前言：今天在生產(chǎn)環(huán)境抓到慢SQL一例，單獨拿出來執(zhí)行速度非?？?，我們拿出該SQL分析一下。

原始SQL：

select  distinct metadatalogid,opertime from xwcmmetadatalog where metadataid='4395597' order by opertime desc;

執(zhí)行計劃：

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)

我們可以看到該執(zhí)行計劃其實問題也不是很大，唯一需要注意的就是用到了文件排序，當(dāng)MySQL SERVER io比較吃緊的時候大量執(zhí)行該SQL就會有問題了，所以我們優(yōu)化思路就是消除這個文件排序即可。
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)

我們看到執(zhí)行計劃用到的索引只包含METADATAID列，而我們的sql篩選條件不僅包含metadataid='4395597' 還用到了opertime來排序，所以我們針對這兩個字段建立索引即可。

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

再次查看執(zhí)行計劃：

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