tranlation
事务：
从失败中回复到正常状态的方法
多个应用并发访问数据库时，提供隔离方法
acid
原子性：要么成功、要么失败
一致性：数据保持“合理性”
隔离型：多个事务同时并发执行，每个事务就像各自独立执行一样
持久性：数据持久化到硬盘
使用事务的话表的引擎为innodb引擎
默认是开启自动提交事务
engine:
存储引擎是基于表的
myisam: 不支持事务、表级锁、全文索引、奔溃恢复不好
innodb: 支持事务、行级锁、全文索引(5.6+)、奔溃恢复好
update table set id = 3 where name like 'a%';
更新行数不确定、此时采用表级锁
总结：一般来说myisam是适合不需要事务的时候：做很多count计算
innodb是适合要去事务，可靠性要求高的
推荐使用innodb
index
sql优化方案：
一、开启慢查询日志
第一步：show variables like '%slow%';
第二步：set slow_query_log = on;
slow_query_log_file:慢日志文件，只存放慢查询sql
show variables like '%long%';
long_query_time 10.00000s
第三步：set long_query_time = 0.4; 设置慢查询时间标准
注：重启mysql服务，配置会被恢复到默认。
永久生效方法，要在配置文件my.cnf中进行配置
执行sql,查找slow_query_log_file中执行慢的sql
分析：
mysql> explain select * from test where username = 'user799999';
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| 1 | SIMPLE | test | NULL | ALL | NULL | NULL | NULL | NULL | 798401 | 10.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
mysql> explain select * from test where id = '0799999';
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | test | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
possible_keys:可能用到的索引
key:实际用到的索引
rows:扫描的行数
mysql> show variables like '%profiling%';
+------------------------+-------+
| Variable_name | Value |
+------------------------+-------+
| have_profiling | YES |
| profiling | OFF |
| profiling_history_size | 15 |
+------------------------+-------+
3 rows in set (0.00 sec)
mysql> set profiling = on;
Query OK, 0 rows affected, 1 warning (0.00 sec)
mysql> show profiles;
Empty set, 1 warning (0.00 sec)
mysql> select * from test where username = 'user799999';
+---------+------------+----------+-------------+---------------------+
| id | username | password | servnumber | createtime |
+---------+------------+----------+-------------+---------------------+
| 0799999 | user799999 | | 18111818911 | 2020-01-13 22:31:18 |
+---------+------------+----------+-------------+---------------------+
1 row in set (0.31 sec)
mysql> select * from test where id = '0799999';
+---------+------------+----------+-------------+---------------------+
| id | username | password | servnumber | createtime |
+---------+------------+----------+-------------+---------------------+
| 0799999 | user799999 | | 18111818911 | 2020-01-13 22:31:18 |
+---------+------------+----------+-------------+---------------------+
1 row in set (0.00 sec)
mysql> show profiles;
+----------+------------+--------------------------------------------------+
| Query_ID | Duration | Query |
+----------+------------+--------------------------------------------------+
| 1 | 0.31259775 | select * from test where username = 'user799999' |
| 2 | 0.00039600 | select * from test where id = '0799999' |
+----------+------------+--------------------------------------------------+
2 rows in set, 1 warning (0.00 sec)
mysql> show profile cpu,block io for query 1;
+----------------------+----------+----------+------------+--------------+---------------+
| Status | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out |
+----------------------+----------+----------+------------+--------------+---------------+
| starting | 0.000084 | 0.000050 | 0.000027 | 0 | 0 |
| checking permissions | 0.000010 | 0.000006 | 0.000003 | 0 | 0 |
| Opening tables | 0.000029 | 0.000019 | 0.000010 | 0 | 0 |
| init | 0.000035 | 0.000023 | 0.000012 | 0 | 0 |
| System lock | 0.000010 | 0.000006 | 0.000003 | 0 | 0 |
| optimizing | 0.000011 | 0.000008 | 0.000004 | 0 | 0 |
| statistics | 0.000020 | 0.000012 | 0.000007 | 0 | 0 |
| preparing | 0.000018 | 0.000013 | 0.000006 | 0 | 0 |
| executing | 0.000005 | 0.000003 | 0.000002 | 0 | 0 |
| Sending data | 0.312217 | 0.307165 | 0.000000 | 0 | 0 |
| end | 0.000026 | 0.000017 | 0.000000 | 0 | 0 |
| query end | 0.000014 | 0.000013 | 0.000000 | 0 | 0 |
| closing tables | 0.000012 | 0.000012 | 0.000000 | 0 | 0 |
| freeing items | 0.000028 | 0.000028 | 0.000000 | 0 | 0 |
| logging slow query | 0.000064 | 0.000065 | 0.000000 | 0 | 8 |
| cleaning up | 0.000015 | 0.000015 | 0.000000 | 0 | 0 |
+----------------------+----------+----------+------------+--------------+---------------+
查看性能详情是否开启
show variables like '%profiling%';
开启性能记录功能
set profiling = on;
查看性能的记录
show profiles;
show profile cpu,block io for query 4;
优化小建议：
查询* 与查询单个字段 查看性能
1、尽量避免使用select * from,尽量精确到想要的结果字段
查询两个条件 用or连接 查看执行计划
2、尽量避免使用or
3、加上limit 限制行数
4、使用like时，%放在前面是会使索引失效 查看执行计划
查询条件字段类型varchar,但条件值类型int时，会进行类型转换 查看执行计划
5、注意条件字段类型的转换会使索引失效