It might be a bit early in the day for me, but isn't that query already
optimised? That is, it's already eliminated a join. It hasn't joined to the
"Especialidad" table, it's only joined to the intermediate table. I *think*
the join to the intermediate table is necessary because there could be
duplicates.
Given the tables:
Usuario(pk):
1
2
Intermediate(usurario_id, especialidad_id):
1, 1
1, 2
Especialidad(pk)
1
2
Joining Usuario to Intermediate will return 4 results in SQL (2 for each pk
on Usuario) unless there was a distinct in there somewhere. I haven't
tested, so I'm not sure if django does duplicate elimination, but I'm
pretty sure it doesn't.
Does this look right to you, or am I missing something?
Cheers
On Thursday, 19 November 2015 11:41:22 UTC+11, Cristiano Coelho wrote:
>
> Hello there,
>
> Lets say I have these two models (sorry about the spanish names!) ( Django
> 1.8.6 and MySQL backend )
>
> class Especialidad(models.Model):
> nombre = models.CharField(max_length=250, blank=False, unique=True)
>
>
>
> class Usuario(AbstractBaseUser):
> permisosEspecialidad = models.ManyToManyField("Especialidad", blank=True)
>
> Let u be some Usuario instance, and the following query:
>
> u.permisosEspecialidad.all().values_list('pk',flat=True)
>
> The actual printed query is:
>
> SELECT `samiBackend_especialidad`.`id`
> FROM `samiBackend_especialidad`
> INNER JOIN `samiBackend_usuario_permisosEspecialidad` ON (
> `samiBackend_especialidad`.`id` =
> `samiBackend_usuario_permisosEspecialidad`.`especialidad_id` )
> WHERE `samiBackend_usuario_permisosEspecialidad`.`usuario_id` = 8
>
> As my understanding, since I'm only selecting the id field which is already
> present in the intermediary table (and is also a FK), the actual join is
> redundant, as I have all the info I need in this case.
>
> So the query could work like this
>
> SELECT `samiBackend_usuario_permisosEspecialidad`.`especialidad_id`
> FROM `samiBackend_usuario_permisosEspecialidad`
> WHERE `samiBackend_usuario_permisosEspecialidad`.`usuario_id` = 8
>
>
> I guess this works this way because this particular case might be hard to
> detect or won't be compatible with any additional query building, however,
> for ForeignKey relations, this optimization is already done (If you select
> the primary key from the selected model only, it wont add a join)
>
> What would be the complications to implement this? Would it worth the effort?
>
>
>
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