What is the difference between subquery and values when passed to NOT IN on Postgresql? - sql

Question

In Rails4 app (versions: rails 4.2.3, postgresql 9.3.5), I have model classes like below
class Message < ActiveRecord::Base
belongs_to :receiver, class_name: 'User'
belongs_to :sender, class_name: 'User'
validate :receiver, presence: true
validate :sender, presence: true
end
class User < ActiveRecord::Base
has_many :received_messages, class_name: 'Message', foreign_key: :receiver_id
has_many :sent_messages, class_name: 'Message', foreign_key: :sender_id
end
I want to get collection of users who are NOT received message from specific user, So I wrote these scopes:
class User < ActiveRecord::Base
...
scope :received_messages_from, -> (user) {
includes(:received_messages).
where('messages.sender_id': user.id).
references(:received_messages)
}
scope :not_received_messages_from, -> (user) {
includes(:received_messages).
where.not(id: received_messages_from(user).select(:id)).
references(:received_messages)
}
end
I have these rows in messages table:
message_00:
sender_user_id: 11
receiver_user_id: 12
message_01:
sender_user_id: 11
receiver_user_id: 12
message_02:
sender_user_id: 12
receiver_user_id: 11
message_11:
sender_user_id: 17
receiver_user_id: 11
message_12:
sender_user_id: 11
receiver_user_id: 17
message_13:
sender_user_id: 18
receiver_user_id: 12
message_14:
sender_user_id: 12
receiver_user_id: 18
message_15:
sender_user_id: 17
receiver_user_id: 12
message_16:
sender_user_id: 17
receiver_user_id: 13
message_17:
sender_user_id: 17
receiver_user_id: 14
So, User.received_messages_from(User.find(17)).pluck(:id) results: [11, 12, 13, 14], and User.not_received_messages_from(User.find(17)).pluck(:id) results sholdn't contain these ids.
But the not_received_messages_from scope dosen't work as it returning users who has received messages from specific user. This generates SQL like this (in this example, user's id is 17):
SELECT "users"."id" FROM "users"
LEFT OUTER JOIN "messages" ON "messages"."receiver_id" = "users"."id"
WHERE ("users"."id"
NOT IN (
SELECT "users"."id" FROM "users"
WHERE "messages"."sender_id" = 17))
User.not_received_messages_from(User.find(17)).pluck(:id) results:
[11, 12, 12, 12, 15, 16, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 17, 18, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100]
So, I tried fixing .select(:id) to .pluck(:id) in where in not_received_messages_from scope and this works.
scope :not_received_messages_from, -> (user) {
includes(:received_messages).
where.not(id: received_messages_from(user).pluck(:id)).
references(:received_messages)
}
SQL:
SELECT "users"."id" FROM "users"
LEFT OUTER JOIN "messages" ON "messages"."receiver_id" = "users"."id"
WHERE ("users"."id" NOT IN (11, 12, 13, 14))
User.not_received_messages_from(User.find(17)).pluck(:id) results:
[15, 16, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 17, 18, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100]
I think the defferece between two SQLs is only subquery or static ids array passed to 'NOT IN'. Why the results differ each other?

Answer 1

This is likely because your sub-select is not returning the expected response.
SELECT "users"."id" FROM "users"
LEFT OUTER JOIN "messages" ON "messages"."receiver_user_id" = "users"."id"
WHERE ("users"."id"
NOT IN (
SELECT "users"."id" FROM "users"
WHERE "messages"."sender_user_id" = 17))
It's been a while since I've looked at PostresQL joins, but I don't know what, if anything that sub-select would produce. It's operating with a join, but... which one? There's no reference in the documentation that explains that.