DB2 huge IREF and lots of scans
The ?'s are Parameter markers. Read abut them here https://www.ibm.com/support/knowledgecenter/en/SSEPGG_10.5.0/com.ibm.db2.luw.apdv.sqlpl.doc/doc/c0020295.html

I would suggest you add an index on column AB

Also consider using IBM Data Server Manager to monitor you database. It includes the ability to see explain plans and also invoke the design advisor