RelToSqlConverter should use ordinal for ORDER BY if the dialect allows

For example:
Query SQL is 

select a, sum(b) from table group by a order by 2

When we convert this sql to relNode and convert it back to SQL by RelToConverter, the result is:

select a, sum(b) from table group by a order by sum(b)

But we want to keep order by column to ordinal.

Would we add a switch in RelToConverter to control whether convert sort by column to expression or ordinal?
Maybe we can use method isSortByOrdinal in dialect SqlConformance?