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
finishVisit( row, query );
			if ( query != value )
			{
				return new IBaseQueryDefinition[]{ query };
			}
			return null;
		}
		/**
		 * handles a cell in a row
		 */
		public Object visitCell( CellDesign cell, Object value )
		{
			BaseQueryDefinition query = createQuery( cell, value );			
			for ( int i = 0; i < cell.getContentCount( ); i++ )
			{
				build( query, cell.getContent( i ) );
			}
			finishVisit( cell, query );
			if ( query != value )
			{
				return new IBaseQueryDefinition[]{ query };
			}
			return null;
		}
		
		protected void addColumBinding( IBaseQueryDefinition transfer,
				ComputedColumnHandle binding )
		{
			String name = binding.getName( );
			String expr = binding.getExpression( );