我的问题是:简单的选择查询需要很长时间(3分钟)。
结构:
mysql> show create table seventhcont_exceptionreport;
seventhcont_exceptionreport | CREATE TABLE `seventhcont_exceptionreport` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`body_html` longtext NOT NULL,
`datetime_created` datetime NOT NULL,
`subject` varchar(256) NOT NULL,
`host` varchar(128) NOT NULL,
`exc_value` varchar(512) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=74607 DEFAULT CHARSET=utf8 |
行数:
mysql> select count(*) from seventhcont_exceptionreport;
+----------+
| count(*) |
+----------+
| 7064 |
+----------+
1 row in set (0.00 sec)
查询1(正常):
mysql> select id, datetime_created from seventhcont_exceptionreport order by id LIMIT 100 OFFSET 6000;
...
100 rows in set (0.30 sec)
查询2(非常慢):
mysql> select id, datetime_created from seventhcont_exceptionreport order by id LIMIT 100 OFFSET 7000;
...
63 rows in set (3 min 40.56 sec)
!!! 3分40秒。
为什么?
更新
解释查询1:
mysql> EXPLAIN select id, datetime_created from seventhcont_exceptionreport order by id LIMIT 100 OFFSET 6000;
+----+-------------+-----------------------------+-------+---------------+---------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------------------------+-------+---------------+---------+---------+------+------+-------+
| 1 | SIMPLE | seventhcont_exceptionreport | index | NULL | PRIMARY | 4 | NULL | 6100 | |
+----+-------------+-----------------------------+-------+---------------+---------+---------+------+------+-------+
1 row in set (0.00 sec)
解释查询2:
mysql> EXPLAIN select id, datetime_created from seventhcont_exceptionreport order by id LIMIT 100 OFFSET 7000;
+----+-------------+-----------------------------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------------------------+------+---------------+------+---------+------+------+----------------+
| 1 | SIMPLE | seventhcont_exceptionreport | ALL | NULL | NULL | NULL | NULL | 7067 | Using filesort |
+----+-------------+-----------------------------+------+---------------+------+---------+------+------+----------------+
1 row in set (0.00 sec)
更新
分析表:
mysql> ANALYZE TABLE seventhcont_exceptionreport;
+--------------------------------+---------+----------+----------+
| Table | Op | Msg_type | Msg_text |
+--------------------------------+---------+----------+----------+
| 7k.seventhcont_exceptionreport | analyze | status | OK |
+--------------------------------+---------+----------+----------+
1 row in set (2.51 sec)
答案 0 :(得分:1)
我不是MySQL专家,但我可以指出你正确的方向。
在第一个查询中,我们可以在解释计划中看到使用了索引访问。相反,对于第二个查询,我们可以看到执行了非索引访问(类型index
vs ALL
)。此外,我们可以看到MySQL正在使用Using filesort
。
这意味着MySQL无法对索引执行排序操作,因此会对数据本身执行排序操作。这可能是因为排序缓冲区太小(另见https://www.percona.com/blog/2009/03/05/what-does-using-filesort-mean-in-mysql/)。
因此,尝试增加排序缓冲区的大小(soft_buffer_size)。