Examples on Github:
What is the "N+1 Queries Problem"?
The N+1 Queries Problem is an O/R mapper specific issue. It may occur when trying to load data from a table (like ProductGroups) along with its related data (like Products) having a one-to-many relationship whith the related data is on the many-side.
An example of a LINQ query that may lead to N+1 Queries with some O/R mappers:
Context.ProductGroups
.Select(g => new
{
ProductGroup = g,
Products = g.Products
})
.ToList();
In this example an O/R mapper may execute 1 SQL statement to load N ProductGroups and after that N SQL statements to load Products for each ProductGroup separately. Hence the name: N+1 Queries.
Let’s revisit the previous blog post’s queries to see whether EF 3.1 still suffers from the N+1 Queries Problem as earlier versions of EF did.
Fetching a collection navigational property
Fetching a collection navigational property in EF 2.1 leads to 2 queries, one for ProductGroups and the other for the Products.
var groups = Context.ProductGroups
.Where(g => g.Name.Contains("Group"))
.Select(g => new
{
ProductGroup = g,
Products = g.Products.Where(p => p.Name.Contains("1")).ToList()
})
.ToList();
With EF 3.1 we get just 1 SQL statement that fetches both, the ProductGroups and Products.
SELECT
[p].[Id], [p].[Name], [p].[RowVersion],
[t].[Id], [t].[GroupId], [t].[Name], [t].[RowVersion]
FROM [ProductGroups] AS [p]
LEFT JOIN
(
SELECT [p0].[Id], [p0].[GroupId], [p0].[Name], [p0].[RowVersion]
FROM [Products] AS [p0]
WHERE CHARINDEX(N'1', [p0].[Name]) > 0
) AS [t] ON [p].[Id] = [t].[GroupId]
WHERE CHARINDEX(N'Group', [p].[Name]) > 0
ORDER BY [p].[Id], [t].[Id]
This example proves that neither EF 2.1 nor 3.1 causes the N+1 Queries Problem, yet there are some differences:
- EF 2.1 requires
ToList()when fetchingProducts, EF 3.1 does not - EF 2.1 executes 2 SQL statements, EF 3.1 just 1
Using "FirstOrDefault" inside "Select"
When selecting the first Product for each ProductGroup using FirstOrDefault() EF 2.1 executes 1 SQL statement for loading N ProductGroups and N SQL statements for loading the Products.
var groups = Context.ProductGroups
.Where(g => g.Name.Contains("Group"))
.Select(g => new
{
ProductGroup = g,
Product = g.Products.FirstOrDefault()
})
.ToList();
As in the previous section, EF 3.1 executes just 1 SQL statement for loading all requested data.
SELECT
[p].[Id], [p].[Name], [p].[RowVersion],
[t0].[Id], [t0].[GroupId], [t0].[Name], [t0].[RowVersion]
FROM [ProductGroups] AS [p]
LEFT JOIN
(
SELECT [t].[Id], [t].[GroupId], [t].[Name], [t].[RowVersion]
FROM
(
SELECT
[p0].[Id], [p0].[GroupId], [p0].[Name], [p0].[RowVersion],
ROW_NUMBER() OVER(PARTITION BY [p0].[GroupId] ORDER BY [p0].[Id]) AS [row]
FROM [Products] AS [p0]
) AS [t]
WHERE [t].[row] <= 1
) AS [t0] ON [p].[Id] = [t0].[GroupId]
WHERE CHARINDEX(N'Group', [p].[Name]) > 0
With respect to the N+1 Queries Problem, the new behavior of the SQL statement generator is an improvement. We can now use FirstOrDefault without producing dozens or even hundreds of SQL statements.
Usage of „FirstOrDefault“ along with „GroupBy“
The usage of FirstOrDefault along with GroupBy in EF 2.1 works, kind of… The LINQ query below leads to client-side evaluation, which again leads to a massive performance loss due to the fact that the whole table Products is fetched into memory.
var firstProducts = Context.Products
.GroupBy(p => p.GroupId)
.Select(g => g.FirstOrDefault())
.ToList();
As you may have read in my blog post Entity Framework Core 3.0 – „Hidden“ GROUP BY Capabilities (Part 1), GroupBy has very limited support in EF 3.0/3.1. With the query above we get an InvalidOperationException.
Lazy Loading
Nothing has changed concerning Lazy Loading. Whether EF 2.1 or EF 3.1, we still have to be careful when using this feature.
Here is an example: for 100 Products and 10 ProductGroups the following LINQ query executes 11 SQL statements: 1 for loading the Products and 10 for loading the ProductGroups.
var lookup = Context.Products
.ToLookup(p => p.Group.Name);
Summary
Personally, I like the behavior of the SQL generator in EF 3.1 better than in EF 2.1 because it gives me more control over SQL statement generation. Previously, if a LINQ query is not translatable to SQL, EF switched to client-side evaluation which could lead to bad performance if it was overlooked. Now, we get an exception and have to do something about it.
Let‘ get back to my opening questions:
- Is N+1 Queries Problem still a big issue in EF 3.1? – In short no. Sure, Lazy Loading may lead to more executed SQL statements than expected but that is the price we have to pay in order to use this feature.
- Did the new behavior introduced new issues we have to be aware of? – Unfortunately yes, this new behavior inevitably causes the so-called Cartesian Explosion Problem
