这是MySQL SQL优化的第三篇。公司某个业务系统频繁抛出问题SQL，我们对此类SQL做了基本面统计：
此类SQL近期共执行了12次，最长一次花费480秒，最短286秒
t1表的rows有90多万，始终会扫描这么多不需要的数据
这是由于MySQL查询优化器在处理相关子查询方面存在局限性
MySQL总是会将相关的外层表压到子查询中，它认为这可以更高效地查找数据行。
如果是个小表，情况可能还不会引起我们注意，但是如果外层表示一个非常大的表，那么这个查询的性能会非常槽糕，
很不幸，我们的场景刚好应了后者
我们优化后的执行效果：
Good Luck！
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