Tables that are read only, or that are rarely changed, can completely fill the space available on a segment, and do not require maintenance. If a table does not require a clustered index, you can use parallel bulk copy to completely fill the space on the segment.
If a clustered index is needed, the table’s data pages can occupy up to 80% of the space in the segment. The clustered index tree requires about 20% of the space used by the table.
This size varies, depending on the length of the key. Loading the data into the table initially and creating the clustered index requires several steps, but once you have performed these steps, maintenance is minimal.