查询A
Select id from jobs;
| 55966 |
| 55971 |
+-------+
10705 rows in set (0.00 sec)
查询 B
Select id from jobs where status = 0;
| 55966 |
| 55971 |
+-------+
7933 rows in set (**20.22 sec**)
有一个状态索引。
mysql> explain select id from jobs where status = 0;
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| 1 | SIMPLE | jobs | ALL | status | NULL | NULL | NULL | 10705 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
1 row in set (0.01 sec)
mysql> show profile for query 1;
+--------------------------------+-----------+
| Status | Duration |
+--------------------------------+-----------+
| starting | 0.000023 |
| checking query cache for query | 0.000039 |
| checking permissions | 0.000006 |
| Opening tables | 0.000008 |
| System lock | 0.000004 |
| Table lock | 0.000016 |
| init | 0.000021 |
| optimizing | 0.000007 |
| statistics | 0.000904 |
| preparing | 0.000023 |
| executing | 0.000003 |
| Sending data | 19.751547 |
| end | 0.000009 |
| query end | 0.000002 |
| freeing items | 0.001561 |
| storing result in query cache | 0.000122 |
| logging slow query | 0.000002 |
| logging slow query | 0.000002 |
| cleaning up | 0.000003 |
+--------------------------------+-----------+
mysql> show index from jobs;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| jobs | 1 | status | 1 | status | A | 6 | NULL | NULL | YES | BTREE | |
| jobs | 1 | date | 1 | dateinit | A | 1784 | NULL | NULL | YES | BTREE | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
10 rows in set (0.02 sec)
我不明白为什么查询 B 需要 20 秒而查询 A 需要 0 秒。 “状态”索引。生产和开发的结果相同。服务器。
最佳答案
尝试将索引更改为两列 (status, id)
据我所知，您没有任何关于 id 的索引
ANSI standard (查找“Scalar expressions 125”)声明 COUNT(*) 给出表的行数:它旨在从一开始就进行优化。
If COUNT(*) is specified, then the result is the cardinality of T.
这就是为什么 COUNT(*) 比 COUNT(id) 快得多。 COUNT(*) 可以使用status 索引。 COUNT(id) 不会使用这个索引并且没有其他有效的索引
什么是聚簇索引，什么是PK？你有两者(即使索引相同)，对吧？
关于Mysql - where子句比完全扫描慢，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/6721944/