Several maintenance operations require room to make a copy of the data pages of a table:
create clustered index
alter table...lock
Some alter table commands that add or modify columns
alter table...partition by
reorg rebuild on a table
In most cases, these commands also require space to recreate any indexes, so you must determine:
The size of the table and its indexes
The amount of space available on the segment where the table is stored
The space management properties set for the table and its indexes
