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/extension/internal/ExtensionManager.java
return;
		IExtension[] exts = extPoint.getExtensions();
		logger.log(Level.FINE, "Start load extension point: {0}", EXTENSION_POINT_EMITTERS); 
		for (int i = 0; i < exts.length; i++)	
		{
			String namespace = exts[i].getNamespace( );
			IConfigurationElement[] configs = exts[i].getConfigurationElements();
			for (int j = 0; j < configs.length; j++)	 
			{				
				String format = configs[j].getAttribute("format");	
				String mimeType = configs[j].getAttribute("mimeType");	
				String id = configs[j].getAttribute("id"); 
				String pagination = configs[j].getAttribute("pagination");
				String icon = configs[j].getAttribute( "icon" );
				EmitterInfo emitterInfo = new EmitterInfo( format, id,
						pagination, mimeType, icon, namespace, configs[j] );
				emitterExtensions.add(emitterInfo);
				assert( format != null );
				formats.put(format.toLowerCase( ), emitterInfo);
				emitters.put( id, emitterInfo );
				logger.log(Level.FINE, "Load {0} emitter {1}", new String[]{format, id}); 
			}
		}
	}
	
	/**
	 * @param format the output format
	 * @return the mime type for the specific format
	 */
	public String getMIMEType( String format )