mysql - No foreign key constraints in database.! -

My organization works on Spring MVC with Hibernate We always specify foreign key constraints in the mapping file, such as a person Person.hbm.xml

  & lt; Set name = "ContactList" & gt; & Lt; Major column = "PersonId" foreign-key = "Fk_Peson_Contact" /> & Lt; One-to-many classes = "sample.model.contact" / & gt; & Lt; / Set & gt;   
 This mapping will create multiple relationships between  person  and  contacts , and  PersonID  < In the form of a foreign key column in the Code> Contacts  table.  
 However, now the organization has decided not to mention any kind of relationship in the mapping file, in this case  person  near the map  contact  Instead of mapping, there will be no one-to-many mappings of any type  property name = "FK_PersonID" />  which will be  personID  to capture columns Create this scenario table In  person  and  contact  will look the same but the difference is that their Coz no mapping is specified, there is no connection between  person  and  contact .  
 In such a case, if we want to get the person's contact list, then we will have to ask two questions so that for a person's contact list, suppose that we want to receive the person's list. Contact list will then have to do  for loop  for the person's list and will bring its contact list which will reduce the number of questions.  
 When I ask why not specify a relationship then the senior said,  
 
 
 If the foreign key is in DB, then we do not divide the pieces and divide Can.   
 
 When we set fire to join the query DB takes much time to execute it. Which can slow down the DB server.    
 But, my question is -  
 
 
 If I type  for looping  I do it, is it possible that many questions will be asked to bring the contacts?   
 
 This type of looping can be slowed or     
 
 What if I The person wants to bring its list, list address, merit list, with the issue of n + 1 due to this?    
  Whether or not to specify a mapping, this situation is more beneficial.    
 
 
 OK, I divide one of the early days into Microsoft SQL (MS) The states doing this have shown a query, which is selected from a partition table, which contains the population data in many parties, and its indexes and statistics are not used to reach those divisions, which have no relevant data was not. Of course if you do not have the FK stable then it can not use the index and the data so that you know about FK. I am pretty sure that this will be covered by Cullen Delanye in his book (for the relevant edition of MSSQL). The key is that the data will include the necessary information to prevent unnecessary efforts. Demo actually used a union to display the effect