-05-08 00:00:00 | +----+---------------------+ 3 rows in set (0.00 sec) mysql> select * from myisam_table2; +----+---------------------+ | id | data | +----+---------------------+ | 1 | 2016-05-07 00:00:00 | | 2 | 2016-05-06 00:00:00 | +----+---------------------+ 2 rows in set (0.00 sec)</code></code></code></code></code>
<code class="hljs haml"><code class="hljs cs"><code class="hljs asciidoc"><code class="hljs asciidoc">INSERT_METHOD的指定起作用了,如果没有指定,那么当试图往Merge表中insert数据时,都会发生错误。通常使用merge表来透明的对多个表进行查询和更新。
<code class="hljs haml"><code class="hljs cs"><code class="hljs asciidoc"><code class="hljs asciidoc">BerkeleyDB存储引擎
<code class="hljs haml"><code class="hljs cs"><code class="hljs asciidoc"><code class="hljs asciidoc">简称BDB,创建该类型的表时,会有2个数据文件,一个.frm文件存储表元数据,另一个.db文件存储数据和索引文件,类似innodb。它的实现事务安全有redo日志。在每次启动的时候,都会做一次检查操作,将所有的redo日志清空。它和Memory引擎一样,都是页级锁定。