Overview

This article summarizes the explanation and response to the N+1 problem.

What is the N+1 Problem?

• A problem where N SQL statements are issued to retrieve all records, resulting in 1 + N SQL statements.
• It is easier to interpret as 1 + N rather than N + 1.

Example

• A case of retrieving data for display
  • Issue a SELECT statement once to retrieve all data for the list (returning N records).
  • Issue N SELECT statements to retrieve related data for the N records.

Response

• 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

• The N+1 Problem is the 1+N Problem
• N+1 Problem
• What is the N+1 Problem / Eager Loading
• Differences between ActiveRecord's joins, preload, includes, and eager_load