SQL Performance: Using Union and Subqueries - sql

Question

Hi stackoverflow(My first question!),
We're doing something like an SNS, and got a question about optimizing queries.
Using mysql 5.1, the current table was created with:
CREATE TABLE friends(
user_id BIGINT NOT NULL,
friend_id BIGINT NOT NULL,
PRIMARY KEY (user_id, friend_id)
) ENGINE INNODB;
Sample data is populated like:
INSERT INTO friends VALUES
(1,2),
(1,3),
(1,4),
(1,5),
(2,1),
(2,3),
(2,4),
(3,1),
(3,2),
(4,1),
(4,2),
(5,1),
(5,6),
(6,5),
(7,8),
(8,7);
The business logic: we need to figure out which users are friends or friends of friends for a given user.
The current query for this for a user with user_id=1 is:
SELECT friend_id FROM friends WHERE user_id = 1
UNION
SELECT DISTINCT friend_id FROM friends WHERE user_id IN (
SELECT friend_id FROM friends WHERE user_id = 1
);
The expected result is(order doesn't matter):
2
3
4
5
1
6
As you can see, the above query performs the subquery "SELECT friend_id FROM friends WHERE user_id = 1" twice.
So, here is the question. If performance is your primary concern, how would you change the above query or schema?
Thanks in advance.

Answer 1

In this particular case, you can use a JOIN:
SELECT DISTINCT f2.friend_id
FROM friends AS f1
JOIN friends AS f2 ON f1.friend_id=f2.user_id OR f2.user_id=1
WHERE f1.user_id=1;
Examining each query suggests the JOIN will about as performant as the UNION in a big-O sense, though perhaps faster by a constant factor. Jasie's query looks like it might be big-O faster.
EXPLAIN SELECT friend_id FROM friends WHERE user_id = 1
UNION
SELECT DISTINCT friend_id FROM friends WHERE user_id IN (
SELECT friend_id FROM friends WHERE user_id = 1
);
+----+--------------------+------------+--------+---------------+---------+---------+------------+------+-------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+------------+--------+---------------+---------+---------+------------+------+-------------------------------------------+
| 1 | PRIMARY | friends | ref | PRIMARY | PRIMARY | 8 | const | 4 | Using index |
| 2 | UNION | friends | index | NULL | PRIMARY | 16 | NULL | 16 | Using where; Using index; Using temporary |
| 3 | DEPENDENT SUBQUERY | friends | eq_ref | PRIMARY | PRIMARY | 16 | const,func | 1 | Using index |
| NULL | UNION RESULT | <union1,2> | ALL | NULL | NULL | NULL | NULL | NULL | |
+----+--------------------+------------+--------+---------------+---------+---------+------------+------+-------------------------------------------+
EXPLAIN SELECT DISTINCT f2.friend_id
FROM friends AS f1
JOIN friends AS f2
ON f1.friend_id=f2.user_id OR f2.user_id=1
WHERE f1.user_id=1;
+----+-------------+-------+-------+---------------+---------+---------+-------+------+---------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+---------------------------------------------+
| 1 | SIMPLE | f1 | ref | PRIMARY | PRIMARY | 8 | const | 4 | Using index; Using temporary |
| 1 | SIMPLE | f2 | index | PRIMARY | PRIMARY | 16 | NULL | 16 | Using where; Using index; Using join buffer |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+---------------------------------------------+
EXPLAIN SELECT DISTINCT friend_id FROM friends WHERE user_id IN (
SELECT friend_id FROM friends WHERE user_id = 1
) OR user_id = 1;
+----+--------------------+---------+--------+---------------+---------+---------+------------+------+-------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+---------+--------+---------------+---------+---------+------------+------+-------------------------------------------+
| 1 | PRIMARY | friends | index | PRIMARY | PRIMARY | 16 | NULL | 16 | Using where; Using index; Using temporary |
| 2 | DEPENDENT SUBQUERY | friends | eq_ref | PRIMARY | PRIMARY | 16 | const,func | 1 | Using index |
+----+--------------------+---------+--------+---------------+---------+---------+------------+------+-------------------------------------------+

Answer 2

No need for the UNION. Just include an OR with the user_id of the beginning user:
SELECT DISTINCT friend_id FROM friends WHERE user_id IN (
SELECT friend_id FROM friends WHERE user_id = 1
) OR user_id = 1;
+-----------+
| friend_id |
+-----------+
| 2 |
| 3 |
| 4 |
| 5 |
| 1 |
| 6 |
+-----------+