code: func (p *PhysicalUnionScan) ResolveIndices() (err error) {
  	err = p.basePhysicalPlan.ResolveIndices()
  	if err != nil {
  		return err
  	}
  	for i, expr := range p.Conditions {
  		p.Conditions[i], err = expr.ResolveIndices(p.children[0].Schema())
  		if err != nil {
  			return err
  		}
  	}
  	return
  }
docs: // ResolveIndices implements Plan interface.