前言：今天在生产环境抓到慢SQL一例，单独拿出来执行速度非常快，我们拿出该SQL分析一下。
原始SQL：
select distinct metadatalogid,opertime from xwcmmetadatalog where metadataid='4395597' order by opertime desc;
执行计划：
MariaDB [test]> explain select distinct metadatalogid,opertime from xwcmmetadatalog where metadataid='4395597' order by opertime desc;
+------+-------------+-----------------+------+-------------------------------+-------------------------------+---------+-------+------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+-------------+-----------------+------+-------------------------------+-------------------------------+---------+-------+------+-----------------------------+
| 1 | SIMPLE | xwcmmetadatalog | ref | IX_xwcmmetadatalog_METADATAID | IX_xwcmmetadatalog_METADATAID | 5 | const | 1 | Using where; Using filesort |
+------+-------------+-----------------+------+-------------------------------+-------------------------------+---------+-------+------+-----------------------------+
1 row in set (0.00 sec)
我们可以看到该执行计划其实问题也不是很大，唯一需要注意的就是用到了文件排序，当MySQL SERVER io比较吃紧的时候大量执行该SQL就会有问题了，所以我们优化思路就是消除这个文件排序即可。
查看索引
MariaDB [test]> show index from xwcmmetadatalog;
+-----------------+------------+----------------------------------+--------------+---------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-----------------+------------+----------------------------------+--------------+---------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| xwcmmetadatalog | 0 | PRIMARY | 1 | METADATALOGID | A | 0 | NULL | NULL | | BTREE | | |
| xwcmmetadatalog | 1 | IX_xwcmmetadatalog_SRCMETADATAID | 1 | SRCMETADATAID | A | 0 | NULL | NULL | YES | BTREE | | |
| xwcmmetadatalog | 1 | IX_xwcmmetadatalog_METADATAID | 1 | METADATAID | A | 0 | NULL | NULL | YES | BTREE | | |
+-----------------+------------+----------------------------------+--------------+---------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.00 sec)
我们看到执行计划用到的索引只包含METADATAID列，而我们的sql筛选条件不仅包含metadataid='4395597' 还用到了opertime来排序，所以我们针对这两个字段建立索引即可。
MariaDB [test]> create index idx_medataid_opertime on xwcmmetadatalog(metadataid,opertime);
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
再次查看执行计划：
MariaDB [test]> explain select distinct metadatalogid,opertime from xwcmmetadatalog where metadataid='4395597' order by opertime desc;
+------+-------------+-----------------+------+-----------------------------------------------------+-----------------------+---------+-------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+-------------+-----------------+------+-----------------------------------------------------+-----------------------+---------+-------+------+--------------------------+
| 1 | SIMPLE | xwcmmetadatalog | ref | IX_xwcmmetadatalog_METADATAID,idx_medataid_opertime | idx_medataid_opertime | 5 | const | 1 | Using where; Using index |
+------+-------------+-----------------+------+-----------------------------------------------------+-----------------------+---------+-------+------+--------------------------+
1 row in set (0.00 sec)
完美
