code: func (p *PhysicalSelection) ToPB(ctx sessionctx.Context) (*tipb.Executor, error) {
  	sc := ctx.GetSessionVars().StmtCtx
  	client := ctx.GetClient()
  	selExec := &tipb.Selection{
  		Conditions: expression.ExpressionsToPBList(sc, p.Conditions, client),
  	}
  	return &tipb.Executor{Tp: tipb.ExecType_TypeSelection, Selection: selExec}, nil
  }
docs: // ToPB implements PhysicalPlan ToPB interface.