[DDC-1602] Executors for Class Table Inheritance (JOINED) are extremely slow on MySQL Created: 15/Jan/12  Updated: 27/Jun/12

Status: Open
Project: Doctrine 2 - ORM
Component/s: DQL
Affects Version/s: 2.2-BETA2
Fix Version/s: None
Security Level: All

Type: Improvement Priority: Major
Reporter: Michael Moravec Assignee: Benjamin Eberlei
Resolution: Unresolved Votes: 3
Labels: None

Debian, MySQL 5.5.17


Update and delete executors for Class Table Inheritance (JOINED) are extremely slow on MySQL platform. It is most probably due to use of subselect on the temporary table.
The slowdown is really significant as the table size increases. As an example, lets have a root entity with one subclass:

 * @Entity
 * @InheritanceType("JOINED")
 * @DiscriminatorColumn(name="discr", type="string")
 * @DiscriminatorMap({"root" = "Root", "a" = "SubA"})
class Root
	 * @Column(type="integer")
	 * @Id
	 * @GeneratedValue
	private $id;

	 * @Column(type="integer")
	private $xyz;
 * @Entity
class SubA extends Root
	 * @Column(type="integer")
	private $foo;

Now lets perform a simple DQL UPDATE:

UPDATE Entities\Root r SET r.xyz = 123 WHERE r.id > ?

(note: always the upper half of entries)
Which creates following SQLs:

INSERT INTO Root_id_tmp (id) SELECT t0.id FROM Root t0 LEFT JOIN SubA s0_ ON t0.id = s0_.id WHERE t0.id > 25000
UPDATE Root SET xyz = 123 WHERE (id) IN (SELECT id FROM Root_id_tmp)

The time spent on this on MySQL 5.5.17 and PostgreSQL 9.1 is:

no. of entries 500 1000 2500 5000 10000 20000 50000
MySQL 0.26s 0.35s 1.1s 3.68s 14.13s 54.44s 338s
PostgreSQL 0.10s 0.10s 0.13s 0.15s 0.22s 0.35s 1.01s

As you can see, MySQL is drastically slower on even relatively small tables. This currently makes Doctrine unusable for this type of inheritance on MySQL. The solution probably would be to avoid subselect in WHERE clause in Doctrine\ORM\Query\Exec\MultiTableUpdateExecutor and Doctrine\ORM\Query\Exec\MultiTableDeleteExecutor.

Feel free to try/modify the test script yourself, it's here.

Comment by Benjamin Eberlei [ 15/Jan/12 ]

Its not a bug as it works. The performance drawback of JTI is discussed in the manual http://www.doctrine-project.org/docs/orm/2.1/en/reference/inheritance-mapping.html.

Changing this would be an improvement where we would hint if databases prefer subselects or joins for different operations. This would increase complexity of the SQL generation since now we are getting along with just one SQL generation strategy.

Comment by Michael Moravec [ 11/May/12 ]

Any chance to get this implemented before 2.3?

Comment by Michael Moravec [ 11/May/12 ]

I've made a change in DBAL and ORM code to implement a solution issue. It's currently more likely a proof of concept.

With the change, my results are (approximately):

no. of entries 500 1000 2500 5000 10000 20000 50000
MySQL 0.17s 0.19s 0.21s 0.26s 0.27s 0.37s 0.92s

Currently only update executor was changed.
DBAL branch with changes: https://github.com/Majkl578/doctrine-dbal/tree/DDC-1602
ORM branch with changes: https://github.com/Majkl578/doctrine2/tree/DDC-1602

Looking forward for your opinions.

Comment by Michael Moravec [ 27/Jun/12 ]


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