mysl 子查询带来的性能问题

一个带有多个子查询的sql，性能非常慢;通过减少子查询改写后性能大幅度提升。

• 原SQL&执行计划:

SELECT  t1.version_no,  t3.instance_idFROM (SELECT    v.version_no,    v.version_id,    v.message,    v.state,    v.start_time,    v.end_time,    t.task_id,    t.data_freq_unit,    t.team_id  FROM tb_data_version v, tb_task t  WHERE v.task_id = t.task_id AND v.task_id = 5757) t1  LEFT JOIN (    SELECT version_id, MAX(instance_id) AS mx    FROM tb_instance    WHERE task_id = 5757    GROUP BY version_id) t2  ON t2.version_id = t1.version_idLEFT JOIN tb_instance t3 ON t3.instance_id = t2.mx

• 优化后的SQL&执行计划:

SELECTv.version_no,  i.instance_idFROM tb_data_version v  JOIN tb_task t ON (v.task_id = t.task_id)  LEFT JOIN tb_instance i ON (  v.version_id = i.version_id   AND v.task_id = i.task_id   and i.instance_id = (      SELECT MAX(instance_id) FROM tb_instance WHERE version_id = v.version_id AND task_id = t.task_id  )  )WHERE v.task_id = 5757

• 疑惑：怀疑旧SQL中的第二个子查询，对于第一个子查询结果中的每行都会执行一次，但是还是不太确定，如果有类似经验的还请帮忙确认一下！