数据中的模式：对于特定的表空间或分区，数据中重复出现的模式的出现频率将决定压缩的效果。
Patterns in the data: the frequency of recurring patterns in the data determines how effective compression will be for a particular table space or partition.
压缩也可以并行执行，但这会降低活动对象的打包效果，可能会生成几个较小的闲置空间，而不是一整块闲置空间。
Compaction can also be done in parallel, but it results in a less efficient packing of the live objects — instead of a single block of free space, several smaller ones might be created.
原始的数字图像需要大量的存储空间，而直接对其进行压缩编码效果并不理想。
Saving original image data needs a lot of memory space, and direct compression doesn't get a good effect.
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