SELECT from subquery without having to specify all columns in GROUP BY - sql

Question

Idea is to query an article table where an article has a given tag, and then to STRING_AGG all (even unrelated) tags that belong to that article row.
Example tables and query:
CREATE TABLE article (id SERIAL, body TEXT);
CREATE TABLE article_tag (article INT, tag INT);
CREATE TABLE tag (id SERIAL, title TEXT);
SELECT DISTICT ON (id)
q.id, q.body, STRING_AGG(q.tag_title, '|') tags
FROM (
SELECT a.*, tag.title tag_title
FROM article a
LEFT JOIN article_tag x ON a.id = tag.article
LEFT JOIN tag ON tag.id = x.tag
WHERE tag.title = 'someTag'
) q
GROUP BY q.id
Running the above, postgres require that the q.body must be included in GROUP BY:
ERROR: column "q.body" must appear in the GROUP BY clause or be used in an aggregate function
As I understand it, it's because subquery q doesn't include any PRIMARY key.
I naively thought that the DISTINCT ON would supplement that, but it doesn't seem so.
Is there a way to mark a column in a subquery as PRIMARY so that we don't have to list all columns in GROUP BY clause?
If we do have to list all columns in GROUP BY clause, does that incur significant perf cost?
EDIT: to elaborate, since PostgreSQL 9.1 you don't have to supply non-primary (i.e. functionally dependent) keys when using GROUP BY, e.g. following query works fine:
SELECT a.id, a.body, STRING_AGG(tag.title, '|') tags
FROM article a
LEFT JOIN article_tag x ON a.id = tag.article
LEFT JOIN tag ON tag.id = x.tag
GROUP BY a.id
I was wondering if I can leverage the same behavior, but with a subquery (by somehow indicating that q.id is a PRIMARY key).

Answer 1

It sadly doesn't work when you wrap your primary key in subquery and I don't know of any way to "mark it" as you suggested.
You can try this workaround using window function and distinct:
CREATE TABLE test1 (id serial primary key, name text, value text);
CREATE TABLE test2 (id serial primary key, test1_id int, value text);
INSERT INTO test1(name, value)
values('name1', 'test01'), ('name2', 'test02'), ('name3', 'test03');
INSERT INTO test2(test1_id, value)
values(1, 'test1'), (1, 'test2'), (3, 'test3');
SELECT DISTINCT ON (id) id, name, string_agg(value2, '|') over (partition by id)
FROM (SELECT test1.*, test2.value AS value2
FROM test1
LEFT JOIN test2 ON test2.test1_id = test1.id) AS sub;
id name string_agg
1 name1 test1|test2
2 name2 null
3 name3 test3
Demo

Answer 2

Problem is in outer SELECT - you should either aggregate columns either
group by them. Postgres wants you to specify what to do with q.body - group by it or calculate aggregate. Looks little bit awkward but should work.
SELECT DISTICT ON (id)
q.id, q.body, STRING_AGG(q.tag_title, '|') tags
FROM (
SELECT a.*, tag.title tag_title
FROM article a
LEFT JOIN article_tag x ON a.id = tag.article
LEFT JOIN tag ON tag.id = x.tag
WHERE tag.title = 'someTag'
) q
GROUP BY q.id, q.body
-- ^^^^^^
Another way is to make a query to get id and aggregated tags then join body to it. If you wish I can make an example.