mysql存储引擎（二） mysql存储引擎二 MEMORY MERGE BerkeleyDB存储引擎 MEMORY 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，
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<span class="hljs-emphasis">_table( id int primary key, name varchar(20) )engine=memory;
Query OK, 0 rows affected (0.02 sec)
</span>mysql> insert into memory<span class="hljs-emphasis">_table(id,name) values(2,'frank');
Query OK, 1 row affected (0.00 sec)
</span><span class="hljs-header">mysql> select * from memory_table;
+----+-----------+</span>
<span class="hljs-header">| id | name |
+----+-----------+</span>
| 1 | frankstar |
<span class="hljs-header">| 2 | frank |
+----+-----------+</span>
2 rows in set (0.00 sec)
mysql> show table status like <span class="hljs-emphasis">'memory_table'</span> \G;
<span class="hljs-bullet">*************************** </span>1. row ***************************
<span class="hljs-code"> Name: memory_table</span>
<span class="hljs-code"> Engine: MEMORY</span>
<span class="hljs-code"> Version: 10</span>
<span class="hljs-code"> Row_format: Fixed</span>
<span class="hljs-code"> Rows: 2</span>
<span class="hljs-code"> Avg_row_length: 66</span>
<span class="hljs-code"> Data_length: 127008</span>
Max<span class="hljs-emphasis">_data_</span>length: 12582900
<span class="hljs-code"> Index_length: 126992</span>
<span class="hljs-code"> Data_free: 0</span>
<span class="hljs-code"> Auto_increment: NULL</span>
<span class="hljs-code"> Create_time: 2016-05-09 22:23:47</span>
<span class="hljs-code"> Update_time: NULL</span>
<span class="hljs-code"> Check_time: NULL</span>
<span class="hljs-code"> Collation: utf8_bin</span>
<span class="hljs-code"> Checksum: NULL</span>
<span class="hljs-code"> Create_options:</span>
<span class="hljs-code"> Comment:</span>
1 row in set (0.00 sec)
ERROR:
No query specified
mysql> show index from memory<span class="hljs-emphasis">_table \G;
*************************** 1. row ***************************
Table: memory_</span>table
<span class="hljs-code"> Non_unique: 0</span>
<span class="hljs-code"> Key_name: PRIMARY</span>
<span class="hljs-code"> Seq_in_index: 1</span>
<span class="hljs-code"> Column_name: id</span>
<span class="hljs-code"> Collation: NULL</span>
<span class="hljs-code"> Cardinality: 2</span>
<span class="hljs-code"> Sub_part: NULL</span>
<span class="hljs-code"> Packed: NULL</span>
<span class="hljs-code"> Null:</span>
<span class="hljs-code"> Index_type: HASH</span>
<span class="hljs-code"> Comment:</span>
Index<span class="hljs-emphasis">_comment:
1 row in set (0.00 sec)
</span>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。
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。
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 haml">mysql> create table myisam_table1(
-<span class="ruby">> id int primary key,
</span> -<span class="ruby">> data datetime
</span> -<span class="ruby">> )engine=myisam;
</span>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(
-<span class="ruby">> id int primary key,
</span> -<span class="ruby">> data datetime
</span> -<span class="ruby">> )engine=merge union=(myisam_table1,myisam_table2) insert_method=first;
</span>Query OK, 0 rows affected (0.01 sec)</code>向2个字表分别添加数据，如下：
<code class=" hljs cs">mysql> insert <span class="hljs-keyword">into</span> myisam_table1 values(<span class="hljs-number">1</span>,<span class="hljs-string">'2016-5-7'</span>); Query OK, <span class="hljs-number">1</span> row affected (<span class="hljs-number">0.00</span> sec) mysql> insert <span class="hljs-keyword">into</span> myisam_table1 values(<span class="hljs-number">2</span>,<span class="hljs-string">'2016-5-6'</span>); Query OK, <span class="hljs-number">1</span> row affected (<span class="hljs-number">0.00</span> sec) mysql> insert <span class="hljs-keyword">into</span> myisam_table2 values(<span class="hljs-number">1</span>,<span class="hljs-string">'2016-5-7'</span>); Query OK, <span class="hljs-number">1</span> row affected (<span class="hljs-number">0.00</span> sec) mysql> insert <span class="hljs-keyword">into</span> myisam_table2 values(<span class="hljs-number">2</span>,<span class="hljs-string">'2016-5-6'</span>); Query OK, <span class="hljs-number">1</span> row affected (<span class="hljs-number">0.00</span> sec) </code>
查询merge表，如下：
<code class=" hljs asciidoc"><span class="hljs-header">mysql> select * from table1_merge_table2; +----+---------------------+</span> <span class="hljs-header">| id | data | +----+---------------------+</span> | 1 | 2016-05-07 00:00:00 | | 2 | 2016-05-06 00:00:00 | | 1 | 2016-05-07 00:00:00 | <span class="hljs-header">| 2 | 2016-05-06 00:00:00 | +----+---------------------+</span> 4 rows in set (0.01 sec)</code>
向merge表中添加一条数据，如下：
<code class=" hljs asciidoc">mysql> insert into table1<span class="hljs-emphasis">_merge_</span>table2 values(3,<span class="hljs-emphasis">'2016-5-8'</span>); Query OK, 1 row affected (0.00 sec) <span class="hljs-header">mysql> select * from table1_merge_table2; +----+---------------------+</span> <span class="hljs-header">| id | data | +----+---------------------+</span> | 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 | <span class="hljs-header">| 2 | 2016-05-06 00:00:00 | +----+---------------------+</span> 5 rows in set (0.00 sec) <span class="hljs-header">mysql> select * from myisam_table1; +----+---------------------+</span> <span class="hljs-header">| id | data | +----+---------------------+</span> | 1 | 2016-05-07 00:00:00 | | 2 | 2016-05-06 00:00:00 | <span class="hljs-header">| 3 | 2016-05-08 00:00:00 | +----+---------------------+</span> 3 rows in set (0.00 sec) <span class="hljs-header">mysql> select * from myisam_table2; +----+---------------------+</span> <span class="hljs-header">| id | data | +----+---------------------+</span> | 1 | 2016-05-07 00:00:00 | <span class="hljs-header">| 2 | 2016-05-06 00:00:00 | +----+---------------------+</span> 2 rows in set (0.00 sec)</code>
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。
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。
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