Select from table is slow - mysql

Question

My problem is: simple select query takes a long time (3 minutes).
Structure:
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 |
Rows count:
mysql> select count(*) from seventhcont_exceptionreport;
+----------+
| count(*) |
+----------+
| 7064 |
+----------+
1 row in set (0.00 sec)
Query 1 (normal):
mysql> select id, datetime_created from seventhcont_exceptionreport order by id LIMIT 100 OFFSET 6000;
...
100 rows in set (0.30 sec)
Query 2 (very slow):
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 minutes and 40 sec.
Why?
UPDATE
Explain for query 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)
Explain for query 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)
UPDATE
Analyze table:
mysql> ANALYZE TABLE seventhcont_exceptionreport;
+--------------------------------+---------+----------+----------+
| Table | Op | Msg_type | Msg_text |
+--------------------------------+---------+----------+----------+
| 7k.seventhcont_exceptionreport | analyze | status | OK |
+--------------------------------+---------+----------+----------+
1 row in set (2.51 sec)

Answer 1

I am no MySQL specialist, but I might be able to point you in the right direction.
In the first query, we can see in the Explain Plan, that an index access was used. In contrast, for the second query, we can see that a non-index access is performed (type index vs ALL). Also, we can see that MySQL is using Using filesort.
This means MySQL cannot perform the sort operation on the index and is therefore performing it on the data itself. This could be because the sort buffer is too small (also see https://www.percona.com/blog/2009/03/05/what-does-using-filesort-mean-in-mysql/).
Therefore, try to increase the size of your sort buffer (soft_buffer_size).