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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