mysql----存储引擎

1，创建表的时候，使用存储引擎

mysql> create table ai(id int) engine=Myisam;
Query OK, 0 rows affected (0.03 sec)

2，查看存储引擎

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

3，存储引擎—MyISAM
mysql默认存储引擎，不支持事务，不支持外键，访问速度快，对事物完整性没有要求的比如select，insert为主的应用可以使用此引擎来创建表
每个MyISAM在磁盘上存储成3个文件，扩展名分别为
.frm(存储表定义)
.MYD(MYData, 存储数据)
.MYI(MYIndex, 存储索引)
表可能会被损坏
可以通过

mysql> check table ai;
+---------+-------+----------+----------+
| Table   | Op    | Msg_type | Msg_text |
+---------+-------+----------+----------+
| user.ai | check | status   | OK       |
+---------+-------+----------+----------+
1 row in set (0.01 sec)

来检查表是否损坏

4，innodb mysql仅有的支持外建的存储引擎（创建外键的时候，要求父表必须有对应的索引，子表在创建外键的时候也会自动创建对应的索引）

mysql> create table country(country_id smallint unsigned not null auto_increment, country varchar(50) not null, last_update timestamp not null default current_timestamp on update current_timestamp, primary key (country_id))engine=innodb default charset=utf8;
Query OK, 0 rows affected (0.03 sec)

mysql> desc country;
+-------------+----------------------+------+-----+-------------------+-----------------------------+
| Field       | Type                 | Null | Key | Default           | Extra                       |
+-------------+----------------------+------+-----+-------------------+-----------------------------+
| country_id  | smallint(5) unsigned | NO   | PRI | NULL              | auto_increment              |
| country     | varchar(50)          | NO   |     | NULL              |                             |
| last_update | timestamp            | NO   |     | CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP |
+-------------+----------------------+------+-----+-------------------+-----------------------------+
3 rows in set (0.01 sec)

mysql> create table city(city_id smallint unsigned not null auto_increment, city varchar(50) not null, country_id smallint unsigned not null, last_update timestamp not null default current_timestamp on update current_timestamp, primary key (city_id), key idx_fk_country_id(country_id), constraint fk_city_country foreign key (country_id) references country (country_id) on delete restrict on update cascade)engine=innodb default charset=utf8;
Query OK, 0 rows affected (0.07 sec)

mysql> desc city;
+-------------+----------------------+------+-----+-------------------+-----------------------------+
| Field       | Type                 | Null | Key | Default           | Extra                       |
+-------------+----------------------+------+-----+-------------------+-----------------------------+
| city_id     | smallint(5) unsigned | NO   | PRI | NULL              | auto_increment              |
| city        | varchar(50)          | NO   |     | NULL              |                             |
| country_id  | smallint(5) unsigned | NO   | MUL | NULL              |                             |
| last_update | timestamp            | NO   |     | CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP |
+-------------+----------------------+------+-----+-------------------+-----------------------------+
4 rows in set (0.01 sec)

子表的外键指定是on delete restrict on update cascade 主表删除记录时，对应子表有对应记录，不允许删除，主要是更新操作时，子表有记录，子表对应更新
5,memory

mysql> create table memoryDemo engine=memory select city_id, city, country_id from city group by city_id;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> select count(*) from memoryDemo;
+----------+
| count(*) |
+----------+
|        0 |
+----------+
1 row in set (0.01 sec)

hash索引，存储在内存中
6，merge
底层是myisam

存储引擎选择
myisam
默认的mysql插件式存储，如果应用以读操作和插入操作为主，只有很少的更新和删除操作，并且对事务的完整性，并发要求不高的情况下选择
innodb
用于事务处理应用程序，支持外键，如果应用对事务的完整性有比较高的要求，在并发条件下要求数据的一致性，数据操作处理查询，插入以外还有更新和删除操作，innodb存储引擎除了有效的降低由于删除和更新导致的锁定，还能保证事务的完整提交(commit)和回滚（RollBack）对于类似计费系统或者财务系统等对数据准确性要求比较高的系统
memory
将所有的数据都保存在ram中，在需要快速定位记录和其他类似数据的环境下，可提供极快的访问，memory的缺陷是对表的大小有限制，太大的表无法cache在内存中，其次是要确保表的数据可以恢复，数据库异常终止后表的数据是可以恢复的，memory表通常用于更新不太频繁的小表
merge
用于将一系列相同的myisam表以逻辑方式组合在一起，并作为一个对象引用他们，merge表的优点在于可以突破单个myiscam表大小的限制，并且通过将不同的表分布在多个磁盘上，可以有效的改善merge表的访问效率，这对于诸如多数据仓储等vldb环境