Hello, the fix for https://code.djangoproject.com/ticket/27241 caused significant performance drop for my project, because it pretty much prevents posgresql from using indexes on the particular query. I use unmanaged tables because Django is used to display data from another project's database, so it has just read-only access to the database and I don't want to create any migrations for it.
Another solution I came up while writing this post would be to replace the > feature flag by a callable that takes a model as a single parameter and > returns > whether or not the optimization can be performed against it. The default > implementation would return `mode._meta.managed` but it would make it > easier for > users affected by this to override in order to opt-in or out based on their > application logic. > This would be perfect for my (arguably niche) use case. Should I try to prepare a patch? On Monday, May 22, 2017 at 5:05:38 AM UTC+2, charettes wrote: > > Hello fellow developers, > > As some of you may know PostgreSQL 9.1 added support for GROUP'ing BY > selected table primary keys[0] only. Five years ago it was reported[1] that > Django could rely on this feature to speed up aggregation on models backed > up by tables with either many fields or a few large ones. > > Being affected by this slow down myself I decided to dive into the ORM > internals > and managed to get a patch that made it in 1.9[2] thanks to Anssi's and > Josh's > review[3]. > > One subtle thing I didn't know back in the time is that PostgreSQL query > planner > isn't able to introspect database views columns' functional dependency > like it > does with tables and thus prevents the primary key GROUP'ing optimization > from > being used. > > While Django doesn't support database views officially it documents that > unmanaged models can be used to query them[4] and thereby perform > aggregation on > them and generating an invalid query. > > This was initially reported as a crashing bug 9 months ago[5] and the > consensus > at this time was that it was an esoteric edge case since there was few > reports > of breakages and it went off my radar. Fast-forward to a month ago, this is > reported again[6] and it takes the reporter quite a lot of effort to > determine > the origin of the issue, pushing me to come up with a solution as I > introduced > this behavior. > > Before Claude makes me realize this is a duplicate of the former report > (which I > completely forgot about in the mean time) I implement a patch and commit > it once > it's reviewed [7]. > > When I closed the initial ticket as "fixed" the reporter brought to my > attention > that this was now introducing a performance regression for unmanaged models > relying on aggregation and that we should document how to disable this > optimization by creating a backend subclass as a workaround instead. > > In my opinion the current situation is as follow. The optimization > introduced a > break in backward compatibility in 1.9 as we've always documented that > database > views could be queried against using unmanaged models. If this issue had > been > discovered during the 1.9 release cycle it would have been eligible for a > backport because it was a bug in a newly introduced feature. Turning this > optimization off for unmanaged models by assuming they could be views is > only > going to degrade performance of queries using unmanaged models to perform > aggregation on tables with either a large number of columns or large > columns > using PostgreSQL. > > Therefore I'd favor we keep the current adjustment in the master branch as > it > restores backward compatibility but I don't have strong feelings about > reverting > it either if it's deemed inappropriate. > > Another solution I came up while writing this post would be to replace the > feature flag by a callable that takes a model as a single parameter and > returns > whether or not the optimization can be performed against it. The default > implementation would return `mode._meta.managed` but it would make it > easier for > users affected by this to override in order to opt-in or out based on their > application logic. > > Thank you for your time, > Simon > > [0] https://www.postgresql.org/docs/9.1/static/sql-select.html#SQL-GROUPBY > [1] https://code.djangoproject.com/ticket/19259 > [2] > https://github.com/django/django/commit/dc27f3ee0c3eb9bb17d6cb764788eeaf73a371d7 > [3] https://github.com/django/django/pull/4397 > [4] https://docs.djangoproject.com/en/1.11/ref/models/options/#managed > [5] https://code.djangoproject.com/ticket/27241 > [6] https://code.djangoproject.com/ticket/28107 > [7] > https://github.com/django/django/commit/daf2bd3efe53cbfc1c9fd00222b8315708023792 > > -- 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 django-developers+unsubscr...@googlegroups.com. To post to this group, send email to django-developers@googlegroups.com. Visit this group at https://groups.google.com/group/django-developers. To view this discussion on the web visit https://groups.google.com/d/msgid/django-developers/80b91e05-bd06-46ac-ad26-1dc5a47f378a%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
