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
if ( object instanceof IReportItemExecutor )
			{
				return (IReportItemExecutor) object;
			}
			else if (object instanceof IReportItemGeneration)
			{
				return new ExtendedItemGenerationExecutor( (IReportItemGeneration)object);
			}
		}
		return null;
	}
	
	/**
	 * @param itemType the type of the extended item, i.e., "chart"
	 * @return an object that is used for presentation time extended item processing 
	 */
	public IReportItemPresentation createPresentationItem(String itemType)
	{
		IConfigurationElement config = (IConfigurationElement)presentationExtensions.get(itemType);
		if (config != null)
		{
			Object object = createObject(config, "class"); 
			if (object instanceof IReportItemPresentation)
			{
				return (IReportItemPresentation)object;
			}
		}
		return null;
	}