The N+1 Problem Explained: How to Detect and Fix It in Go/Rails

This article was originally published on bmf-tech.com.

Overview

A summary of the N+1 problem and how to address it.

What is the N+1 Problem

• The issue where 1 SQL query is issued for retrieving all records plus N SQL queries for each record.
• It is easier to understand if interpreted as 1+N rather than N+1.

Example

• Case of retrieving data for list display
  • Issue 1 SELECT query to retrieve all data for the list (returns N records)
  • Issue N SELECT queries to retrieve related data for N records

Solutions

• Join
  • SELECT "users".* FROM "users" INNER JOIN "posts" ON "posts"."user_id" = "users"."id" WHERE "posts"."id" = 1
  • Eager Loading
  • SELECT "users".* FROM "users"
  • SELECT "posts".* FROM "posts" WHERE "posts"."id" IN (1, 2, 3, 4, 5)

References

• N+1問題は1+N問題
• N+1 問題
• N+1問題 / Eager Loading とは
• ActiveRecordのjoinsとpreloadとincludesとeager_loadの違い