· If the column value is NULL, do not set aside the defined, fixed amount of space.
· If the column value can be easily known or determined (like default values) and if the value is available to the database manager during record formatting and column extraction.
DB2 compression has been improved as of DB2 10 for LUW. IBM calls the new compression in DB2 10, adaptive compression (http://www-01.ibm.com/support/knowledgecenter/SSEPGG_10.1.0/com.ibm.db2.luw.admin.dbobj.doc/doc/c0059054.html). With adaptive compression you can expect improved compression rates.
Adaptive compression combines table level compression with an additional page level compression. The compression at the page level can change over time based on your actual data – hence, “adaptive” compression. Significant improvement in compression rates can be achieved with adaptive compression.
For DB2 LUW, you use Data Studio to estimate how much storage you could save via adaptive compression. You will use the Data Source Explorer to find Schemas, find SYSPROC and then run the ADMIN_GET_TAB_COMPRESS_INFO user-defined function. Simply provide the name of the table to estimate and the UDF does the rest.
Of course, before you consider compression be sure to examine all of its details -- and be sure to understand all of the nuances of your particular data and applications. I mean, we didn’t even touch upon index compression (http://it.toolbox.com/blogs/db2zos/index-compression-and-ibm-redpapers-18160) in this article…
But don't be afraid of investigating data compression in DB2... it can be a very handy tool in the DBA's arsenal!