N+1 Problem in Entity Framework

 What is Select N+1 Problem?

ORMs can help you to address the impedance mismatch between relational databases and object oriented models and by that make your life simpler. But not knowing about some of their pitfalls can decrease your performance dramatically. One of those pitfalls is the select N+1 problem. This problem is being caused mainly because most of the ORMs out there are enabling lazy loading behavior by default. When we have a parent-children relation, the problem can raise its ugly head. The problem is happening when we are executing a single query and then N following queries (N is the number of parent entities) in order to query for something. As you can expect, doing N+1 queries instead of a single one will flood your database with queries that we can and should avoid. This is very unacceptable.       

 
How to Avoid the N+1 Problem in Entity Framework?

One of the main solutions to the select N+1 problem in Entity Framework is to use the Include method. The Include method is making an eager load for the children that you indicate to it. You give the method a path of all the children you like to load in the query (as long as you have a relation between the entities) and one query will be generated to bring back all the relevant entities. This isn’t a bullet proof solution! There are serious implications that you should understand when you use the Include method. The main implication is that it is doing a join between all the tables that you want to return and the data is retrieved in a flatten manner in order to materialize all the entities from it. Also the materialization process when having a lot of included entities can cause a downgrade of performance. So you will have to weigh the balance between using Include or lazy loading.