关于MySQL时间执行差异的SQL查询

Question

我对twe非常相似的查询的执行时间有疑问。

下面的第二个查询花费的时间比第一个（10次或更多），任何线索为什么？数据大小？

提前谢谢。

QUERY 1：

SELECT DistinctUUsersID.user_guid AS uUserID, d.user_guid AS dUserID, count(*) AS numrows
FROM (SELECT DISTINCT u.user_guid 
      FROM users u) AS DistinctUUsersID 
LEFT JOIN dogs d
  ON DistinctUUsersID.user_guid=d.user_guid
GROUP BY DistinctUUsersID.user_guid
ORDER BY numrows DESC;

QUERY 2：

SELECT DistinctUUsersID.user_guid AS uUserID, DistinctDUsersID.user_guid AS dUserID, count(*) AS numrows
FROM (SELECT DISTINCT u.user_guid 
      FROM users u) AS DistinctUUsersID 
LEFT JOIN (SELECT DISTINCT d.user_guid 
      FROM dogs d) AS DistinctDUsersID 
  ON DistinctUUsersID.user_guid=DistinctDUsersID.user_guid
GROUP BY DistinctUUsersID.user_guid
ORDER BY numrows DESC;