The Cartesian Explosion Issue in Entity Framework

Learn about SQL cartesian explosion issues, how it affects performance and what you can do to fix cartesian explosions in Entity Framework.

You may unwittingly produce a SQL Cartesian explosion when constructing your LINQ queries or writing query expressions in Entity Framework Core (EF Core).

As an example, let's say you have two related tables in your EF Core data model: Orders and OrderItems, and you want to retrieve a list of orders and their associated items. You might write a LINQ query similar to this:

var query = context.Orders
    .Include(o => o.OrderItems)
    .ToList();

Nothing wrong with that, you may be thinking. You intend to retrieve orders along with their associated items, and you might expect that each order will have a collection of related OrderItems. However, you haven't configured the relationship properly using navigation properties and foreign keys. In that case, EF Core might generate a Cartesian product query, which retrieves all orders and all order items and then attempts to combine them, leading to a much larger result set than you expect.

If we look at the generated SQL for this LINQ query, you might expect something like this:

SELECT *
FROM OrderItems
LEFT JOIN
    Orders
    ON Orders.OrderId = OrderItems.OrderId 

However, EF Core (and prior Entity Frameworks) have much going on behind the scenes, and the generated query is more similar to this monster.

SELECT
    [p].[OrderId], [p].[OrderId], [p].[OrderName],
    [p0].[OrderItemId], [p0].[Name] 
FROM
    [Orders] AS [p]
LEFT JOIN
    [OrderItems] AS [p0]
    ON [p].[OrderId] = [p0].[OrderId]
ORDER BY
    [p].[OrderId], [p0].[OrderId]

In the results for this query, we can see that every OrderItem row is returned for each order. So, if we had two orders, each with two items, we'd pull back four rows. That means if each of these orders had five items, it would return ten items. Now, say we had three orders. This one only has 1 item, and we pull back 18 items.

To avoid this potentially disastrous Cartesian explosion in EF Core, it is important that your data model correctly defines the relationship between your entities using navigation properties and foreign keys. You'll have a very good understanding of how this will help you avoid such occurrences and keep your application performance intact.

Below is how you would configure the entities in EF Core to avoid a Cartesian explosion:

public class Order
{
    public int OrderId { get; set; }
    public ICollection<OrderItem> OrderItems { get; set; }
}

public class OrderItem
{
    public int OrderItemId { get; set; }
    public int OrderId { get; set; }
    public Order Order { get; set; }
}

In this configuration, the OrderItems entity has a foreign key property OrderId, and the Order entity has a navigation property OrderItems. This configuration tells EF Core how the two entities are related. When you fetch orders with their associated items, EF Core will generate the appropriate SQL query without causing a Cartesian explosion.

Identifying a Cartesian explosion in SQL Profiler can be difficult. Still, it is possible by evaluating the SQL query being executed and the query execution plan. A Cartesian explosion typically results in a query returning many rows, which you can detect by checking for specific patterns in the SQL statements and query plans. The following steps can help you identify a Cartesian explosion using SQL Profiler.

1. In SQL Profiler, In the "Events Selection" tab, choose events related to SQL queries, such as "SQL:BatchCompleted" or "RPC:Completed."
2. After you've collected the data, go over the SQL query text to find queries that could cause a Cartesian explosion. Look at queries that join tables without defining a correct join condition or appear to have many-to-many links between tables.
3. Examine the estimated and actual row counts in the execution plan. A Cartesian explosion typically results in an extremely high row count in the execution plan. You can also use tools like SQL Server Management Studio to view graphical query plans and highlight the operators contributing to the explosion.
4. Entity Framework Core allows you to enable logging, which can be helpful for debugging and performance analysis. You can set up a logger to capture and inspect SQL commands - context.Database.Log = Console.Write
5. If you suspect a Cartesian explosion, review your LINQ query and make sure it includes the appropriate join conditions. You may need to revise your query to avoid generating a cross-join. Ensure that you have navigation properties and foreign keys properly configured in your EF Core data model.

Generally speaking, detection means that with the help of LINQ or Entity Framework Core, you check your LINQ and SQL queries produced for any mistake- a missing or erroneous join. Appropriate use of join conditions, relationships, and navigation attributes in your data model will make it possible to avoid a Cartesian explosion and optimize the performance of an application.