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/api/DataEngineContext.java
case SUBQUERY_INFO_STREAM :
				relativePath = "SubQueryInfo";
				break;
			case QUERY_DEFN_STREAM :
				relativePath = "QueryDefn";
				break;
			case ORIGINAL_QUERY_DEFN_STREAM:
				relativePath = "OriginalQueryDefn";
				break;
			case ROW_INDEX_STREAM:
				relativePath = "RowIndex";
				break;
			case QUERYID_INFO_STREAM :
				relativePath = "QueryIDInfo";
				break;
			case SUBQUERY_PARENTINDEX_STREAM :
				relativePath = "ParentIndex";
				break;
				
			default :
				assert false; 
		}
		
		String streamRoot = "/" + streamID + "/";
		if ( subStreamID != null )
			streamRoot += subStreamID + "/";
		return streamRoot + relativePath;
	}	
}