Bug 177504 Engine doesn't provide SubQuery for extended item
Engine doesn't provide SubQuery for extended item, so chart can't use container's binding to execute the query. If possible, engine could provide an option for extended item to choose whether engine could create the query, so extended item won't create SubQuery by itself.
engine/org.eclipse.birt.report.engine/src/org/eclipse/birt/report/engine/data/dte/ReportQueryBuilder.java
String name = String.valueOf( item.getID( ) );
				query = new SubqueryDefinition( name, parentQuery );
				parentQuery.getSubqueries( ).add( query );				
			}
			
			
			query.setMaxRows( maxRows );
			if ( item.getHandle( ) instanceof ReportItemHandle )
			{
				ReportItemHandle designHandle = (ReportItemHandle) item
						.getHandle( );
				Iterator iter = designHandle.columnBindingsIterator( );
				while ( iter.hasNext( ) )
				{
					ComputedColumnHandle binding = (ComputedColumnHandle) iter
							.next( );
					addColumBinding( query, binding );
				}
			}
			addSortAndFilter( item, query );			
		
			return query;
		}
		/**
		 * An item needs query when it satisfies following conditions:
		 * <li>Has column bindings.
		 * <li>Is a table or a list.
		 * <li>Has hightlight rules and doesn't have parent query.
		 * 
		 * @param item