我有一个名为CacheSync的模型，mysql显示它有一个索引：

mysql> show indexes from cache_syncs;
+-------------+------------+---------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table       | Non_unique | Key_name                        | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------------+------------+---------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| cache_syncs |          0 | PRIMARY                         |            1 | id          | A         |       90878 |     NULL | NULL   |      | BTREE      |         |               |
| cache_syncs |          1 | index_cache_syncs_on_created_at |            1 | created_at  | A         |       18175 |     NULL | NULL   | YES  | BTREE      |         |               |
+-------------+------------+---------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.01 sec)

但是当我去解释时，它说它没有使用索引：

CacheSync.where("created_at < ?", (Time.now - 1.hour).to_time).explain
 =>
EXPLAIN for: SELECT `cache_syncs`.* FROM `cache_syncs` WHERE (created_at < '2022-06-13 19:37:23.316439')
+----+-------------+-------------+------+---------------------------------+------+---------+------+-------+-------------+
| id | select_type | table       | type | possible_keys                   | key  | key_len | ref  | rows  | Extra       |
+----+-------------+-------------+------+---------------------------------+------+---------+------+-------+-------------+
|  1 | SIMPLE      | cache_syncs | ALL  | index_cache_syncs_on_created_at | NULL | NULL    | NULL | 93651 | Using where |
+----+-------------+-------------+------+---------------------------------+------+---------+------+-------+-------------+
1 row in set (0.00 sec)

为什么不使用索引？

感谢您的帮助， 凯文