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"