MySql查询截取分析

小表驱动大表

总结
子查询为小表时用in，因为此时是子查询驱动主查询
子查询为大表时用exists，因为此时是主查询驱动子查询

---

order by

order by满足两种情况， 会使用index方式排序
1.order by语句使用索引最左前列
2.使用where子句与order by子句条件列组合满足索引最前列

filesort有2种排序算法

1.双路排序
mysql4.1之前使用双路排序，两次扫描磁盘，最终得到数据
读取行指针和orderby列，对他们进行排序，然后扫描已经排序号的列表，按照列表中的值重新从列表中读取数据
从磁盘读取排序字段，在buffer进行排序，再从磁盘取其他字段

2.单路排序
从磁盘读取查询锁需要的所有列，按照order by在buffer对他们进行排序，然后进行输出
效率高，只需要一次IO,把随机IO变成顺序IO(这里随机IO应该是指单路排序第二次根据排序好的取数据)，
但是会使用更多空间，因为他把每一行放到内存。

如果数据列太大，sort_buffer装不下，那就凉了，要进行多次读取，多次排序，还要合并

如果需要排序的列的总大小加上order by列的大小超过了 max_length_for_sort_data定义的字节，mysql就会使用双路排序，当任何需要的列甚至不是用order by的列（text.blob的时候），也会使用双路排序

myisam存储引擎无法使用主键进行排序，会变成filesort，innodb可以

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慢查询日志

mysql提供的一种日志记录，用来记录在mysql中响应记录超过阈（yu）值的语句
具体指运行时间超过long_query_time值的SQL，则会被记录到慢查询中

//查看是否开启了慢查询，结果是没有
mysql> show variables like '%slow_query_log%';

+---------------------+-----------------------------------------------------------+
| Variable_name       | Value                                                     |
+---------------------+-----------------------------------------------------------+
| slow_query_log      | OFF                                                       |
| slow_query_log_file | C:\Program Files\mysql-5.7.17-winx64\data\Calvin-slow.log |
+---------------------+-----------------------------------------------------------+

//开启慢查询，重启后失效，永久生效请修改配置文件
mysql> set global slow_query_log = 1;

//默认大于10秒计入慢查询
mysql> show variables like '%long_query_time%';

+-----------------+-----------+
| Variable_name   | Value     |
+-----------------+-----------+
| long_query_time | 10.000000 |
+-----------------+-----------+

//更改时间，重新连接才能看到改变
mysql> set global long_query_time=3;

//模拟慢查询
mysql> select sleep(4);

//查看日志
MySQL, Version: 5.7.17 (MySQL Community Server (GPL)). started with:
TCP Port: 3306, Named Pipe: MySQL
Time                 Id Command    Argument
# Time: 2018-08-02T06:13:52.037803Z
# User@Host: root[root] @ localhost [::1]  Id:    85
# Query_time: 4.004477  Lock_time: 0.000000 Rows_sent: 1  Rows_examined: 0
use test;
SET timestamp=1533190432;
select sleep(4);

//查看当前系统中有多少条慢查询记录
mysql> show global status like '%Slow_queries%';

+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| Slow_queries  | 1     |
+---------------+-------+

---

批量插入数据库脚本

//先建个表

mysql> create table dept(
    -> id int primary key auto_increment,
    -> dename varchar(20) not null default "",
    -> deno int);

//开启二进制日志，要不然函数报错
mysql> set global log_bin_trust_function_creators=1;

//定义一个产生随机字符串的函数
//更改结束字符
mysql> delimiter $

mysql> DROP FUNCTION IF EXISTS rand_str;
    -> create FUNCTION rand_str(strlen SMALLINT ) RETURNS VARCHAR(255)
    -> BEGIN
    -> DECLARE randStr VARCHAR(255) DEFAULT 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890';
    -> DECLARE i SMALLINT DEFAULT 0;
    -> DECLARE resultStr VARCHAR(255) DEFAULT '';
    -> WHILE iDO
    -> SET resultStr=CONCAT(SUBSTR(randStr,FLOOR(RAND()*LENGTH(randStr))+1,1),resultStr);
    -> SET i=i+1;
    -> END WHILE;
    -> RETURN resultStr;
    -> END $

//测试
mysql> select rand_str(10);

+--------------+
| rand_str(10) |
+--------------+
| 6PYirAsQcZ   |
+--------------+

//定义一个随机数函数
mysql> create function rand_num() returns int(5)
    -> begin
    -> declare i int default 0;
    -> set i = floor(100 + rand() * 10);
    ->  return i;
    -> end $

//测试

+------------+
| rand_num() |
+------------+
|        103 |
+------------+

//创建一个存储过程
mysql> create procedure insert_emp(in max_num int(10))
    -> begin
    -> declare i int default 0;
    -> set autocommit=0;
    ->  repeat
    -> set i = i + 1;
    ->  insert into dept(dename, deno) values(rand_str(10), rand_num());
    -> until i = max_num
    -> end repeat;
    -> commit;
    -> end $

//调用存储过程
//多来点
mysql> call insert_emp(500000);
Query OK, 0 rows affected (32.30 sec)

show profile

//查看profile状态，默认关闭
mysql> show variables like "%profiling%";

+------------------------+-------+
| Variable_name          | Value |
+------------------------+-------+
| have_profiling         | YES   |
| profiling              | OFF   |
| profiling_history_size | 15    |
+------------------------+-------+

//打开
mysql> set profiling=on;

//查看
mysql>  show profiles;

+----------+------------+------------------------------------------------+
| Query_ID | Duration   | Query                                          |
+----------+------------+------------------------------------------------+
|        1 | 0.00107075 | show variables like "%profiling%"              |
|        2 | 0.00043875 | show tables                                    |
|        3 | 0.00015875 | SELECT DATABASE()                              |
|        4 | 0.00059625 | show tables                                    |
|        5 | 0.00116725 | select * from tbl_dept                         |
|        6 | 0.00011975 | SELECT DATABASE()                              |
|        7 | 0.00025225 | select * from dept limit 1                     |
|        8 | 0.00088375 | select * from dept limit 1000                  |
|        9 | 0.00740350 | select * from tbl_dept group by id%2 limit 100 |
|       10 | 0.20496625 | select * from dept group by id%2 limit 100     |
+----------+------------+------------------------------------------------+

//进行诊断，查看某条语句的cpu和block io。。看不懂

mysql> show profile cpu,block io for query 10;

+----------------------+----------+----------+------------+--------------+---------------+
| Status               | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out |
+----------------------+----------+----------+------------+--------------+---------------+
| starting             | 0.000063 | 0.000000 |   0.000000 |         NULL |          NULL |
| checking permissions | 0.000006 | 0.000000 |   0.000000 |         NULL |          NULL |
| Opening tables       | 0.000012 | 0.000000 |   0.000000 |         NULL |          NULL |
| init                 | 0.000024 | 0.000000 |   0.000000 |         NULL |          NULL |
| System lock          | 0.000006 | 0.000000 |   0.000000 |         NULL |          NULL |
| optimizing           | 0.000004 | 0.000000 |   0.000000 |         NULL |          NULL |
| statistics           | 0.000024 | 0.000000 |   0.000000 |         NULL |          NULL |
| preparing            | 0.000010 | 0.000000 |   0.000000 |         NULL |          NULL |
| Creating tmp table   | 0.000506 | 0.000000 |   0.000000 |         NULL |          NULL |
| Sorting result       | 0.000009 | 0.000000 |   0.000000 |         NULL |          NULL |
| executing            | 0.000002 | 0.000000 |   0.000000 |         NULL |          NULL |
| Sending data         | 0.204175 | 0.203125 |   0.000000 |         NULL |          NULL |
| Creating sort index  | 0.000040 | 0.000000 |   0.000000 |         NULL |          NULL |
| end                  | 0.000002 | 0.000000 |   0.000000 |         NULL |          NULL |
| query end            | 0.000005 | 0.000000 |   0.000000 |         NULL |          NULL |
| removing tmp table   | 0.000005 | 0.000000 |   0.000000 |         NULL |          NULL |
| query end            | 0.000002 | 0.000000 |   0.000000 |         NULL |          NULL |
| closing tables       | 0.000004 | 0.000000 |   0.000000 |         NULL |          NULL |
| freeing items        | 0.000057 | 0.000000 |   0.000000 |         NULL |          NULL |
| cleaning up          | 0.000012 | 0.000000 |   0.000000 |         NULL |          NULL |
+----------------------+----------+----------+------------+--------------+---------------+