How to clean / Purge DBQL Tables

delete from DBC.DBQLObjTbl Where CollectTimeStamp < '2009-10-01 00:00:00';
delete from DBC.DBQLogTbl where LogonDateTime < '2009-10-01 00:00:00';
delete from DBC.DBQLSqlTbl Where CollectTimeStamp < '2009-10-01 00:00:00';
delete from DBC.DBQLExplainTbl where CollectTimeStamp < '2009-10-01 00:00:00';
delete from DBC.DBQLStepTbl Where CollectTimeStamp < '2009-10-01 00:00:00';
delete from DBC.DBQLSummaryTbl where CollectTimeStamp < '2009-10-01 00:00:00';