Flattening nested query in WHERE clause with NOT IN - sql

Question

Suppose I have these two tables, simplified for the purpose of the question:
CREATE TABLE merchandises
(
id BIGSERIAL PRIMARY KEY,
name VARCHAR(255) NOT NULL,
price INT NOT NULL
)
CREATE TABLE gifts
(
id BIGSERIAL NOT NULL PRIMARY KEY,
from_user VARCHAR(255) REFERENCES users(id),
to_user VARCHAR(255) REFERENCES users(id),
with_merchandise BIGINT REFERENCES merchandises(id)
)
The merchandises table lists available merchandises. The gifts table show records that a user has sent a merchandise to another user as gift (proper index is in place to avoid duplication).
What I would like to query is a list of merchandises that a user can send to another user, provided that the merchandises should not have been gifted before.
This is a query that works, but I hope that I can find one that does not have a nested query, thinking that it might give better performance thanks to the optimizer of POSTGRESQL.
SELECT DISTINCT ON (m.id) m.id, m.name, m.description
FROM merchandises m
WHERE m.id NOT IN (
SELECT g.with_merchandise
FROM gifts g
WHERE g.from_user = 'some_user_id' AND g.to_user = 'some_other_user_id'
)
ORDER BY m.id ASC
LIMIT 20 OFFSET 0
In the previous attempt, I had this query, but I found out that it does not work:
SELECT DISTINCT ON (m.id) m.id, m.name, m.description
FROM merchandises m
LEFT JOIN gifts g
ON m.id = g.with_merchandise
WHERE g.id IS NULL
OR g.from_user <> 'some_user_id' AND g.to_user <> 'some_other_user_id'
ORDER BY m.id ASC
LIMIT 20 OFFSET 0
This query does not work because even though the WHERE clause filters out gift entries from two specific users, two other users might have given gifts with the same merchandise (same merchandise_id).

Answer 1

Even though you asked to remove the subquery, using a not exists subquery might run faster than not in especially if the not in query returns a lot of values:
SELECT m.id, m.name, m.description
FROM merchandises m
WHERE NOT EXISTS (
SELECT 1
FROM gifts g
WHERE g.with_merchandise = m.id
AND g.from_user = 'some_user_id'
AND g.to_user = 'some_other_user_id'
)
This query can take advantage of a composite key on gifts(with_merchandise,from_user,to_user)
If you still rather use left join, then move your conditions for from_user and to_user from the where to the on clause
SELECT m.id, m.name, m.description
FROM merchandises m
LEFT JOIN gifts g ON m.id = g.with_merchandise
AND g.from_user = 'some_user_id' AND g.to_user = 'some_other_user_id'
WHERE g.id IS NULL
ORDER BY m.id ASC
LIMIT 20 OFFSET 0

Answer 2

This uses a left outer join and should perform well.
SELECT m.*
FROM merchandises m
LEFT OUTER JOIN (SELECT with_merchandise FROM gifts WHERE from_user = 'some_user_id' AND to_user = 'some_other_user_id' GROUP BY with_merchandise) g ON m.id = g.with_merchandise
WHERE g.with_merchandise IS NULL
ORDER BY m.id ASC
LIMIT 20 OFFSET 0