Let's say you have a collection of Car objects (database rows), and each Car has a collection of Wheel objects (also rows). In other words, Car → Wheel is a 1-to-many relationship.
Now, let's say you need to iterate through all the cars, and for each one, print out a list of the wheels. The naive O/R implementation would do the following:
SELECT * FROM Cars;
And then for each Car:
SELECT * FROM Wheel WHERE CarId = ?
In other words, you have one select for the Cars, and then N additional selects, where N is the total number of cars.
Alternatively, one could get all wheels and perform the lookups in memory:
SELECT * FROM Wheel
This reduces the number of round-trips to the database from N+1 to 2.
Most ORM tools give you several ways to prevent N+1 selects.
Reference: Java Persistence with Hibernate, chapter 13.
1.1) First-level cache
First-level cache always Associates with the Session object. Hibernate uses this cache by default. Here, it processes one
transaction after another one, means wont process one transaction many
times. Mainly it reduces the number of SQL queries it needs to
generate within a given transaction. That is instead of updating after
every modification done in the transaction, it updates the transaction
only at the end of the transaction.
1.2) Second-level cache
Second-level cache always associates with the Session Factory object. While running the transactions, in between it loads the
objects at the Session Factory level, so that those objects will be
available to the entire application, not bound to single user. Since
the objects are already loaded in the cache, whenever an object is
returned by the query, at that time no need to go for a database
transaction. In this way the second level cache works. Here we can use
query level cache also.
Quoted from: http://javabeat.net/introduction-to-hibernate-caching/
Best Answer
LLBLGen is an exquisitely case hardened tool that generates ultimately extensible code. Customer service is second to none and bugs are fixed as they are found by one of the finest minds in the business. I have had 5 hour turnarounds on obscure, affecting only me, bugs.
I am currently cutting my teeth on nhibernate and have to say that the apprehension that kept me from it for so long was not misplaced. It is a great tool with a huge user base but inherently, the mapping file / mapping classes / mapping attributes strategy is at the same time soft and brittle.
This is my opinion, no need for punitive downvotes because I am criticizing nhibernate.