code: func convert2Group(node plannercore.LogicalPlan) *memo.Group {
  	e := memo.NewGroupExpr(node)
  	e.Children = make([]*memo.Group, 0, len(node.Children()))
  	for _, child := range node.Children() {
  		childGroup := convert2Group(child)
  		e.Children = append(e.Children, childGroup)
  	}
  	g := memo.NewGroup(e)
  	// Stats property for `Group` would be computed after exploration phase.
  	g.Prop = &property.LogicalProperty{Schema: node.Schema()}
  	return g
  }
docs: // convert2Group converts a logical plan to expression groups.