MySQL离开连接不使用索引

Question

我因索引问题而陷入困境......我有两张桌子：

产品：

+----+
| id |
+----+
|  1 |
|  2 |
|  3 |
+----+

products_relations

+------------+-----------------+----------+
| product_id | product_link_id | priority |
+------------+-----------------+----------+
|       4084 |            3554 |        1 |
|       8515 |            6680 |        1 |
|       4084 |            1248 |        2 |
+------------+-----------------+----------+

当我运行此查询时

SELECT 
  `products`.*,
  products_relations.product_link_id 
FROM
  `products` 
  LEFT JOIN products_relations 
    ON products_relations.product_id = `products`.id 

未使用索引：

+----+-------------+--------------------+------------+-------+---------------+---------+---------+------+------+----------+----------------------------------------------------+
| id | select_type | table              | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra                                              |
+----+-------------+--------------------+------------+-------+---------------+---------+---------+------+------+----------+----------------------------------------------------+
|  1 | SIMPLE      | products           | NULL       | index | NULL          | PRIMARY | 4       | NULL | 5790 |   100.00 | Using index                                        |
|  1 | SIMPLE      | products_relations | NULL       | ALL   | product_id    | NULL    | NULL    | NULL |    4 |   100.00 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+--------------------+------------+-------+---------------+---------+---------+------+------+----------+----------------------------------------------------+

为什么？

这是我的架构，如果你想尝试一下。

为了使其更具可读性，我对其进行了简化，但索引问题仍然存在。

CREATE TABLE `products_relations` (
  `product_id` int(11) NOT NULL,
  `product_link_id` int(11) NOT NULL,
  `priority` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

ALTER TABLE `products_relations`
  ADD KEY `product_id` (`product_id`);


CREATE TABLE `products` (
  `id` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;


ALTER TABLE `products`
  ADD PRIMARY KEY (`id`);


ALTER TABLE `products`
  MODIFY `id` int(11) NOT NULL AUTO_INCREMENT;