第二十四节、为什么SQL语句逻辑相同，性能却差异巨大？

1、条件字段函数操作：

对索引字段做函数操作，可能会破坏索引值的有序性，因此优化器就决定放弃走树搜索功能

2、隐式类型转换

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3、隐式字符编码转换

mysql> CREATE TABLE `tradelog` (

  `id` int(11) NOT NULL,

  `tradeid` varchar(32) DEFAULT NULL,

  `operator` int(11) DEFAULT NULL,

  `t_modified` datetime DEFAULT NULL,

  PRIMARY KEY (`id`),

  KEY `tradeid` (`tradeid`),

  KEY `t_modified` (`t_modified`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

mysql> CREATE TABLE `trade_detail` (

  `id` int(11) NOT NULL,

  `tradeid` varchar(32) DEFAULT NULL,

  `trade_step` int(11) DEFAULT NULL, /* 操作步骤 */

  `step_info` varchar(32) DEFAULT NULL, /* 步骤信息 */

  PRIMARY KEY (`id`),

  KEY `tradeid` (`tradeid`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8;

insert into tradelog values(1, 'aaaaaaaa', 1000, now());

insert into tradelog values(2, 'aaaaaaab', 1000, now());

insert into tradelog values(3, 'aaaaaaac', 1000, now());

insert into trade_detail values(1, 'aaaaaaaa', 1, 'add');

insert into trade_detail values(2, 'aaaaaaaa', 2, 'update');

insert into trade_detail values(3, 'aaaaaaaa', 3, 'commit');

insert into trade_detail values(4, 'aaaaaaab', 1, 'add');

insert into trade_detail values(5, 'aaaaaaab', 2, 'update');

insert into trade_detail values(6, 'aaaaaaab', 3, 'update again');

insert into trade_detail values(7, 'aaaaaaab', 4, 'commit');

insert into trade_detail values(8, 'aaaaaaac', 1, 'add');

insert into trade_detail values(9, 'aaaaaaac', 2, 'update');

insert into trade_detail values(10, 'aaaaaaac', 3, 'update again');

insert into trade_detail values(11, 'aaaaaaac', 4, 'commit');

mysql> select d.* from tradelog l, trade_detail d where d.tradeid=l.tradeid and l.id=2; /* 语句 Q1*/

执行结果分析：

优化方式1：

把trade_detai表上的tradeid字段的字符集也改成utf8mb4，这样就没有字符集转换的问题了

alter table trade_detail modify tradeid varchar(32) CHARACTER SET utf8mb4 default null;

优化方式二：

修改sql语句

mysql> select d.* from tradelog l , trade_detail d where d.tradeid=CONVERT(l.tradeid USING utf8) and l.id=2;

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无法使用索引：

mysql> select d.* from tradelog l, trade_detail d where d.tradeid=l.tradeid and l.id=2; /* 语句 Q1*/

select * from trade_detail where CONVERT(traideid USING utf8mb4)=$L2.tradeid.value;

可以使用索引：（CONVERT函数在输入处执行）

mysql>select l.operator from tradelog l , trade_detail d where d.tradeid=l.tradeid and d.id=4;

select operator from tradelog where traideid =CONVERT($R4.tradeid.value USING utf8mb4);