索引失效

函数操作

对条件字段做函数操作，可能破坏了索引值的有序性，走不了索引。

select * from t1 where date(c) ='2019-05-21';

优化：改成范围查询

select * from t1 where c>='2019-05-21 00:00:00' and c<='2019-05-21 23:59:59';

隐式转换

操作符与不同类型的操作对象一起使用时，就会发生类型转换以使操作兼容。

select user_name,tele_phone from user_info where tele_phone =11111111111; /* tele_phone varchar */

实际会做函数操作：

select user_name,tele_phone from user_info where cast(tele_phone as singed int) =11111111111; 

优化：类型统一

select user_name,tele_phone from user_info where tele_phone ='11111111111';//字符串转数字

模糊查询

通配符在前面

select * from t1 where a like '%1111%';

优化:模糊查询必须包含条件字段前面的值

select * from t1 where a like '1111%';

范围查询

范围查询数据量太多，需要回表，因此不走索引。

select * from t1 where b>=1 and b <=2000;

优化：降低单次查询范围，分多次查询。（实际可能速度没得快太多,建议走索引）

select * from t1 where b>=1 and b <=1000;

 show profiles;
+----------+------------+------------------------------------------+
| Query_ID | Duration   | Query                                    |
+----------+------------+------------------------------------------+
|        1 | 0.00534775 | select * from t1 where b>=1 and b <=1000 |
|        2 | 0.00605625 | select * from t1 where b>=1 and b <=2000 |
+----------+------------+------------------------------------------+
2 rows in set, 1 warning (0.00 sec)

计算操作

即使是简单的计算

explain select * from t1 where b-1 =1000;

优化：将计算操作放在等号后面

explain select * from t1 where b =1000 + 1;