MySQL 索引命中问题

Question

SELECT
    a.*,
    b.food_name,
    d.`name`
FROM
    a
    LEFT JOIN b ON a.number = b.order_number
    LEFT JOIN c ON b.order_number = c.number
    LEFT JOIN d ON c.store_id = d.id 
WHERE
    a.hide = 0 
    AND a.deleted_at IS NULL  
     AND a.wid = 1996 
GROUP BY
    a.id 
ORDER BY
    a.id DESC 
    LIMIT 5

此时 where 条件中 AND a.wid = 1996 ， b表就全表扫描了

如果我换成 AND b.wid = 1996

这是为啥啊
b 是一个大表
a.number 可能会对应 b 表的多条