Understanding, identifying and fixing the N+1 query problem

One of the most common performance pitfalls for applications accessing data from databases is the so-called “N+1 query problem”, or sometimes also called the “N+1 selects problem”. It is the first thing you should look for when an application has performance issues related to database access. It is especially easy to run into with object-relational mappers (ORMs).

The problem

The problem typically arises when your entity-relationship model has a 1:n or n:m association. It exists when application code executes one query to get objects of one entity and then executes another query for each of these objects to get the objects of an associated entity. An example would be a blog application that executes one query to fetch all authors whose names start with the letter ‘B’, and then another query for each of these authors to fetch their articles. In pseudocode:

# The 1 query
authors = sql("SELECT * FROM author WHERE name LIKE 'B%'");

# The N queries
articles = []
FOR EACH author IN authors:
    articles += sql("SELECT * FROM article WHERE author_id=:aid", aid: author.id)

The first query is the “1” in “N+1”, the following queries in the loop are the “N”.

Of course, to anybody who knows SQL this is a very naive way to get the desired result. However, OR mappers often seduce their users into writing inefficient database access code by hiding the SQL queries and allowing their users to reach for the normal tools of their favorite programming language like loops or collection operations such as map. A lot of popular web application frameworks come along with OR mappers: Rails with Active Record, Grails with GORM (Hibernate based), Laravel with Eloquent.

How to detect

The easiest way to detect the problem in an application is to log the database queries. Virtually all ORMs have a configuration option to enable query logging.

For Grails/GORM the logging can be enabled per data source in the application.yml config file:

dataSource:
    logSql: true
    formatSql: true

For Rails/ActiveRecord query logging is automatically enabled in the development environment. Since Grails 5.2 the Verbose Query Logs format is enabled by default, which you had to explicitly enable in earlier versions.

For Laravel/Eloquent you can enable and access the query log with these two methods/functions:

DB::connection()->enableQueryLog();
DB::getQueryLog();

Once query logging is enabled you will quickly see if the same query is executed over and over again, usually indicating the presence of the N+1 problem.

How to fix

The goal is to replace the N+1 queries with a single query. In SQL this means joining. The example above would be written as a single query:

SELECT article.*
FROM article
JOIN author
  ON article.author_id=author.id
WHERE author.name LIKE 'B%'

The query interface of ORMs usually allows you to write joins as well. Here the example in ActiveRecord:

Article.joins(:authors).where("authors.name LIKE ?", "B%")

Another option when using ORMs is to enable eager loading for associations. In GORM this can be enabled via the fetchMode static property:

class Author {
    static hasMany = [articles: Article]
    static fetchMode = [articles: 'eager']
}

REST APIs

The problem isn’t limited to SQL databases and SQL queries. For REST APIs it’s the “N+1 requests problem”, describing the situation where a client application has to call the server N+1 times to fetch one collection resource + N child resources. Here the REST-API has to be extended or modified to serve the client’s use cases with a single request. Another option is to offer a GraphQL API instead of a REST API. GraphQL is a query language for HTTP APIs that allows complex queries, so the client application can specify exactly what resources it needs with in a single request.