注意：返回0行。
我的查询是17.88秒。我不知道为什么。知道我能做些什么来加快速度吗？我特别有点困惑为什么derived2没有索引。我知道子查询返回3887行。也许它说有数（*）没有索引？其余的可能有索引。我尝试在一个新的mysqld实例上运行子查询（所以没有缓存），它花了＆lt; 1sec。我之后立即在同一个连接上运行完整查询，花了17.58秒。
我被困了，因为它必须通过115K行？我跑了select count(*) from file;并获得了2457062（大约5％的行）。读115K行是慢吗？我不认为我可以加快速度吗？
select f.id, d.name as dir, n.name as name, d.volume from file f
join NameList n on n.id=f.name
join directory d on d.id=f.dir
join (
select size
from file
where size>123 and size<456000
group by size
having count(*)>1
) cs on cs.size=f.size
left join Collision c on c.id=f.id
where c.id is null
+----+-------------+------------+--------+------------------+---------+---------+----------------+--------+--------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+------------------+---------+---------+----------------+--------+--------------------------------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 3887 | |
| 1 | PRIMARY | f | ref | dir,name,ix_size | ix_size | 8 | cs.size | 1 | |
| 1 | PRIMARY | n | eq_ref | PRIMARY | PRIMARY | 8 | thedb.f.name | 1 | |
| 1 | PRIMARY | d | eq_ref | PRIMARY | PRIMARY | 8 | thedb.f.dir | 1 | |
| 1 | PRIMARY | c | eq_ref | PRIMARY | PRIMARY | 8 | thedb.f.id | 1 | Using where; Using index; Not exists |
| 2 | DERIVED | file | range | ix_size | ix_size | 8 | NULL | 115394 | Using where; Using index |
+----+-------------+------------+--------+------------------+---------+---------+----------------+--------+--------------------------------------+
+--------------------------------+-----------+
| Status | Duration |
+--------------------------------+-----------+
| starting | 0.000020 |
| Waiting for query cache lock | 0.000007 |
| checking query cache for query | 0.000179 |
| checking permissions | 0.000006 |
| checking permissions | 0.000007 |
| checking permissions | 0.000001 |
| checking permissions | 0.000002 |
| checking permissions | 0.000005 |
| Opening tables | 0.629606 |
| System lock | 0.000204 |
| optimizing | 0.000030 |
| statistics | 0.060076 |
| preparing | 0.000026 |
| executing | 0.000009 |
| Sorting result | 0.000002 |
| Sending data | 0.378205 |
| Waiting for query cache lock | 0.000011 |
| Sending data | 0.000034 |
| init | 0.000032 |
| optimizing | 0.000024 |
| statistics | 0.000050 |
| preparing | 0.000026 |
| executing | 0.000002 |
| Sending data | 16.806435 |
| end | 0.000012 |
| query end | 0.000004 |
| closing tables | 0.000002 |
| removing tmp table | 0.000013 |
| closing tables | 0.000021 |
| freeing items | 0.000021 |
| Waiting for query cache lock | 0.000002 |
| freeing items | 0.000049 |
| Waiting for query cache lock | 0.000002 |
| freeing items | 0.000001 |
| storing result in query cache | 0.000003 |
| logging slow query | 0.000001 |
| logging slow query | 0.000002 |
| cleaning up | 0.000008 |
+--------------------------------+-----------+
38 rows in set (0.00 sec)