mysql - 当我添加 ORDER BY 时解释“行”跳转

Question

感谢 MrCleanX 将我指向 EXPLAIN 函数。（由于此处的措辞更好，我删除了另一个问题）......当我将 ORDER BY 子句添加到我的查询时，我基本上在“检查的行”中得到了巨大的跳跃。

表：

packages 和 package_to_tag 都运行 MyISAM

结构如下：

包裹 日期被索引。

+----------------+------------------+----------------+
|   aid(primary) |     source       |   date(index)  |
+----------------+------------------+----------------+
|   1            |    CA            |   2013-04-05   |
+----------------+------------------+----------------+
|   2            |    FL            |   2013-05-05   |
+----------------+------------------+----------------+
|   3            |    UT            |   2012-06-13   |
+----------------+------------------+----------------+
|   4            |    VT            |   2011-04-29   |
+----------------+------------------+----------------+
|   5            |    CT            |   2013-04-10   |
+----------------+------------------+----------------+

package_to_tag package-tag 上的唯一索引，并且package_aid 和 tag 都有索引

+---------------+------------------+
|  package_aid  |     tag          |
+---------------+------------------+
|   2           |    sports        |
+---------------+------------------+
|   2           |    nba           |
+---------------+------------------+
|   1           |    food          |
+---------------+------------------+
|   1           |    burrito       |
+---------------+------------------+
|   4           |    hockey        |
+---------------+------------------+
|   4           |    sports        |
+---------------+------------------+
|   3           |    news          |
+---------------+------------------+
|   5           |    sports        |
+---------------+------------------+
|   5           |    nba           |
+---------------+------------------+

我的基本查询是：

SELECT package_aid, source 
FROM package_to_tag
RIGHT JOIN packages ON packages.aid = package_to_tag.package_aid
WHERE tag IN("sports","nba")
GROUP BY package_aid
HAVING COUNT(*) = 2
ORDER BY date DESC
LIMIT 500;

如果我“解释”给出的结果表明我正在检查 package_to_tag 表中的每条记录（在 150 万左右）。

但是，如果我去掉 ORDER BY 子句，这将减少到更易于管理的 30k。

完整的结果屏幕大写。

包：

package_to_tag：