However, vacuum columnoperations present an arduous environment for efficient fractionation, which makes upgrading product from bottoms to distillate more difficult.
然而，有效馏分的提取会使减压操作处境变得困难，也使塔底物料中蒸馏出更多的产品变得更加困难。
2
So far we've considered only basic model operations, like displaying or editing simple column values.
到现在为止，我们只考虑了基本的模型操作，例如显示或编辑简单的列值。
3
Note that if several counting functions or tests operate on the same column, then the sort and count operations only need to be done once.