EXAMPLE ONLY: DML on load of VF page, which technically you're not supposed to do but that makes it fun... More @ https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_compref_page.htm

ExampleDmlAsap.cls

/* ' ### You'll need something like this in your visual force page, main focus being action="{!markActiveDetails}" ###
' <apex:page docType="html-5.0" action="{!markActiveDetails}" standardController="Service_Inquiry_SP__c" recordSetVar="sit" extensions="ServiceInquiryOSPExt" standardStylesheets="false" sidebar="false">
' </apex:page>
*/

public PageReference  markActiveDetails(){
        
  System.debug('#$#$# markActiveDetails running with siId ' + siId);

  List<Service_Inquiry_Details__c> deetsToUpdate = new List<Service_Inquiry_Details__c>();
  Set<String> speedList = new Set<String>();
  Set<String> locList = new Set<String>();
  Boolean wasThisNeeded = false;

  List<Service_Inquiry_Details__c> siDetails = [
      SELECT Id, ServiceLocationStreet__c, Active__c, Service_Location__c, Cost_Line__c, Speed__c
      FROM Service_Inquiry_Details__c
      WHERE Service_Inquiry__c = :siId
      ORDER BY ServiceLocationStreet__c
  ];

  for(Service_Inquiry_Details__c d : siDetails){

      if(!locList.contains(d.ServiceLocationStreet__c)){

          system.debug('#$#$# NEW STREET ' + d.ServiceLocationStreet__c);
          locList.add(d.ServiceLocationStreet__c);
          d.Active__c = true;

          speedList = new Set<String>();
      }

      if(!speedList.contains(d.Speed__c)){

          system.debug('#$#$# NEW SPEED ' + d.Speed__c);
          speedList.add(d.Speed__c);

            if(d.Cost_Line__c != true){
                d.Cost_Line__c = true;
                deetsToUpdate.add(d);
            }
        }
      }

      if(deetsToUpdate.size() > 0){
          wasThisNeeded = true;
      }

      if(wasThisNeeded){

          system.debug('#$#$# Updating deets size: '+ deetsToUpdate.size() +' Was This Needed? '+ wasThisNeeded);

          update deetsToUpdate;

          PageReference pageRef = new PageReference(ApexPages.currentPage().getUrl());
          pageRef.setRedirect(true);

          return pageRef;

      }
      else {
          return null;
      }
  }