mysql索引及优化学习

Posted 2020-10-23 songgj

一般优化mysql首先看查找的数据有没有用到索引，没有索引就加索引，有索引时候避免索引失效。

看下面两条语句

MySQL [release_test_oa]> EXPLAIN SELECT * FROM WORKFLOW_LOGS WHERE ID -100 < 300000;
+----+-------------+---------------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table         | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+---------------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | WORKFLOW_LOGS | ALL  | NULL          | NULL | NULL    | NULL | 3998 | Using where |
+----+-------------+---------------+------+---------------+------+---------+------+------+-------------+

 

MySQL [release_test_oa]> EXPLAIN SELECT * FROM WORKFLOW_LOGS WHERE ID < 300000-100 ;
+----+-------------+---------------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table         | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+---------------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | WORKFLOW_LOGS | range | PRIMARY       | PRIMARY | 8       | NULL | 1999 | Using where |
+----+-------------+---------------+-------+---------------+---------+---------+------+------+-------------+

 上面那句没有使用到ID字段上的索引，而下面使用到了，所以要避免在列上进行表达式计算。