mysql不走索引的几个问题小结

----创建表：
CREATE TABLE `test_a`(
    `id` int(11) NOT NULL,
    `work_no` varchar(11) DEFAULT NULL,
    PRIMARY KEY (`id`),
    KEY `index_work_no`(`work_no`)
)ENGINE=InnoDB;

----查询工号，执行一下-----
explain SELECT * from test_a where work_no=1000;
explain  SELECT * from test_a where work_no="1000";

----创建表：
CREATE TABLE `test_t`(
    `id` int(11) NOT NULL,
    `a` int(11) DEFAULT NULL,
    `b` int(11) DEFAULT NULL,
    PRIMARY KEY (`id`),
    KEY `index_a`(`a`),
    KEY`index_b`(`b`)
)ENGINE=InnoDB;
-----定义初始化10w数据函数：
delimiter;;
create procedure idata()
begin
    declare i int;
    set i=1;
    while(i<=100000) do
    insert into test_t values(i,i,i);
    set i=i+1;
    end while;
end;;
delimiter;
call idata();
----删除存储过程
drop procedure idata;

----查询条件
explain select * from test_t where (a BETWEEN 1 and 1000) and (b BETWEEN 50000 and 100000) order by b limit 1;
explain select * from test_t force index(index_a) where (a BETWEEN 1 and 1000) and (b BETWEEN 50000 and 100000) order by b limit 1;

----创建表：
CREATE TABLE `trade_log`(
    `id` int(11) NOT NULL,
    `created_time` datetime NOT NULL DEFAULT,
    `b` int(11) DEFAULT NULL,
    PRIMARY KEY (`id`),
    KEY `index_created_time`(`created_time`)
)ENGINE=InnoDB;
-----定义初始化10w数据函数：
delimiter;;
create procedure c_trade_log()
begin
    declare i int;
    set i=1;
    while(i<=100000) do
    insert into trade_log values(i,from_unixtime(1539123415 + 1000*i),i);
    set i=i+1;
    end while;
end;;
delimiter;
call c_trade_log();
----删除存储过程
DROP PROCEDURE c_trade_log

explain SELECT count(*) from trade_log where month(created_time)=7;
SELECT count(*) from trade_log where month(created_time)=7;
-----优化-----
explain SELECT count(*) from trade_log where created_time BETWEEN "2018-07-01" and "2018-08-01"
or created_time BETWEEN "2019-07-01" and "2019-08-01"
or created_time BETWEEN "2020-07-01" and "2020-08-01"
or created_time BETWEEN "2021-07-01" and "2021-08-01" ;

--不走索引
explain SELECT * from trade_log where id -1000=3030300
--走索引
explain SELECT * from trade_log where id =3030300-1000
 

--不走索引 因为有max，不要究竟这儿为什么要加max公式，主要是为了突出这儿不走索引给的一个例子
explain select * FROM test_t WHERE id IN
( SELECT max(id) FROM test_t WHERE a BETWEEN 1000 and 2000 GROUP BY a )
--走索引
explain select * FROM test_t WHERE id IN
( SELECT id FROM test_t WHERE a BETWEEN 1000 and 2000 GROUP BY a )

----慢查询开启情况---
show variables like "slow_query_log";
----设置开启----
set global slow_query_log = "ON";
---慢查询日志地址---
show variables like "slow_query_log_file";