PostgreSQL: Select the group with specific members - sql

Question

Given the tables below:
CREATE TABLE users (
id bigserial PRIMARY KEY,
name text NOT NULL
);
CREATE TABLE groups (
id bigserial PRIMARY KEY
);
CREATE TABLE group_members (
group_id bigint REFERENCES groups ON DELETE CASCADE,
user_id bigint REFERENCES users ON DELETE CASCADE,
PRIMARY KEY (group_id, user_id)
);
How do we select a group with a specific set of users?
We want an SQL function that takes an array of user IDs and returns the group ID (from the group_members table) with the exact same set of user IDs.
Also, please add indexes if they will make your solution faster.

Answer 1

First, we need to get "candidate" rows from group_members relation, and then with additional run ensure that group size is the same as user_ids array size (here I use CTE https://www.postgresql.org/docs/current/static/queries-with.html):
with target(id) as (
select * from unnest(array[2, 3]) -- here is your input
), candidates as (
select group_id
from group_members
where user_id in (select id from target) -- find all groups which include input
)
select group_id
from group_members
where group_id in (select group_id from candidates)
group by group_id
having array_length(array_agg(user_id), 1)
= array_length(array(select id from target), 1) -- filter out all "bigger" groups
;
Demonstration with some sample data: http://dbfiddle.uk/?rdbms=postgres_9.6&fiddle=a98c09f20e837dc430ac66e01c7f0dd0
This query will utilize indexes you already have, but probably it's worth to add a separate index on group_members (user_id) to avoid intermediate hashing in the first stage of the CTE query.
SQL function is straightforward:
create or replace function find_groups(int8[]) returns int8 as $$
with candidates as (
select group_id
from group_members
where user_id in (select * from unnest($1))
)
select group_id
from group_members
where group_id in (select group_id from candidates)
group by group_id
having array_length(array_agg(user_id), 1) = array_length($1, 1)
;
$$ language sql;
See the same DBfiddle for demonstration.