columns_for_distinct

# File activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb, line 819
        def columns_for_distinct(columns, orders) # :nodoc:
          order_columns = orders.compact_blank.map { |s|
            # Convert Arel node to string
            s = visitor.compile(s) unless s.is_a?(String)
            # Remove any ASC/DESC modifiers
            s.gsub(/\s+(?:ASC|DESC)\b/, "")
             .gsub(/\s+NULLS\s+(?:FIRST|LAST)\b/, "")
          }.compact_blank.map.with_index { |column, i| "#{column} AS alias_#{i}" }

          (order_columns << super).join(", ")
        end