MySQL索引失效：为何`shop_id`索引在数据量增多后失效？

<code class="sql">CREATE TABLE `ns_delivery_shop` (
  `id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
  `goods_id` INT(10) NOT NULL COMMENT '商品id',
  `sku_id` INT(10) NOT NULL COMMENT '商品sku',
  `shop_id` INT(10) NOT NULL COMMENT '代理商id',
  `stock` INT(10) NOT NULL COMMENT '库存',
  `sales` INT(10) NOT NULL COMMENT '销量',
  `create_time` INT(10) NOT NULL COMMENT '时间',
  PRIMARY KEY (`id`),
  KEY `idx_shop_id` (`shop_id`) USING BTREE
) ENGINE=INNODB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8 COMMENT='代理商打货表';</code>

<code class="sql">INSERT INTO `ns_delivery_shop`(`id`, `goods_id`, `sku_id`, `shop_id`, `stock`, `sales`, `create_time`) VALUES (1, 86, 99, 1, 10, 5, 1613960196);
INSERT INTO `ns_delivery_shop`(`id`, `goods_id`, `sku_id`, `shop_id`, `stock`, `sales`, `create_time`) VALUES (2, 86, 100, 1, 15, 10, 1613960196);
INSERT INTO `ns_delivery_shop`(`id`, `goods_id`, `sku_id`, `shop_id`, `stock`, `sales`, `create_time`) VALUES (3, 86, 101, 1, 25, 10, 1613960196);
INSERT INTO `ns_delivery_shop`(`id`, `goods_id`, `sku_id`, `shop_id`, `stock`, `sales`, `create_time`) VALUES (4, 101, 119, 9, 30, 5, 1613960196);
INSERT INTO `ns_delivery_shop`(`id`, `goods_id`, `sku_id`, `shop_id`, `stock`, `sales`, `create_time`) VALUES (5, 101, 119, 11, 30, 5, 1613960196);
INSERT INTO `ns_delivery_shop`(`id`, `goods_id`, `sku_id`, `shop_id`, `stock`, `sales`, `create_time`) VALUES (6, 101, 119, 12, 30, 5, 1613960196);</code>

<code class="sql">EXPLAIN SELECT stock FROM `ns_delivery_shop` WHERE shop_id = 1;</code>