@MySQL的存储引擎

1.存储引擎

查看MySQL提供了哪些存储引擎

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

查看MySQL当前默认的存储引擎

mysql> show variables like '%storage_engine%';
+----------------------------+--------+
| Variable_name              | Value  |
+----------------------------+--------+
| default_storage_engine     | InnoDB |
| default_tmp_storage_engine | InnoDB |
| storage_engine(当前存储引擎) | InnoDB |
+----------------------------+--------+
3 rows in set (0.00 sec)

对比项	MyISAM	InnoDB
外键	不支持	支持
事务	不支持	支持
行表锁	表锁，即使操作一条记录也会锁住整张表， 不适合高并发的操作	行锁，操作时只锁定某一行，不对其他的行有影响， 适合高并发的操作
缓存	只缓存索引，不缓存真实数据	不仅缓存索引还缓存真实数据，对内存要求较高， 而且内存大小对性能有决定性影响
表空间	小	大
关注点	性能	事务

2.sql性能下降的原因

1).查询语句写的烂。

2).索引失效，可能常用的搜索字段没有建索引。

3).关联查询太多的join。

4).MySQL服务器调优及各个参数的设置(缓冲，线程数等)。

3.sql的执行顺序

手写的sql

SELECT 
    <select_list>
FROM 
    <left_table><join_type>
JOIN <right_table> ON <join_condition>
WHERE 
    <where_condition>
GROUP BY
    <group_by_list>
HAVING
    <having_condition>
ORDER BY
    <order_by_condition>
LIMIT <limit_number>

MySQL是怎么读的呢？(从from开始读)

FROM  <left_table>
ON <join_condition>   
<join_type> JOIN <right_table> 
WHERE <where_condition>
GROUP BY <group_by_list>
HAVING <having_condition>
SELECT <select_list>
ORDER BY <order_by_condition>
LIMIT <limit_number>

WHERE和GROUP BY是一对哟~