SQLite subquery: "IN" the result of the outer query - sqlite

Question

I have two tables user and pair. I want to get the number of duplicate pairs (a, b) for each user.name.
user
name | id
-------------
"Alice" | 0
"Bob" | 1
"Alice" | 2
pair
id | a | b
-----------
0 | 0 | 1
0 | 1 | 3
1 | 0 | 1
2 | 1 | 3
In the above example, the result should be:
name | id | c
-------------------
"Alice" | 0,2 | 1
"Bob" | 1 | 0
When there is only one id for each user, I can do this:
SELECT name, id, (
SELECT COUNT(*) FROM pair JOIN pair AS p USING (id, a, b)
WHERE id = user.id AND pair.rowid < p.rowid
) AS c FROM user;
When there is multiple ids, I can get the correct result from the below query, but it is quite slow when there is more rows and more subqueries.
SELECT name, GROUP_CONCAT(id), (
WITH t AS (SELECT id FROM user AS u WHERE name = user.name)
SELECT COUNT(*) FROM pair JOIN pair AS p USING (a, b)
WHERE pair.id IN t AND p.id IN t AND pair.rowid < p.rowid
) AS c FROM user GROUP BY name;
I want to know that is there a simple and efficient way for this, like changing the WHERE clause from pair.id = user.id to pair.id IN <<the user.id list>>?
/* This will not work! "Error: no such table: user.id" */
SELECT name, GROUP_CONCAT(id), (
SELECT COUNT(*) FROM pair JOIN pair AS p USING (a, b)
WHERE pair.id IN user.id AND p.id IN user.id AND pair.rowid < p.rowid
) AS c FROM user GROUP BY name;

Answer 1

The GROUP BY name operation can be sped up if the database is able to go through the rows in order, without having to sort the table.
This can be done with an index on the name column (the other column makes this a covering index, which helps only a little more):
CREATE INDEX user_name_id_index ON user(name, id);
The query looks up pair rows by their id, a, and b values; these lookups can be sped up with an index on these columns:
CREATE INDEX pair_id_a_b_index ON pair(id, a, b);
To help the query optimizer make better decisions when selecting indexes, run ANALYZE.
The query optimizer gets improved constantly; get the newest SQLite version, if possible.
To check how your queries are executed, look at the output of the EXPLAIIN QUERY PLAN command.