为什么在MYSQL中这里不使用索引？

Question

非常奇怪如下：

mysql> explain select *from online where last < now()-interval 30 second and type=1; 
+----+-------------+--------+------+---------------------------------------+------+---------+------+------+-------------+ 
| id | select_type | table | type | possible_keys       | key | key_len | ref | rows | Extra  | 
+----+-------------+--------+------+---------------------------------------+------+---------+------+------+-------------+ 
| 1 | SIMPLE  | online | ALL | i_online_type_last,i_online_last_type | NULL | NULL | NULL | 24 | Using where | 
+----+-------------+--------+------+---------------------------------------+------+---------+------+------+-------------+ 

mysql> explain select *from online where last < '2009-06-16 06:48:33' and type=1; 
+----+-------------+--------+------+---------------------------------------+------+---------+------+------+-------------+ 
| id | select_type | table | type | possible_keys       | key | key_len | ref | rows | Extra  | 
+----+-------------+--------+------+---------------------------------------+------+---------+------+------+-------------+ 
| 1 | SIMPLE  | online | ALL | i_online_type_last,i_online_last_type | NULL | NULL | NULL | 120 | Using where | 
+----+-------------+--------+------+---------------------------------------+------+---------+------+------+-------------+ 
1 row in set (0.00 sec) 


mysql> show index from online; 
+--------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ 
| Table | Non_unique | Key_name   | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | 
+--------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ 
| online |   0 | PRIMARY   |   1 | id   | A   |   24 |  NULL | NULL |  | BTREE  |   | 
| online |   0 | account_id   |   1 | account_id | A   |   24 |  NULL | NULL |  | BTREE  |   | 
| online |   1 | i_online_type_last |   1 | last  | A   |   2 |  NULL | NULL | YES | BTREE  |   | 
| online |   1 | i_online_type_last |   2 | type  | A   |   2 |  NULL | NULL |  | BTREE  |   | 
| online |   1 | i_online_last_type |   1 | last  | A   |   2 |  NULL | NULL | YES | BTREE  |   | 
| online |   1 | i_online_last_type |   2 | type  | A   |   2 |  NULL | NULL |  | BTREE  |   | 
+--------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ 
6 rows in set (0.00 sec) 

对于那些谁说，这是因为台面尺寸：

mysql> explain select *from users where email='test@gmail.com'; 
+----+-------------+-------+-------+---------------+---------------+---------+-------+------+-------+ 
| id | select_type | table | type | possible_keys | key   | key_len | ref | rows | Extra | 
+----+-------------+-------+-------+---------------+---------------+---------+-------+------+-------+ 
| 1 | SIMPLE  | users | const | u_users_email | u_users_email | 386  | const | 1 |  | 
+----+-------------+-------+-------+---------------+---------------+---------+-------+------+-------+ 
1 row in set (0.00 sec) 

mysql> select count(*) from users; 
+----------+ 
| count(*) | 
+----------+ 
|  24 | 
+----------+ 
1 row in set (0.00 sec) 

这里有一些更多的线索：

mysql> explain select * from online where `last` > '2009-06-16 06:48:33' and type in (1,2); 
+----+-------------+--------+-------+--------------------+--------------------+---------+------+------+-------------+ 
| id | select_type | table | type | possible_keys  | key    | key_len | ref | rows | Extra  | 
+----+-------------+--------+-------+--------------------+--------------------+---------+------+------+-------------+ 
| 1 | SIMPLE  | online | range | i_online_type_last | i_online_type_last | 13  | NULL | 2 | Using where | 
+----+-------------+--------+-------+--------------------+--------------------+---------+------+------+-------------+ 
1 row in set (0.00 sec) 

mysql> explain select * from online where `last` < '2009-06-16 06:48:33' and type in (1,2); 
+----+-------------+--------+------+--------------------+------+---------+------+------+-------------+ 
| id | select_type | table | type | possible_keys  | key | key_len | ref | rows | Extra  | 
+----+-------------+--------+------+--------------------+------+---------+------+------+-------------+ 
| 1 | SIMPLE  | online | ALL | i_online_type_last | NULL | NULL | NULL | 120 | Using where | 
+----+-------------+--------+------+--------------------+------+---------+------+------+-------------+ 
1 row in set (0.00 sec) 

改变 '<' 到“ >'会使它完全不同，为什么？

最后我找到了修复，这是因为last有一个默认值“null”，将此列更改为“not null”会使索引起作用。

但我不知道为什么这可以使它不同，任何解释？

Answer 1

对于优化器来说，24行是不够的。您需要使用更大的表格进行测试。

Answer 2

• 首先，它是24行 - 你可能会因此而关闭;
• 其次，尝试删除对日期文字now()-interval 30 second的引用。我看到他们扔掉索引。

---

这是我还是你有2个指标，其是完全一样的？

Answer 3

我看到MySQL中的查询优化器用索引选择做了一些奇怪的事情，并且通常只有通过试验和错误才能找到修复。一对夫妇可以尝试（不作任何保证，他们可能会帮助）：

• 删除冗余的指标之一（i_online_*）;保留第一列具有更高特异性的那个（可能是第一列为last的那个）。
• 尝试看看是否作出last列NOT NULL有差别（使用最低日期，而不是null）。
• 我第二次罗伯特蒙代亚努的建议，试图取代now() ...表达式。尝试使用之前设置的变量。

它也将有助于查看您的表的整个架构;也许有一些奇怪的副作用要发现？