id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
2306	Many-to-many queries very slow on MySQL 3.23.58	anonymous	Adrian Holovaty	"We have a MySQL 3.23.58 database with about 1000 Room and Photo objects, connected by a many-to-many relationship.

Given a room r, asking Django to evaluate r.photos.all() runs an explicit join:

SELECT `kords3_photo`.`id`,`kords3_photo`.`image` FROM `kords3_photo` LEFT OUTER JOIN `kords3_room_photos` AS `m2m_kords3_photo__room` ON `kords3_photo`.`id` = `m2m_kords3_photo__room`.`photo_id` WHERE (`m2m_kords3_photo__room`.`room_id` = 1) ORDER BY `kords3_photo`.`image` ASC

which takes around 1 second, when the same query rewritten as an implicit join:

SELECT `kords3_photo`.`id`,`kords3_photo`.`image` FROM `kords3_photo` , `kords3_room_photos` WHERE `kords3_photo`.`id` = `kords3_room_photos`.`photo_id` AND `kords3_room_photos`.`room_id` = 1 ORDER BY `kords3_photo`.`image` ASC

would take under a millisecond.
"	defect	new	Database layer (models, ORM)	dev	normal				Unreviewed	0	0	0	0	0	0