code: func (p *LogicalUnionAll) DeriveStats(childStats []*property.StatsInfo) (*property.StatsInfo, error) {
  	p.stats = &property.StatsInfo{
  		Cardinality: make([]float64, p.Schema().Len()),
  	}
  	for _, childProfile := range childStats {
  		p.stats.RowCount += childProfile.RowCount
  		for i := range p.stats.Cardinality {
  			p.stats.Cardinality[i] += childProfile.Cardinality[i]
  		}
  	}
  	return p.stats, nil
  }
docs: // DeriveStats implement LogicalPlan DeriveStats interface.