code: func getCardinality(cols []*expression.Column, schema *expression.Schema, profile *property.StatsInfo) float64 {
  	indices := schema.ColumnsIndices(cols)
  	if indices == nil {
  		logutil.Logger(context.Background()).Error("column not found in schema", zap.Any("columns", cols), zap.String("schema", schema.String()))
  		return 0
  	}
  	var cardinality = 1.0
  	for _, idx := range indices {
  		// It is a very elementary estimation.
  		cardinality = math.Max(cardinality, profile.Cardinality[idx])
  	}
  	return cardinality
  }
docs: // getCardinality will return the Cardinality of a couple of columns. We simply return the max one, because we cannot know
  // the Cardinality for multi-dimension attributes properly. This is a simple and naive scheme of Cardinality estimation.