Salesfore Experts, 
 
I am working on a requirement where I need to trigger email with all attachments on a ticket when owner gets changed. The below mentioned code works as expected in Sandbox and I get 87% code coverage but when we try to deploy in production the code coverage is reduced to 25% which I am unable to figure out what is causing it. 
And We are getting error is 
"  ystem.DmlException: Insert failed. First exception on row 0; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, zGLOBAL_New_Owner_Notifi_Trigger: execution of BeforeUpdate caused by: System.QueryException: Non-selective query against large object type (more than 200000 rows). Consider an indexed filter or contact salesforce.com about custom indexing. Even if a field is indexed a filter might still not be selective when: 1. The filter value includes null (for instance binding with a list that contains null) 2. Data skew exists whereby the number of matching rows is very large (for instance, filtering for a particular foreign key value that occurs many times) Class.zGLOBAL_New_Owner_Notifi_Attachment.emailSender: line 9, column 1 Trigger.z  "
 

If any one have experianced such an issue earlier, any pointers in fixing this would be greatly appriciated.
 
Here is my APEX class 
 

public class zGLOBAL_New_Owner_Notifi_Attachment  {

public static void emailSender(Map<Id,Case> oldMap, Map<Id,Case> newMap) {
    // this is a list for storing email for settoAddress 
    List<String>toAddress =new List<String>();
    // this is a list for storing email for setccAddress 
    List<String>CCAddress =new List<String>();
    
    List<contact> lstcont=[select id ,Email,name,phone from contact];
    Map<String,String> mapContact=new Map<String,String>();
    list<String> ccAddresslst=new List<String>();
    map<string,String> mapuser=new map<string,string>();    
    Set<Id> accIds=new Set<Id>();
    Set<Id> recId=new Set<Id>();
   
    String ownerName=''; 
    String baseUrl = URL.getSalesforceBaseUrl().toExternalForm();
    
    //for getting user email and store in map where user id is key and email is value
    for(User u:[select id, name, Email from user]) 
    {
        mapuser.put(u.id,u.email);
    }
    
    //for getting Contact name  and store in map where  id is key and name is value
    for(contact con:lstcont)
    {
        mapContact.put(con.id,con.name);
    }
    
    
    //Change Trigger.new with udtCases
    for(Id key : oldMap.keySet())
    {
        case oldcase=oldMap.get(key);
       case cs=newMap.get(key);
        
        if(cs.zGloblal_Total_attachments__c>=1 && cs.APEX_Owner_notifications_Attachments__c==true)
        {
            //Changed Trigger.oldMap
          
            

            if(oldcase.OwnerId!=cs.OwnerId)
            {
               if(string.valueOf(cs.OwnerId).startsWith('005'))
                {
                    toAddress.add(mapuser.get(cs.OwnerId));  
                    ownerName=Getqueuemember.getOwnername(cs.OwnerId);
                }
                
                else{ 
                    CCAddress=Getqueuemember.getEmailfromuser(cs.OwnerId);
                    ownerName=Getqueuemember.getqueueOwnername(cs.OwnerId);
                }
                
                
                List<Messaging.Emailfileattachment> fileAttachments = new List<Messaging.Emailfileattachment>();
                List<Attachment> attlist=[select Name, Body, BodyLength from Attachment where ParentId = :cs.id ];
                for (Attachment a :attlist){
                    
                    Messaging.Emailfileattachment efa = new Messaging.Emailfileattachment();
                    
                    efa.setFileName(a.Name);
                    efa.setBody(a.Body);
                    fileAttachments.add(efa);
                }
                
                
                String Subject='Ticket #'+cs.CaseNumber+' has been assigned to  '+' '+ownerName;
                
                
                
                try{
                    
                   
                    Messaging.SingleEmailMessage semail = new Messaging.SingleEmailMessage();
                    
                    
                    
                    semail.setSubject(Subject); 
                    semail.setSenderDisplayName('Salesforce Support'); 
                    semail.setTargetObjectId(cs.ContactId);
                    //semail.setTemplateId(et.id);
                    semail.setCcAddresses(CCAddress);
                    semail.setToAddresses(toAddress); 
                    
                    semail.setHtmlBody('***THIS IS A SYSTEM GENERATED MESSAGE! DO NOT REPLY!***<br/><br/>Please be advised that the following SalesForce ticket has been assigned to  '+ownerName+'<br/><br/> Ticket #:'+cs.CaseNumber+
                                       '<br/>Subject:'+cs.subject+'<br/><br/>Contact Name: '+mapContact.get(cs.Contactid)+'<br/>Contact Phone (primary): '+Getqueuemember.r(cs.ContactPhone)+'<br/>Contact Mobile (primary): '+Getqueuemember.r(cs.ContactMobile)+
                                       '<br/> Contact Email:'+Getqueuemember.r(cs.ContactEmail)+'<br/><br/>Category: '+Getqueuemember.r(cs.Ticket_Category__c)+Getqueuemember.r(cs.Service_Category__c)+Getqueuemember.r(cs.Ticket_Category2__c)+               
                                       '<br/>Type: '+Getqueuemember.r(cs.Ticket_Type__c)+Getqueuemember.r(cs.Service_Type__c)+Getqueuemember.r(cs.Ticket_Type2__c)+                           
                                       '<br/>Item: '+Getqueuemember.r(cs.Ticket_Item__c)+Getqueuemember.r(cs.Service_Item__c)+Getqueuemember.r(cs.Ticket_Item2__c)+                   
                                       '<br/>Ticket Source: '+Getqueuemember.r(cs.Case_Source__c)+'<br/><br/><br/>Building: '+Getqueuemember.r(cs.building__c)+'<br/>Room: '+Getqueuemember.r(cs.Room_Number__c)+'<br/>Area Description: '+Getqueuemember.r(cs.Area_description__c)+
                                       '<br/><br/><br/>Click the appropriate link below to view the ticket directly:<br/><br/>Partner Portal Users: <a href='+baseUrl+'/'+cs.id+'>'+baseUrl+'/'+cs.id+'</a><br/>BlackbeltHelp (Community Users): <a href='+Getqueuemember.getcommunityurl(cs.AccountId)+cs.id+
                                       '>'+Getqueuemember.getcommunityurl(cs.AccountId)+cs.id+'</a><br/><br/>Description:'+cs.Description+'<br/><br/>Internal Notes: '+Getqueuemember.r(cs.Internal_Notes__c)); 
                    semail.setFileAttachments(fileAttachments);     
                    semail.setWhatId(cs.id);
                 Messaging.sendEmail(new Messaging.SingleEmailMessage[] {semail}); 
                    
                }
                catch(exception e)    
                {}
                
            }
        }
    }
}
}

And Test Class is 

@isTest
public class zGLOBAL_New_Owner_Notifi_AttachmentTest {
    @isTest
    public Static void emailTest()
    {
       // create test account
        Account acc = TestUtilityController.getAccount('MassBay');
        acc.APEX_Owner_notifications_Attachments__c=true;
        insert acc;
        
        // create test contact
        Contact con = TestUtilityController.getContact('Test Last Name','testing@testing.com',acc.Id);
        insert con;
        
        // fill utility
        UtilityController.fillUtilities();
        UtilityController.isUtility = false;
        
        trigger_control__c tc = new trigger_control__c();
        tc.name='test';
        tc.Enable_Owner_Notification_Trigger__c=true;
        insert tc;
        
        // Create test cases 
        List<Case> listCase = new List<Case>();
        
        if(UtilityController.caseRecordType.containsKey('MassBayCC_Ticket')){
            String recordTypeId = UtilityController.caseRecordType.get('MassBayCC_Ticket').Id;   
                 
             
            Case csanew = TestUtilityController.getCase(recordTypeId,con.Id,acc.Id);
            csanew.status = 'Action - Automated Escalation';
            csanew.zGloblal_Total_attachments__c=2;
            csanew.Ticket_Category2__c = 'Blackboard Learn - LMS';
            csanew.Ticket_Type2__c = 'Faculty/Staff';
            csanew.Ticket_Item2__c='Tutorial';
            listCase.add(csanew);
            
    //Added additional code here
     System.test.startTest();

            insert listCase;


System.Test.stopTest();

            Getqueuemember.getcommunityurl(acc.id);
            Getqueuemember.getEmailfromuser('0050W0000061RWD');
            Getqueuemember.getOwnername('0050W0000061RWD');
            Getqueuemember.getqueueOwnername('00Gd00000027kH7');
            Getqueuemember.r(csanew.Ticket_Category2__c);
        
    }

}
    
   

}