Bug 180189 Support in memory data set cache.
Currently, Design-Time Data Cache is stored on disks. By performance tests using ramdisk, we found that caching in physical memory can improve the performance of RunAndRender by 30%.
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/impl/PreparedJointDataSourceQuery.java
private IBaseDataSetDesign dataSetDesign;
	private Map appContext;
	
	private Collection parameterBindings;
	/**
	 * Constructor.
	 * 
	 * @param dataEngine
	 * @param queryDefn
	 * @param dataSetDesign
	 * @param appContext
	 * @throws DataException
	 */
	PreparedJointDataSourceQuery( DataEngineImpl dataEngine,
			IQueryDefinition queryDefn, IBaseDataSetDesign dataSetDesign,
			Map appContext ) throws DataException
	{
		super( dataEngine, queryDefn, dataSetDesign, appContext );
		
		this.dataEngine = dataEngine;
		this.dataSetDesign = dataSetDesign;
		this.appContext = appContext;
		this.parameterBindings = queryDefn.getInputParamBindings( );
	}
	/**
	 * Initialize the instance. The method includes heavyweight operations
	 * such as ResultIterator population.
	 * 
	 * @param dataEngine
	 * @param appContext