protected DataSetGroupIndex singleGroup(DataSetGroup op, InternalContext context) {
ColumnGroup columnGroup = op.getColumnGroup();
DataColumn sourceColumn = context.dataSet.getColumnById(columnGroup.getSourceId());
ColumnType columnType = sourceColumn.getColumnType();
GroupStrategy groupStrategy = columnGroup.getStrategy();
IntervalBuilder intervalBuilder = intervalBuilderLocator.lookup(columnType, groupStrategy);
if (intervalBuilder == null) throw new RuntimeException("Interval generator not supported.");
// No index => Build required
if (context.index == null) {