mysql索引和优化，使用where；使用临时的；使用文件排序

Question

哪些表和列应该有索引？我在 flow_permanent_id 和 entry_id 上有一个索引，但它似乎正在使用文件排序？

我可以做些什么来优化它？

mysql> explain SELECT COUNT(*) AS count_all, entry_id AS entry_id FROM `votes` WHERE `votes`.`flow_permanent_id` = '4fab490cdc1c82cfa800000a' GROUP BY entry_id;
+----+-------------+-------+------+----------------------------------+----------------------------------+---------+-------+------+----------------------------------------------+
| id | select_type | table | type | possible_keys                    | key                              | key_len | ref   | rows | Extra                                        |
+----+-------------+-------+------+----------------------------------+----------------------------------+---------+-------+------+----------------------------------------------+
|  1 | SIMPLE      | votes | ref  | index_votes_on_flow_permanent_id | index_votes_on_flow_permanent_id | 74      | const |    1 | Using where; Using temporary; Using filesort |
+----+-------------+-------+------+----------------------------------+----------------------------------+---------+-------+------+----------------------------------------------+
1 row in set (0.00 sec)

Answer 1

为了避免文件排序，您需要一个关于 (flow_permanent_id, entry_id) 的复合索引，以便 MySQL 可以将索引用于 WHERE 和 GROUP BY .