Bug 182895 Support sort on Level attribute.
Currently in OLAP engine we only support sort on level key value. We should go ahead to support sort on level attribute value.
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/olap/data/impl/aggregation/sort/AggregationSortHelper.java
base.getAggributeNames( ) );
	}
	/**
	 * Populate the aggregation disk array which is then used to combine with base disk array.
	 * @param base
	 * @param sortKeys
	 * @param filteredAggrResultSet
	 * @return
	 * @throws IOException
	 */
	private static IDiskArray populateAggrDiskArray(
			IAggregationResultSet base, SortKey[] sortKeys )
			throws IOException
	{
		IDiskArray keyDiskArray = new BufferedStructureArray( AggregationResultRow.getCreator( ),
				4096 );
		for ( int i = 0; i < base.length( ); i++ )
		{
			base.seek( i );
			
			keyDiskArray.add( new AggrValueOnlyResultRow( toObjArray( createMatchedAggrRow( base,
					sortKeys ) ) ) );
		}
		return keyDiskArray;
	}
	/**
	 * Find all matched AggrRows and populate the aggregation value to a list.
	 * 
	 * @param base
	 * @param sortKeys