Thanks for that extensive write up! As the reporter of #27241 it seems I would be arguing against my own self interest when I say that I'm not in favour of the patch, but my reasonings are as follows:
* The current behaviour is preferable in the vast majority of cases, only a couple of projects are affected by the change of behaviour in Django 1.9. * Anyone using unmanaged models on views in a way that stops working when upgrading Django to >= 1.9 is actively maintaining their code and should be able to implement a workaround. * The easiest workaround is only ~5 lines of code and pretty much restores the behaviour of Django 1.8 so it's perfectly acceptable for people upgrading from <= 1.8. * Other workarounds are also possible, especially with the new subquery and other ORM improvements that have been introduced since Django 1.8. * Upgrading from <= 1.8 doesn't necessarily mean upgrading to 2.0 (i.e. ready to upgrade to the next LTS but not yet ready to migrate to Python 3). These people will still end up looking for a workaround anyway. * Any project using unmanaged models that works with Django >= 1.9 will suddenly see a performance hit when they upgrade to 2.0. * The reporter of #28107, #26758 (which is probably the same issue) and I (reporter of #27241) have worked around the issue, so we none of us will benefit from a patch. So while I think that fixing this bug is noble, in this case I think there's way more downsides than upsides. Thanks! Jaap Roes 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. 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