Large object data types store data ranging in size from zero bytes to two gigabytes - 1.
The three large object data types have the following definitions:
A separate database location stores all large object values outside their records in the table. There is a large object descriptor for each large object in each row in a table. The large object descriptor contains control information used to access the large object data stored elsewhere on disk. It is the storing of large object data outside their records that allows LOBs to be 2 GB in size. Accessing the large object descriptor causes a small amount of overhead when manipulating LOBs. (For storage and performance reasons you would likely not want to put small data items into LOBs.)
The maximum size for each large object column is part of the declaration of the large object type in the CREATE TABLE statement. The maximum size of a large object column determines the maximum size of any LOB descriptor in that column. As a result, it also determines how many columns of all data types can fit in a single row. The space used by the LOB descriptor in the row ranges from approximately 60 to 300 bytes, depending on the maximum size of the corresponding column. For specific sizes of the LOB descriptor, refer to the CREATE TABLE statement in the SQL Reference.
The lob-options-clause on CREATE TABLE gives you the choice to log (or not) the changes made to the LOB column(s). This clause also allows for a compact representation for the LOB descriptor (or not). This means you can allocate only enough space to store the LOB or you can allocate extra space for future append operations to the LOB. The tablespace-options-clause allows you to identify a LONG table space to store the column values of long field or LOB data types. For more information on the CREATE TABLE and ALTER TABLE statements, refer to the SQL Reference.
With their potentially very large size, LOBs can slow down the performance of your database system significantly when moved into or out of a database. Even though DB2 does not allow logging of a LOB value greater than 1 GB, LOB values with sizes near several hundred megabytes can quickly push the database log to near capacity. An error, SQLCODE -355 (SQLSTATE 42993), results from attempting to log a LOB greater than 1 GB in size. The lob-options-clause in the CREATE TABLE and ALTER TABLE statements allows users to turn off logging for a particular LOB column. Although setting the option to NOT LOGGED improves performance, changes to the LOB values after the most recent backup are lost during roll-forward recovery. For more information on these topics, refer to the Administration Guide.