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
/**
	 * Check whether a column is a temp column according to column name.
	 * @param columnName
	 * @return
	 */
	private boolean isTempColumn( String columnName )
	{
		if ( columnName.length( ) < 7 )
		{
			return false;
		}
		if ( !columnName.matches( TEMP_COLUMN_STRING ) )
		{
			return false;
		}
		return true;
	}
	/**
	 * Populate the JointResultMetadata.
	 * 
	 * @param left
	 * @param leftPrefix
	 * @param right
	 * @param rightPrefix
	 * @return
	 * @throws DataException
	 */
	private JointResultMetadata populatorJointResultMetadata(
			IResultMetaData left, String leftPrefix, IResultMetaData right,
			String rightPrefix ) throws DataException