Mysql 不使用 group by 和 order by 索引

Question

我有带有以下列的表用户。

ID、名称、更新时间

带有解释计划的查询

mysql> explain select * from users group by users.id order by users.updated_at desc limit 10;
+----+-------------+-------------+------+---------------+------+---------+------+--------+----------------+
| id | select_type | table       | type | possible_keys | key  | key_len | ref  | rows   | Extra          |
+----+-------------+-------------+------+---------------+------+---------+------+--------+----------------+
|  1 | SIMPLE      | users | ALL  | NULL          | NULL | NULL    | NULL | 190551 | Using filesort |
+----+-------------+-------------+------+---------------+------+---------+------+--------+----------------+
1 row in set (0.00 sec)

创建了一个新索引

create index test_id_updated_at on users (id, updated_at);

创建新索引后，仍然通过解释计划获得相同的结果。

mysql> explain select * from users group by users.id order by users.updated_at desc limit 10;
+----+-------------+-------------+------+---------------+------+---------+------+--------+----------------+
| id | select_type | table       | type | possible_keys | key  | key_len | ref  | rows   | Extra          |
+----+-------------+-------------+------+---------------+------+---------+------+--------+----------------+
|  1 | SIMPLE      | users | ALL  | NULL          | NULL | NULL    | NULL | 190551 | Using filesort |
+----+-------------+-------------+------+---------------+------+---------+------+--------+----------------+
1 row in set (0.00 sec)

在查询中强制使用新索引后，仍然得到相同的结果。

我不明白为什么在创建新索引后显示“使用文件排序”。

Answer 1

我复制了你的场景，mysql使用了索引:

mysql> explain SELECT * FROM test_index GROUP BY id ORDER BY updated_at DESC;
+----+-------------+------------+------+---------------+------+---------+------+--------+---------------------------------+
| id | select_type | table      | type | possible_keys | key  | key_len | ref  | rows   | Extra                           |
+----+-------------+------------+------+---------------+------+---------+------+--------+---------------------------------+
|  1 | SIMPLE      | test_index | ALL  | NULL          | NULL | NULL    | NULL | 393520 | Using temporary; Using filesort |
+----+-------------+------------+------+---------------+------+---------+------+--------+---------------------------------+
1 row in set (0.00 sec)

mysql> CREATE INDEX index1 ON test_index(id, updated_at);
Query OK, 0 rows affected (1.85 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> explain SELECT * FROM test_index GROUP BY id ORDER BY updated_at DESC;
+----+-------------+------------+-------+---------------+--------+---------+------+--------+---------------------------------+
| id | select_type | table      | type  | possible_keys | key    | key_len | ref  | rows   | Extra                           |
+----+-------------+------------+-------+---------------+--------+---------+------+--------+---------------------------------+
|  1 | SIMPLE      | test_index | index | NULL          | index1 | 12      | NULL | 393520 | Using temporary; Using filesort |
+----+-------------+------------+-------+---------------+--------+---------+------+--------+---------------------------------+
1 row in set (0.00 sec)

也许你的mysql版本？在 5.5 上测试过。

如果不知道完整的表结构以及您想要用此检索的内容(指定字段而不是“*”)，则很难优化此查询(删除 using filesort 和临时)