document that order_by('?') is a huge performance issue
|Reported by:||Owned by:||Matt Boersma|
|Component:||Database layer (models, ORM)||Version:|
|Cc:||gomo@…||Triage Stage:||Ready for checkin|
|Has patch:||yes||Needs documentation:||no|
|Needs tests:||no||Patch needs improvement:||no|
order_by('?') generates an SQL query that is horrendous from a performance point of view (the "ORDER BY RAND() LIMIT" type query).
For the current state of affairs, I think at the very least a warning should be added to http://www.djangoproject.com/documentation/db-api/#order-by-fields .
That page happily states that you can use the method for obtaining a random row, but in a real scenario that is a very bad idea, and should be avoided at all costs.
On a more useful approach, maybe extra code could be added to a model's Meta class if you plan on grabbing random rows from that particular table. This could set up needed tables/columns/constraints in order to extract a random row without such a big performance hit. If you use order_by('?') on a model with this Meta setting, the enhancement would be transparent. How and if this improvement could be implemented is open for discussion, and is probably database dependent. The page I linked above has some discussion on the topic.
Change History (4)
comment:1 Changed 9 years ago by
|Patch needs improvement:||unset|
|Summary:||order_by('?') is a huge performance issue → document that order_by('?') is a huge performance issue|
|Triage Stage:||Unreviewed → Accepted|
comment:2 Changed 9 years ago by
|Owner:||changed from nobody to Matt Boersma|
|Status:||new → assigned|
|Triage Stage:||Accepted → Ready for checkin|