"Plan isn't ready yet":"This value occurs with EXPLAIN FOR CONNECTION when the optimizer has not finished creating the execution plan for the statement executing in the named connection. If execution plan output comprises multiple lines, any or all of them could have this Extra value, depending on the progress of the optimizer in determining the full execution plan.",
"Select tables optimized away":"仅通过使用索引,优化器可能仅从聚合函数结果中返回一行。如:在没有 GROUP BY 子句的情况下,基于索引优化 MIN/MAX 操作,或者对于 MyISAM 存储引擎优化 COUNT(*) 操作,不必等到执行阶段再进行计算,查询执行计划生成的阶段即完成优化。",