Create a JSON object from parent -> child relationship without duplication - sql

Question

I want to query a database, get ALL of the user's data, and send it to my front end in a JSON object (with many layers of nesting).
e.g.
{
user_id: 1,
username: james,
messages: [
{
message_id: 'fewfef',
message: 'lorum ipsum'
... : {
...
}
}
]
}
Sample schema/data:
--user table (parent)
CREATE TABLE userdata (
user_id integer,
username text
);
INSERT INTO userdata VALUES (1, 'james');
-- messages table (child) connected to user table
CREATE TABLE messages(
message_id integer,
fk_messages_userdata integer,
message text
);
INSERT INTO messages VALUES (1, 1, 'hello');
INSERT INTO messages VALUES (2, 1, 'lorum ipsum');
INSERT INTO messages VALUES (3, 1, 'test123');
-- querying all data at once
SELECT u.username, m.message_id, m.message FROM userdata u
INNER JOIN messages m
ON u.user_id = m.fk_messages_userdata
WHERE u.user_id = '1';
This outputs the data as so:
username|message_id|message |
--------+----------+-----------+
james | 1|hello |
james | 2|lorum ipsum|
james | 3|test123 |
The issue is I have the username is repeated for each message. For larger databases and more layers of nesting this would cause a lot of useless data being queried/sent.
Is it better to do one query to get all of this data and send it to the backend, or make a seperate query for each table, and only get the data I want?
For example I could run these queries:
-- getting only user metadata
SELECT username from userdata WHERE user_id = '1';
-- output
username|
--------+
james |
-- getting only user's messages
SELECT m.message_id, m.message as message_id FROM userdata u
INNER JOIN messages m
ON u.user_id = m.fk_messages_userdata
WHERE u.user_id = '1';
--output
message_id|message_id |
----------+-----------+
1|hello |
2|lorum ipsum|
3|test123 |
This way I get only the data I need, and its a little easier to work with, as it comes to the backed more organized. But is there a disadvantage of running separate queries instead of one big one? Are there any other ways to do this?

Answer 1

Is it better to do one query to get all of this data and send it to the backend, or make a seperate query for each table, and only get the data I want?
It's best to run only one query and get only the data you want. As long as it doesn't get too complicated - which it doesn't IMO:
SELECT to_json(usr)
FROM (
SELECT u.user_id, u.username
, (SELECT json_agg(msg) -- aggregation in correlated subquery
FROM (
SELECT m.message_id, m.message
FROM messages m
WHERE m.fk_messages_userdata = u.user_id
) msg
) AS messages
FROM userdata u
WHERE u.user_id = 1 -- provide user_id here once!
) usr;
fiddle
There are many other ways.
A (LEFT) JOIN LATERAL instead of the correlated subquery. See:
What is the difference between a LATERAL JOIN and a subquery in PostgreSQL?
json_build_object() instead of converting whole rows from subselects. See:
Return multiple columns of the same row as JSON array of objects
LEFT JOIN query with JSON object array aggregate
But this version above should be shortest and fastest.
Related:
What are the pros and cons of performing calculations in sql vs. in your application