How Data Compression Works in DB2 UDB
Compression in DB2 UDB v8.1 can take place at the table level, as well as at the column level, depending on the data you want to compress. There are two ways in which DB2 UDB tables can occupy less space when stored on disk:
If the column value is NULL or a zero-length varying character, the database manager can use a mechanism at the table level by which the defined fixed amount of space reserved for the data is not allocated and no data is stored on disk.
If the column value can be easily known or determined (like system default values) and if the value is available to the database manager during record formatting and column extraction, the database manager can replace this value at run time and not store ...