code: func (a *aggPushDownSolver) getAggFuncChildIdx(aggFunc expression.AggregationFunction, schema expression.Schema) int {
  	fromLeft, fromRight := false, false
  	var cols []*expression.Column
  	for _, arg := range aggFunc.GetArgs() {
  		cols = append(cols, expression.ExtractColumns(arg)...)
  	}
  	for _, col := range cols {
  		if schema.GetColumnIndex(col) != -1 {
  			fromLeft = true
  		} else {
  			fromRight = true
  		}
  	}
  	if fromLeft && fromRight {
  		return -1
  	} else if fromLeft {
  		return 0
  	}
  	return 1
  }
docs: // getAggFuncChildIdx gets which children it belongs to, 0 stands for left, 1 stands for right, -1 stands for both.