The ILM Engine uses native database technology to compress a segment. If you want to compress multiple segments at once, create a segment set and run the compress segments standalone job.

If the segmentation group that contains the segments you want to compress has a bitmap index and any of the segments in the group are read-only, you must either turn the read-only segments to read-write mode or drop the bitmap index before you compress any segments. This is because Oracle requires that you mark bitmap indexes as unusable before you compress segments for the first time. When compression is complete, the segmentation process cannot rebuild the index for segments that are read-only.