之前用mysql一直也没觉得有特别慢的感觉，最近发现新开发的系统有个页面打开速度非常慢，有时候1分钟都打不开。查了一下系统，定位到是一条sql语句执行慢造成的。该sql如下：
- SELECT
- COUNT(1) AS value
- , document.sourceType AS lable
- FROM
- document
- WHERE
- document.id
- IN
- (SELECT
- document_id
- FROM
- subject_document
- WHERE
- subject_id = 345
- )
- GROUP BY
- document.sourceType
粗略看一下，真没觉得有非常严重的问题，只是本来该用内连接的写成了IN。查看执行计划结果如下：
+----+--------------------+------------------+------+-------------------------------------------+---------------------------+---------+------------+--------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+------------------+------+-------------------------------------------+---------------------------+---------+------------+--------+----------------------------------------------+
| 1 | PRIMARY | document | ALL | NULL | NULL | NULL | NULL | 117287 | Using where; Using temporary; Using filesort |
| 2 | DEPENDENT SUBQUERY | subject_document | ref | uk_subject_id_document_id,idx_document_id | uk_subject_id_document_id | 10 | const,func | 1 | Using where; Using index |
+----+--------------------+------------------+------+-------------------------------------------+---------------------------+---------+------------+--------+----------------------------------------------+
2 rows in set (0.41 sec)
嵌套的那个查询是用了索引的，但是外层没有用上索引，所以导致查询速度严重下降。
+----+-------------+-------+--------+-------------------------------------------+---------------------------+---------+----------------------+------+-----------------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+-------------------------------------------+---------------------------+---------+----------------------+------+-----------------------------------------------------------+
| 1 | SIMPLE | sd | ref | uk_subject_id_document_id,idx_document_id | uk_subject_id_document_id | 5 | const | 455 | Using where; Using index; Using temporary; Using filesort |
| 1 | SIMPLE | d | eq_ref | PRIMARY | PRIMARY | 4 | pscms.sd.document_id | 1 | |
+----+-------------+-------+--------+-------------------------------------------+---------------------------+---------+----------------------+------+-----------------------------------------------------------+
由于这个程序是从一个oracle数据库上代码移植过来的，所以特别去看了一下oracle的执行速度。结果在oracle上执行速度非常快，忘记看执行计划了，但显然是利用上索引。看来oracle的优化器做的要强大许多。
最后查看了一下自己的历史代码，里面还是有一些地方用到了IN。上面那个嵌套查询里面的子查询结果就算为空，居然也要执行很久。那如果直接写成 IN (1)呢？结果是速度又变得很快了，这要再不快，IN条件真一无是处了。查看了一下这个的执行计划：
+----+-------------+----------+-------+---------------+---------+---------+------+------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------+-------+---------------+---------+---------+------+------+----------------------------------------------+
| 1 | SIMPLE | document | range | PRIMARY | PRIMARY | 4 | NULL | 2 | Using where; Using temporary; Using filesort |
+----+-------------+----------+-------+---------------+---------+---------+------+------+----------------------------------------------+
总结一下，数据库的查询速度和索引有很大关系，正确利用索引可以有效的加快查询速度，另外还要多用执行计划去分析。
本文转自passover 51CTO博客，原文链接：http://blog.51cto.com/passover/608499，如需转载请自行联系原作者