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 dataSetDesign
	 */
	private void setCurrentDataSet( IBaseDataSetDesign dataSetDesign )
	{
		dataSet = (IJointDataSetDesign) dataSetDesign;
	}
	/**
	 * Return an instance of JointResultMeta.
	 * 
	 * @param left
	 * @param right
	 * @return
	 * @throws DataException
	 */
	private JointResultMetadata getJointResultMetadata( IResultMetaData left,
			IResultMetaData right ) throws DataException
	{
		
		if ( left == null || right == null )
		{
			throw new DataException( ResourceConstants.UNEXPECTED_ERROR );
		}
		try
		{
			String leftPrefix = getDataSetName(dataSet.getLeftDataSetDesignName( ));
			String rightPrefix = getDataSetName(dataSet.getRightDataSetDesignName( ));
			if ( leftPrefix.equals( rightPrefix ) )
			{
				leftPrefix = leftPrefix + "1";
				rightPrefix = rightPrefix + "2";