Key compression lets you compress portions of the primary key column values in an
index or index-organized table, which reduces the storage overhead of repeated
values.

Generally, keys in an index have two pieces, a grouping piece and a unique piece. If
the key is not defined to have a unique piece, Oracle provides one in the form of a
rowid appended to the grouping piece. Key compression is a method of breaking off
the grouping piece and storing it so it can be shared by multiple unique pieces.

键压缩
1. 利用键压缩可以将索引或索引组织表中键值的部分内容进行压缩 ，以便减少重复值带来的存储开销
2. 索引键包含两个片段 ：分组片段和唯一片段
3. 如果索引键不是定义为唯一片段 ，Oracle将在分组片段后追加rowid作为唯一片段
4. 键压缩是将键的分组片段从键中拆分出来并存储供多个唯一片段使用