If it does not free enough heap space, it proceeds to collect higher generations until enough memory has been reclaimed.

如果它不能释放足够的堆空间，那么它就继续收集上一代，直到回收了足够的内存。

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Note: If there is too little space on a line for the floating element, it will jump down on the next line, and continue until a line has enough space.

注意：如果在一行空间上的浮动元素过多，超出了空间的容纳范围，它就会自动跳到下一行中，直到一行有足够的空间容纳它。

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RETENTION parameter works best if the current undo tablespace has enough space for the active transactions.

retention能做好地工作如果——当前的回滚表空间对活动事务有足够的空间。

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