DEV Community

Kaziu
Kaziu

Posted on

How to resolve N+1 problem on Rails

#rails #sql #performance #ruby
kaziusan

🀨 What is N+1 problem?

Kaziu ・ Apr 1 '22

#sql #beginners #database #mysql

πŸ”‘ 4 key methods

  • joins
  • eager_load
  • preload
  • includes

πŸ’Ž joins

integrate by INNER JOIN

  • doesn't cache association, so if you don't need data which is created again, you should use it.
  • save memory allocated spaces because ActiveRecord object doesn't cache. 
Skill.joins(:skill_category).limit(5) ↓ SELECT "skills".* FROM "skills" INNER JOIN "skill_categories" ON "skill_categories"."id" = "skills"."skill_category_id" LIMIT ? [["LIMIT", 5]]
Enter fullscreen mode Exit fullscreen mode

πŸ’Ž eager_load

integrate by LEFT OUTER JOIN with cache

  • faster than preload() because it creates only one SQL
  • it can use WHERE in table which is integrated by JOIN (preload() can't do it) 
Skill.eager_load(:skill_category).limit(5) ↓ SELECT "skills"."id" AS t0_r0, "skills"."name" AS t0_r1, "skills"."user_id" AS t0_r2, "skills"."skill_category_id" AS t0_r3, "skills"."created_at" AS t0_r4, "skills"."updated_at" AS t0_r5, "skill_categories"."id" AS t1_r0, "skill_categories"."name" AS t1_r1, "skill_categories"."reccomend" AS t1_r2, "skill_categories"."created_at" AS t1_r3, "skill_categories"."updated_at" AS t1_r4 FROM "skills" LEFT OUTER JOIN "skill_categories" ON "skill_categories"."id" = "skills"."skill_category_id" LIMIT ? [["LIMIT", 5]]
Enter fullscreen mode Exit fullscreen mode

πŸ’Ž preload

use multiple SQLs with cache

  • recommend to use it when you have big table which you don't wanna JOIN
  • * it's impossible to use WHERE because it is not integrated by JOIN 
Skill.preload(:skill_category).limit(5) ↓ # this one SELECT "skills".* FROM "skills" LIMIT ? [["LIMIT", 5]] # and this one SELECT "skill_categories".* FROM "skill_categories" WHERE "skill_categories"."id" IN (?, ?, ?, ?, ?) [[nil, 483], [nil, 583], [nil, 901], [nil, 181], [nil, 147]]
Enter fullscreen mode Exit fullscreen mode

πŸ’Ž includes

Image description

if you use where, join, references method at least one, executes as eager_load, otherwise preload

# just includes Skill.includes(:skill_category).limit(5) ↓ # this one SELECT "skills".* FROM "skills" LIMIT ? [["LIMIT", 5]] # and this one SELECT "skill_categories".* FROM "skill_categories" WHERE "skill_categories"."id" IN (?, ?, ?, ?, ?) [[nil, 483], [nil, 583], [nil, 901], [nil, 181], [nil, 147]]
Enter fullscreen mode Exit fullscreen mode 
# using where() Skill.includes(:skill_category).where(skill_categories: { name: 'baseball' }) ↓ # just one SQL like eager_load πŸ‘ SELECT "skills"."id" AS t0_r0, "skills"."name" AS t0_r1, "skills"."user_id" AS t0_r2, "skills"."skill_category_id" AS t0_r3, "skills"."created_at" AS t0_r4, "skills"."updated_at" AS t0_r5, "skill_categories"."id" AS t1_r0, "skill_categories"."name" AS t1_r1, "skill_categories"."reccomend" AS t1_r2, "skill_categories"."created_at" AS t1_r3, "skill_categories"."updated_at" AS t1_r4 FROM "skills" LEFT OUTER JOIN "skill_categories" ON "skill_categories"."id" = "skills"."skill_category_id" WHERE "skill_categories"."name" = ? [["name", "baseball"]]
Enter fullscreen mode Exit fullscreen mode

includes method is convenience as you see, but when other developer will see includes method, they need to think it means "preload" or "eager_load"

Top comments (1)