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
* @param dataEngine
	 * @param dataSetDesign
	 * @param appContext
	 * @throws DataException
	 */
	private void initializeResultClass( DataEngineImpl dataEngine, Map appContext )
			throws DataException
	{
		try
		{
			IQueryResults left = getResultSetQuery( dataEngine,
					dataSet.getLeftDataSetDesignName( ),
					appContext,
					dataSet.getJoinConditions( ),
					true );
			IQueryResults right = getResultSetQuery( dataEngine,
					dataSet.getRightDataSetDesignName( ),
					appContext,
					dataSet.getJoinConditions( ),
					false );
			JointResultMetadata meta = getJointResultMetadata( left.getResultMetaData( ),
					right.getResultMetaData( ) );
			resultClass = meta.getResultClass( );
		}
		catch ( BirtException be )
		{
			throw DataException.wrap( be );
		}
	}
	/**