db2 tables altered, reorg
The reorg is necessary when the table content has changed (lots of inserts, lots of updates, many deletes, a combination of all of that) and you can query which tables need reorganization via 'REORGCHK' 

http://publib.boulder.ibm.com/infocenter/db2luw/v9r7/topic/com.ibm.db2.luw.admin.cmd.doc/doc/r0001971.html