【mysql】mysql索引及优化学习

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。(如果优化器觉得不需要索引就能返回所需要的数据暂不考虑)

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。

模糊搜索时候前置%不能使用索引

mysql> EXPLAIN SELECT * FROM workflow_order WHERE WF_ID LIKE '%HRCZ%';
+----+-------------+----------------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| id | select_type | table          | partitions | type | possible_keys | key  | key_len | ref  | rows  | filtered | Extra       |
+----+-------------+----------------+------------+------+---------------+------+---------+------+-------+----------+-------------+
|  1 | SIMPLE      | workflow_order | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 21491 |    11.11 | Using where |
+----+-------------+----------------+------------+------+---------------+------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.01 sec)

mysql> EXPLAIN SELECT * FROM workflow_order WHERE WF_ID LIKE '%HRCZ';
+----+-------------+----------------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| id | select_type | table          | partitions | type | possible_keys | key  | key_len | ref  | rows  | filtered | Extra       |
+----+-------------+----------------+------------+------+---------------+------+---------+------+-------+----------+-------------+
|  1 | SIMPLE      | workflow_order | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 21491 |    11.11 | Using where |
+----+-------------+----------------+------------+------+---------------+------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

下面可以用到

mysql> EXPLAIN SELECT * FROM workflow_order WHERE WF_ID LIKE 'HRCZ%';
+----+-------------+----------------+------------+-------+-------------------------------+-------------------------------+---------+------+------+----------+-----------------------+
| id | select_type | table          | partitions | type  | possible_keys                 | key                           | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+----------------+------------+-------+-------------------------------+-------------------------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | workflow_order | NULL       | range | workflow_process_wf_id_unique | workflow_process_wf_id_unique | 98      | NULL |  425 |   100.00 | Using index condition |
+----+-------------+----------------+------------+-------+-------------------------------+-------------------------------+---------+------+------+----------+-----------------------+

数据区分度不大的时候，索引也会失效

比如字段只有两个值，如性别

SELECT * FROM `USERS` WHERE `sex` = 1;