code: func onPhaseImplementation(sctx sessionctx.Context, g *memo.Group) (plannercore.Plan, error) {
  	prop := &property.PhysicalProperty{
  		ExpectedCnt: math.MaxFloat64,
  	}
  	// TODO replace MaxFloat64 costLimit by variable from sctx, or other sources.
  	impl, err := implGroup(g, prop, math.MaxFloat64)
  	if err != nil {
  		return nil, err
  	}
  	if impl == nil {
  		return nil, plannercore.ErrInternal.GenWithStackByArgs("Can't find a proper physical plan for this query")
  	}
  	return impl.GetPlan(), nil
  }
docs: // onPhaseImplementation starts implementation physical operators from given root Group.