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Yannick PEREIRA-REIS

Full stack web developer Symfony2, VueJs and AngularJs in Lyon (France), I also play with DevOps tools like Docker, Vagrant, Elasticsearch, Logstash, Kibana, Supervisor, ...

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As you may know, Doctrine can be a very good ally, but if we do not use it correctly we can have big performance problems really easily and quickly.

Symfony

Let’s take these two tables as an example:

Players

+------------------------------+--------------+------+-----+---------+-------+
| Field                        | Type         | Null | Key | Default | Extra |
+------------------------------+--------------+------+-----+---------+-------+
| id                           | int(11)      | NO   | PRI | NULL    |       |
| name                         | varchar(255) | YES  |     | NULL    |       |
+------------------------------+--------------+------+-----+---------+-------+

Players Actions

Players can do actions, each action is defined by a name and a format. Of course, an action is done by a single player. A Player can do an action of a given format only once. (Not really important actually)

+------------+-------------+------+-----+---------+----------------+
| Field      | Type        | Null | Key | Default | Extra          |
+------------+-------------+------+-----+---------+----------------+
| id         | int(11)     | NO   | PRI | NULL    | auto_increment |
| player_id  | int(11)     | NO   | MUL | NULL    |                |
| action     | varchar(20) | NO   |     | NULL    |                |
| format     | varchar(20) | NO   |     | NULL    |                |
+------------+-------------+------+-----+---------+----------------+

Bad DQL example

We could write this king of DQL query, it works and it will generate a valid SQL query :

DQL Query

In the example, let’s say that the player has id 52.

<?php

$qb = $this->doctrine->getRepository('AppBundle:ActionPlayer')
    ->createQueryBuilder('ap')
    ->select('ap.format, ap.action, COUNT(ap.player) AS value')
    ->andWhere('ap.player = :player')->setParameter('player', $player)
    ->addGroupBy('ap.player')
    ->addGroupBy('ap.format')
    ->addGroupBy('ap.action')
;

Generated SQL (Mysql)

SELECT g0_.format AS format_0, g0_.action AS action_1, COUNT(g0_.player_id) AS sclr_2
FROM action_player g0_
WHERE g0_.player_id = '52'
GROUP BY g0_.player_id, g0_.format, g0_.action;

Good DQL example (not really actually)

DQL Query

In the example, let’s say that the player has id 52.

<?php

$qb = $this->doctrine->getRepository('CommonBundle:Stats\StatActionPlayer')
    ->createQueryBuilder('ap')
    ->select('ap.format, ap.action, COUNT(ap.player) AS value')
    ->innerJoin('ap.player', 'player')
    ->andWhere('player.id = :player_id')->setParameter('player_id', $player)
    ->addGroupBy('ap.player')
    ->addGroupBy('ap.format')
    ->addGroupBy('ap.action')
;

Generated SQL (Mysql)

SELECT g0_.format AS format_0, g0_.action AS action_1, COUNT(g0_.player_id) AS sclr_2
FROM action_player g0_
INNER JOIN player g1_ ON g0_.player_id = g1_.id
WHERE g1_.id = '52'
GROUP BY g0_.player_id, g0_.format, g0_.action;

So! What’s the problem ?

Let’s say we have millions of rows in our action_player table… We need an index to improve SELECT performance:

ALTER TABLE action_player ADD INDEX action_player_idx (player_id, format, action);

Important to note that the index must contains GROUP BY fields AND field used in WHERE clause.

Actually, if we go back to our two previous SQL queries (auto generated) and we try to execute them on our table with millions of rows and the above index:

  • The first one gives results in about 3,9s.
  • The second one gives results in about 0,36s.

The difference is HUGE….

How are those two queries really executed by MySQL ?

mysql> EXPLAIN SELECT g0_.format AS format_0, g0_.action AS action_1, COUNT(g0_.player_id) AS sclr_2
    -> FROM action_player g0_
    -> WHERE g0_.player_id = '52'
    -> GROUP BY g0_.player_id, g0_.format, g0_.action;
+----+-------------+-------+------+-----------------------------------------+----------------------+...
| id | select_type | table | type | possible_keys                           | key                  |...
+----+-------------+-------+------+-----------------------------------------+----------------------+...
|  1 | SIMPLE      | g0_   | ref  | IDX_4D92B9D399E6F5DF, action_player_idx | IDX_4D92B9D399E6F5DF |...
+----+-------------+-------+------+-----------------------------------------+----------------------+...
1 row in set (0.00 sec)
mysql> EXPLAIN SELECT g0_.format AS format_0, g0_.action AS action_1, COUNT(g0_.player_id) AS sclr_2
    -> FROM action_player g0_
    -> INNER JOIN player g1_ ON g0_.player_id = g1_.id
    -> WHERE g1_.id = '52'
    -> GROUP BY g0_.player_id, g0_.format, g0_.action;
+----+-------------+-------+-------+-----------------------------------------+-------------------+...
| id | select_type | table | type  | possible_keys                           | key               |...
+----+-------------+-------+-------+-----------------------------------------+-------------------+...
|  1 | SIMPLE      | g1_   | const | PRIMARY                                 | PRIMARY           |...
|  1 | SIMPLE      | g0_   | ref   | IDX_4D92B9D399E6F5DF, action_player_idx | action_player_idx |...
+----+-------------+-------+-------+-----------------------------------------+-------------------+...
2 rows in set (0.00 sec)
  • In the first case the created index IS NOT used.
  • The index IS used in the second case.

Explanation

  • When an index is created, it takes into account the type of columns used in the index.
  • The player id field is an integer, so the index is created with an integer for the player_id field.
  • In the bad DQL example, Doctrine generates an SQL query with WHERE g0_.player_id = '52' and NO join, the index is not used !
  • In the good DQL example, Doctrine generates an SQL query with WHERE g0_.player_id = '52' AND a join on player table, the index is used for the inner join !

The solution, improve your DQL queries with type hinting.

In these two examples, we can see that the generated SQL contains this part WHERE g1_.id = '52'.

Actually, THIS IS the problem, not really the join between tables. In both DQL queries the player_id is used as a string but it should be as an integer.

Always add type hinting to your DQL parameters:

<?php

$qb = $this->doctrine->getRepository('AppBundle:ActionPlayer')
    ->createQueryBuilder('ap')
    ->select('ap.format, ap.action, COUNT(ap.player) AS value')
    ->andWhere('ap.player = :player')->setParameter('player', (int) $player)
    ->addGroupBy('ap.player')
    ->addGroupBy('ap.format')
    ->addGroupBy('ap.action')
;

Or you could use the third parameter of the setParameter() method:

<?php

->andWhere('ap.player = :player')->setParameter('player', $player, \Doctrine\DBAL\Types\Type::INTEGER)

No more join needed to filter by player_id and the index will be used as it should be.