Factors Affecting Space Requirements for Modify Operations
The following are important factors that affect disk space requirements:
• You need at least twice the disk space of the table (“2X”), one copy of the original table and one copy of the new table.
• The new table size can be increased if an index is being added. Conversely, space can be freed if an index is no longer necessary. Index space can vary widely depending on the size of the key.
• If you are modifying to a sorted structure (ISAM, B-tree) or to hash, an additional copy of the original table is needed, thus requiring three times the disk space of the table (“3X”).
• If you are modifying a compressed table, calculate disk space based on the uncompressed size. In the worst case, this is the row size times the number of rows.
Usually going from a compressed structure to an uncompressed structure increases the table size, and going the other way decreases its size. The amount of change cannot be predicted and is dependent on the data in the table. If many NULL values are present and if many string fields have trailing blanks, the use or omission of compression is very noticeable.