How to resolve N+1 problem on Rails

🤨 What is N+1 problem?

Kaziu ・ Apr 1 '22

#sql #beginners #database #mysql

🔑 4 key methods

joins
eager_load
preload
includes

💎 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]]

💎 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]]

💎 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]]

💎 includes

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]]

# using where()
Skill.includes(:skill_category).where(skill_categories: { name: 'baseball' })
↓
# just one SQL like eager_load 👍
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"]]

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"

Blog