标签：冗余 null esc mysq www 公式 default rem date
通过 EXPLAIN 来判断 SQL 的执行计划，发现慢 SQL 或者性能影响业务的 sql
explain [EXTENDED] SELECT...
查看执行计划会有如下信息：
id:1 select_type:simple table:t possible_keys:primary key:primary key_len:4 ref:const rows:1 filtered:100.00 extra:using index
关于 key_len 长度计算公式：
varchar(10) 变长字段且允许 NULL：10_(Character Set:utf-8,gbk=2,latin1=1)+1(NULL)+2(变长字段) varchar(10) 变长字段且不允许 NULL：10_(Character Set:utf-8,gbk=2,latin1=1)+2(变长字段) char(10) 变长字段且允许 NULL：10_(Character Set:utf-8,gbk=2,latin1=1)+1(NULL) char(10) 变长字段且不允许 NULL：10_(Character Set:utf-8,gbk=2,latin1=1)
默认 null，会占用字节，索引长度。 也就是说索引 key_len 长度过大，也会影响 SQL 性能。
min(), max()
order by
group by
distinct
如果列定义为 DEFAULT NULL 时，NULL 值也会有索引，存放在索引树的最前端部分。 案例 1：
CREATE TABLE `base_assets` ( `ID` int(11) unsigned NOT NULL AUTO_INCREMENT, `ASSETS1` int(11) DEFAULT ‘0‘, `ASSETS2` int(10) unsigned DEFAULT NULL, `ASSETS5` int(10) unsigned NOT NULL DEFAULT ‘0‘, `ASSETS3` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_STAMP, `ASSETS4` varchar(200) NOT NULL DEFAULT, PRIMARY KEY (`ID`) KEY ‘idx_A1‘ (`ASSETS1`), KEY `key_c2` (`ASSETS2`) ) ENGINE=InnoDB AUTO_INCREMENT=512976 DEFAULT CHARSET=utf8;
表说明：
# 查询 explain select ASSETS1 from base_assets where ASSETS1 = 100000 limit 1; # 对比 explain select ASSETS5 from base_assets where ASSETS5 = 100000 limit 1; # 统计类业务： explain select max(ASSETS2) from base_assets; # 求平均值，有索引时，扫描索引即可，无需全表扫描（避免回表） explain select avg(ASSETS1) from base_assets;
# 查询 explain select ASSETS5 from base_assets where ASSETS5 > 100000 order by ASSETS5 limit 100; # 有索引，可以快速排序完成 explain select ASSETS5 from base_assets where ASSETS1 > 100000 order by ASSETS1 limit 100; # 读写的列改成c1 explain select ASSETS1 from base_assets where ASSETS1 > 100000 order by ASSETS1 limit 100;
结果可以再次表明不同的执行计划性能差距。（图略）
desc select count(ASSETS1) from base_assets; desc select count(ASSETS2) from base_assets; desc select count(ASSETS1) from base_assets where ASSETS1 is null; desc select count(ASSETS2) from base_assets where ASSETS2 is null;
desc select * form base_assets where ASSETS1 = 2333 or ASSETS2 = 6666
案例 2:
# 测试索引写入效率 create bable base_assets_test ( id int unsigned not null auto_increment, assets1 int not null default ‘0‘, assets2 int not null default ‘0‘, assets3 int not null default ‘0‘, assets4 int not null default ‘0‘, assets5 timestamp null, assets6 varchar(200) not null default ‘‘, primary key(‘id‘), KEY `idx_c2`(`assets2`), KEY `idx_c3` (`assets3`) ); # 测试有无索引对比写入效率存储过程 delimiter $$$ create procedure ‘insert_test‘(in row_num int) begin declare i int default 1; while i <= row_num do insert into base_assets_test(id,assets1,assets2,assets3,assets4,assets5,assets6) values(i,floor(rand()_row_num),floor(rand()_row_num),floor(rand()_row_num), now(),repeat(‘wubx‘,floor(rand()*)20))); set i = i + 1; end while; end $$$
客户端调用：call insert_test (1 000 000);
插入初始化数据：
| 1 | 模式 | 耗时 |
|---|---|---|
| innodb | 无索引 | 110 |
| innodb | 只有主键索引 | 110 |
| innodb | 下全部索引 | 110 |
| myisam | 无任何索引 | 24 |
| myisam | 只有主键索引 | 27 |
| myisam | 全部索引 | 31 |
over
标签：冗余 null esc mysq www 公式 default rem date
原文地址：https://www.cnblogs.com/yizhiamumu/p/9208181.html
