We din't currently do join trimming from the beginning of the query's tables, only from the end of the tables.
Fixing this issue only doesn't seem worth it, as the current join trimmig code can't be adapted to handle trimming from the beginning of the query's tables. But we might be able to trim any non-necessary table from the query in the compiler stage. If we were able to do that, then there might be a point in implementing that. I know we sometimes include non-necessary tables in the query for example in multitable inheritance scenarios. - Anssi On Thursday, November 19, 2015, Cristiano Coelho <[email protected]> wrote: > You are right. I believe an optimization like this would probably help > just a few people, as only fetching data from the intermediary table is a > rare thing. But if it is an easy change, which improves performance, why > not? However I think the change is quite complicated. > > El jueves, 19 de noviembre de 2015, 11:51:51 (UTC-3), charettes escribió: >> >> Hi Cristiano, >> >> If I get it correctly you'd like m2m querying to start with the >> intermediary (FROM) table and JOIN the referenced one only if more fields >> than the primary key are selected. >> >> class Book(models.Model): >> name = models.CharField(max_length=100) >> >> class Author(models.Model): >> books = models.ManyToMany(Book) >> >> author = Author.objects.get(pk=1) >> author.books.values_list('pk') >> >> Would result in the following query: >> SELECT book_id FROM author_books WHERE author_id = 1; >> >> Instead of: >> SELECT id FROM book JOIN author_books ON (book.id = >> author_books.book_id) WHERE author_id = 1; >> >> I think this is a sensible optimization but I wonder about its >> feasibility. It looks like the `pk` reference would require some special >> handling to reference `book_id` since it's not actually a primary key on >> the intermediate table. >> >> Simon >> >> Le mercredi 18 novembre 2015 19:41:22 UTC-5, Cristiano Coelho a écrit : >>> >>> 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? >>> >>> >>> -- > You received this message because you are subscribed to the Google Groups > "Django developers (Contributions to Django itself)" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected] > <javascript:_e(%7B%7D,'cvml','django-developers%[email protected]');> > . > To post to this group, send email to [email protected] > <javascript:_e(%7B%7D,'cvml','[email protected]');>. > Visit this group at http://groups.google.com/group/django-developers. > To view this discussion on the web visit > https://groups.google.com/d/msgid/django-developers/f96c5d2f-3655-461f-8534-de8622cd284d%40googlegroups.com > <https://groups.google.com/d/msgid/django-developers/f96c5d2f-3655-461f-8534-de8622cd284d%40googlegroups.com?utm_medium=email&utm_source=footer> > . > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "Django developers (Contributions to Django itself)" group. 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