build_subselect

# File activerecord/lib/arel/visitors/mysql.rb, line 88
        def build_subselect(key, o)
          subselect = super

          # Materialize subquery by adding distinct
          # to work with MySQL 5.7.6 which sets optimizer_switch='derived_merge=on'
          unless has_limit_or_offset_or_orders?(subselect)
            core = subselect.cores.last
            core.set_quantifier = Arel::Nodes::Distinct.new
          end

          Nodes::SelectStatement.new.tap do |stmt|
            core = stmt.cores.last
            core.froms = Nodes::Grouping.new(subselect).as("__active_record_temp")
            core.projections = [Arel.sql(quote_column_name(key.name))]
          end
        end