performance

`select`, `pluck`

pluck will load an array of column, and is faster than select.

select is ~10X slower than pluck, because of creating objects.

User.select(:id).to_a
# => User Load (0.9ms)  SELECT id FROM "users"
# => [#<User id: 12>, #<User id: 42>, #<User id: 1>, #<User id: 24>, #<User id: 200>, ...]

User.pluck(:id)
# => (0.9ms)  SELECT "users"."id" FROM "users"
# => [12, 42, 1, 24, 200, ..., 365]

User.pluck(:first_name, :last_name, :email)
# =>   (2.2ms)  SELECT "users"."first_name", "users"."last_name", "users"."email" FROM "users"
# => [[nil, nil, "jeff@gmail.com"], ['joe', nil, "joe.brown@gmail.com"], ["Edward", "Stanza", "ed@email.com"], ...]

http://gavinmiller.io/2013/getting-to-know-pluck-and-select/

How long to build ActiveRecord models

10.4ms for db query, but total 143ms for the whole process.

User.all.to_a
# => User Load (10.4ms)  SELECT "users".* FROM "users"
# => [#<User id: 263, ... >, #<User id: 264, ... >, #<User id: 265, ... >, ... ]

puts Benchmark.measure { User.all.to_a }
    user    system     total        real
0.143333  0.000000  0.143333 (  0.163123)

Preload, Eagerload, Includes and Joins

preload, eager_load, includes = eager loading (prevent n+1)

Preload loads the association data in a separate query.

 User.preload(:posts).to_a
 # =>
 SELECT "users".* FROM "users"
 SELECT "posts".* FROM "posts"  WHERE "posts"."user_id" IN (1)

Since preload always generates two sql, we can’t use posts table in where condition.

 User.preload(:posts).where("posts.desc='ruby is awesome'")
 # =>
 SQLite3::SQLException: no such column: posts.desc:

eager_loading loads all association in a single query using LEFT OUTER JOIN.

 User.eager_load(:posts).to_a    
 # =>
 SELECT "users"."id" AS t0_r0, "users"."name" AS t0_r1, "posts"."id" AS t1_r0,
        "posts"."title" AS t1_r1, "posts"."user_id" AS t1_r2, "posts"."desc" AS t1_r3
 FROM "users" LEFT OUTER JOIN "posts" ON "posts"."user_id" = "users"."id"

Includes

Behaves based on situations, intelligent!

Blog.includes(:posts)
# =>
Blog Load (2.8ms)  SELECT "blogs".* FROM "blogs"
Post Load (0.7ms)  SELECT "posts".* FROM "posts" WHERE "posts"."blog_id" IN (1, 2, 3)

Blog.includes(:posts).where(name: 'Blog 1').where(posts: {title: 'Post 1-1'})
# =>
SQL (0.2ms)  SELECT "blogs"."id" AS t0_r0, "blogs"."name" AS t0_r1, "blogs"."author" AS t0_r2, "blogs"."created_at" AS t0_r3, "blogs"."updated_at" AS t0_r4, "posts"."id" AS t1_r0, "posts"."title" AS t1_r1, "posts"."created_at" AS t1_r2, "posts"."updated_at" AS t1_r3, "posts"."blog_id" AS t1_r4 FROM "blogs" LEFT OUTER JOIN "posts" ON "posts"."blog_id" = "blogs"."id" WHERE "blogs"."name" = ? AND "posts"."title" = ?  [["name", "Blog 1"], ["title", "Post 1-1"]]

includes + references = eager_loading

Joins brings association data using inner join.

 User.joins(:posts)
 # =>
 SELECT "users".* FROM "users" INNER JOIN "posts" ON "posts"."user_id" = "users"."id"

Intention for eager loading

# Give me users with polish addresses and preload only polish addresses
User.includes(:addresses).where("addresses.country = ?", "Poland")

# Give me users with polish addresses and preload only polish addresses(same, but inner join)
User.joins(:addresses).where("addresses.country = ?", "Poland").includes(:addresses)

# Give me users with polish addresses but preload all of their addresses.
User.joins(:addresses).where("addresses.country = ?", "Poland").preload(:addresses)