Question

In the web page that I’m working on I need to show some statistics based on a different user details which are in three tables. So I have the following query that I join to more different tables:

SELECT *
FROM `user` `u`
LEFT JOIN `subscriptions` `s` ON `u`.`user_id` = `s`.`user_id`
LEFT JOIN `devices` `ud` ON `u`.`user_id` = `ud`.`user_id`
GROUP BY `u`.`user_id`

When I execute the query with LIMIT 1000 it takes about 0.05 seconds and since I’m using the data from all the three tables in a lot of queries I’ve decided to put it inside a VIEW:

CREATE VIEW `user_details` AS ( the same query from above )

And now when I run:

SELECT * FROM user_details LIMIT 1000

it takes about 7-10 seconds.

So my question is can I do something to optimize the view because the query seems to be pretty quick or I should the whole query instead of the view ?

Edit: this is what EXPLAIN SELECT * FROM user_details returns

+----+-------------+------------+--------+----------------+----------------+---------+------------------------+--------+-------+
| id | select_type | table      | type   | possible_keys  | key            | key_len | ref                    | rows   | Extra |
+----+-------------+------------+--------+----------------+----------------+---------+------------------------+--------+-------+
|  1 | PRIMARY     | <derived2> | ALL    | NULL           | NULL           | NULL    | NULL                   | 322666 |       |
|  2 | DERIVED     | u          | index  | NULL           | PRIMARY        | 4       | NULL                   | 372587 |       |
|  2 | DERIVED     | s          | eq_ref | PRIMARY        | PRIMARY        | 4       | db_users.u.user_id     |      1 |       |
|  2 | DERIVED     | ud         | ref    | device_id_name | device_id_name | 4       | db_users.u.user_id     |      1 |       |
+----+-------------+------------+--------+----------------+----------------+---------+------------------------+--------+-------+

4 rows in set (8.67 sec)

this is what explain retuns for the query:

+----+-------------+-------+--------+----------------+----------------+---------+------------------------+--------+-------+
| id | select_type | table | type   | possible_keys  | key            | key_len | ref                    | rows   | Extra |
+----+-------------+-------+--------+----------------+----------------+---------+------------------------+--------+-------+
|  1 | SIMPLE      | u     | index  | NULL           | PRIMARY        | 4       | NULL                   | 372587 |       |
|  1 | SIMPLE      | s     | eq_ref | PRIMARY        | PRIMARY        | 4       | db_users.u.user_id     |      1 |       |
|  1 | SIMPLE      | ud    | ref    | device_id_name | device_id_name | 4       | db_users.u.user_id     |      1 |       |
+----+-------------+-------+--------+----------------+----------------+---------+------------------------+--------+-------+

3 rows in set (0.00 sec)

Answer 1

Views and joins are extremely bad if it comes to performance. This is more or less true for all relational database management systems. Sounds strange, since that is what those systems are designed for, but it is true nevertheless.

Try to avoid the joins if this is a query in heavy usage on your page: instead create a real table (not a view) that is filled from the three tables. you can automate that process using triggers. So each time an entry is inserted into one of the original tables the triggers takes care that the data is propagated to the physical user_details table.

This strategy certainly means a one time investment for the setup, but you definitely will get a much better performance.