code: func (p *PhysicalStreamAgg) ToPB(ctx sessionctx.Context) (*tipb.Executor, error) {
  	sc := ctx.GetSessionVars().StmtCtx
  	client := ctx.GetClient()
  	aggExec := &tipb.Aggregation{
  		GroupBy: expression.ExpressionsToPBList(sc, p.GroupByItems, client),
  	}
  	for _, aggFunc := range p.AggFuncs {
  		aggExec.AggFunc = append(aggExec.AggFunc, aggregation.AggFuncToPBExpr(sc, client, aggFunc))
  	}
  	return &tipb.Executor{Tp: tipb.ExecType_TypeStreamAgg, Aggregation: aggExec}, nil
  }
docs: // ToPB implements PhysicalPlan ToPB interface.