Right now we are getting exceptions on DB2 if history level none is set and query of process instances with ordering by start time is performed.
to reproduce run org.camunda.bpm.cockpit.plugin.base.ProcessInstanceRestServiceTest#testQueryOrderByStartTime with profile cfghistorynone,db2,db2-105

Hint
statement

<if test="!historyEnabled">
        null START_TIME_
</if>

in org/camunda/bpm/cockpit/plugin/base/queries/processInstance.xml
looks very strange.

Possible solution
http://stackoverflow.com/questions/2509233/db2-wont-allow-null-column
customization of null column is done through org.camunda.bpm.engine.impl.db.sql.DbSqlSessionFactory