How to Identify and Resolve Hibernate N+1 Problem?

Let’s assume that you’re writing code that’d track the price of mobile phones. Now, let’s say you have a collection of objects representing different Mobile phone vendors (MobileVendor), and each vendor has a collection of objects representing the PhoneModels they offer.
To put it simple, there’s exists a one-to-many relationship between MobileVendor:PhoneModel.
MobileVendor Class

view sourceprint?

1.Class MobileVendor{

2.long vendor_id;

3.PhoneModel[] phoneModels;

4....

5.}

Okay, so you want to print out all the details of phone models. A naive O/R implementation would SELECT all mobile vendors and then do N additional SELECTs for getting the information of PhoneModel for each vendor.

view sourceprint?

1.-- Get all Mobile Vendors

2.SELECT * FROM MobileVendor;

3. 

4.-- For each MobileVendor, get PhoneModel details

5.SELECT * FROM PhoneModel WHERE MobileVendor.vendorId=?

As you see, the N+1 problem can happen if the first query populates the primary object and the second query populates all the child objects for each of the unique primary objects returned.
Resolve N+1 SELECTs problem
(i) HQL fetch join

view sourceprint?

1."from MobileVendor mobileVendor join fetch mobileVendor.phoneModel PhoneModels"

Corresponding SQL would be (assuming tables as follows: t_mobile_vendor for MobileVendor and t_phone_model for PhoneModel)

view sourceprint?

1.SELECT * FROM t_mobile_vendor vendor LEFT OUTER JOIN t_phone_model model ON model.vendor_id=vendor.vendor_id

(ii) Criteria query

view sourceprint?

1.Criteria criteria = session.createCriteria(MobileVendor.class);

2.criteria.setFetchMode("phoneModels", FetchMode.EAGER);