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 type = binding.getDataType( );
			int dbType = ModelDteApiAdapter.toDteDataType( type );
			IBaseExpression dbExpr = new ScriptExpression( expr, dbType );
			dbExpr.setGroupName( binding.getAggregateOn( ) );
			transfer.getResultSetExpressions( ).put( name, dbExpr );
		}
		
		/**
		 * create query for non-listing report item
		 * 
		 * @param item
		 *            report item
		 * @return a report query
		 */
		protected BaseQueryDefinition createQuery( ReportItemDesign item, Object value )
		{
			BaseQueryDefinition parentQuery = null;
			if ( value instanceof BaseQueryDefinition )
			{
				parentQuery = (BaseQueryDefinition)value;
			}
			DesignElementHandle handle = item.getHandle( );
			if ( ! ( handle instanceof ReportItemHandle ) )
			{
				if ( !needQuery( item, parentQuery ) )
				{
					
					return parentQuery;
				}