QuerySet 'get' method should clear ordering before executing query

[mattdennewitz@…]

The SQL generated by the ORM when calling get includes ordering parameters specified in Meta.ordering. This might be small in the greater scheme of things, but clearing the ordering before evaluating the cloned QuerySet would prevent unnecessary filesorts by the database.

[nbv4]

I'm not 100% positive, but I'm pretty sure SQL databases internally optimize queries so that "LIMIT 1" automatically "cancels out" any ordering clause so that it doesn't execute the ordering code on that query.

[Luke Plant]

Replying to nbv4:

I'm not 100% positive, but I'm pretty sure SQL databases internally optimize queries so that "LIMIT 1" automatically "cancels out" any ordering clause so that it doesn't execute the ordering code on that query.

If they did, they would produce wrong results. .get() does not do a LIMIT 1 anyway — the only reason we can remove the ORDER BY here is because we retrieve all results and throw an exception if there isn't exactly one i.e. we do the limiting to one after the database has returned results, and excess rows cause no data to be returned.

[Jeremy Dunck]

Updated patch which passes all tests. The existing patch broke .latest() because you can't change ordering after slicing.

[Luke Plant]

(In [11916]) Fixed #12258 - QuerySet.get() should clear ordering.

We only clear ordering when doing so cannot change the result returned by
the get() method i.e. when the query does not already define limits.

Thanks to mattdennewitz@… for the report, and jdunck for the patch