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sql性能优化-mysql子查询是否被重复执行？

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Jun 06, 2016 am 09:44 AM

mysqlsql性能优化

请教！mysql返回固定结果集时，是否被重复执行
例如1：select t.*,(select count(1) from table01) from table01 t;

例如2：select t.*,(select count(1) from table01 t2 where t2.key=t1.key) from table01 t1 ;——像这种嵌套的子查询是会被重复执行的。
疑问：上面例子1中的子查询遍历过程中，都是固定结果集，会被重复执行N遍吗（N条记录）？
执行计划中是如何查看子查询有无重复执行，或分析子查询的执行效率，求教！

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