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Kaziu
Kaziu

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How to resolve N+1 problem on Rails

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]]
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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]]
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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]]
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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]]
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# 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"]]
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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"

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