code: func (p *PhysicalLimit) ToPB(ctx sessionctx.Context) (*tipb.Executor, error) {
  	limitExec := &tipb.Limit{
  		Limit: p.Count,
  	}
  	return &tipb.Executor{Tp: tipb.ExecType_TypeLimit, Limit: limitExec}, nil
  }
docs: // ToPB implements PhysicalPlan ToPB interface.