code: func (p *Apply) SetCorrelated() {
  	corCols := p.GetChildren()[1].extractCorrelatedCols()
  	p.correlated = p.GetChildren()[0].IsCorrelated()
  	for _, corCol := range corCols {
  		// If the outer column can't be resolved from this outer schema, it should be resolved by outer schema.
  		if idx := p.GetChildren()[0].GetSchema().GetColumnIndex(&corCol.Column); idx == -1 {
  			p.correlated = true
  			break
  		}
  	}
  }
docs: // SetCorrelated implements Plan interface.