QuerySet evaluation docs are out of date

[Michal Petrucha]

The ​docs for QuerySet contain a reference to partial evaluation of queryset and a warning that explicitly calling list(qs) requires more memory than just iterating over a queryset.

As far as I can see, anytime a QuerySet is evaluated, all results are pulled from the database at once, stored in _result_cache and most list-like operations are simply proxied to this cache. This means that there is no partial evaluation going on anymore, and list(qs) simply creates a shallow copy of qs._result_cache.

[Michal Petrucha]

I created a simple pull request (​https://github.com/django/django/pull/3507) as well and while I was in there, I also added a note regarding #22503. If anyone wants to suggest a different wording, no problem.

[Tim Graham]

I think the inaccuracy is due to the removal of "chunked reads" from QuerySet.__iter__() in #18702 -- does that make sense?

[Michal Petrucha]

Yes, you're right, that patch indeed appears to be the relevant one.

[Tim Graham <timograham@…>]

In 327df551e89a505c5756becee97c40198f38aff2:

Fixed #23817 -- Updated docs on QuerySet evaluation

Removed inaccurate info about partial evaluation after refs #18702.
Added information on modifying sliced QuerySets; refs #22503.

[Tim Graham <timograham@…>]

In 614dd44d0d2e5f8f12256835c3453f420f54c3b4:

[1.6.x] Fixed #23817 -- Updated docs on QuerySet evaluation

Removed inaccurate info about partial evaluation after refs #18702.
Added information on modifying sliced QuerySets; refs #22503.

Backport of 327df551e89a505c5756becee97c40198f38aff2 from master

[Tim Graham <timograham@…>]

In 593353d8af5b4b8d1a1e627712fe68ed593961d0:

[1.7.x] Fixed #23817 -- Updated docs on QuerySet evaluation

Removed inaccurate info about partial evaluation after refs #18702.
Added information on modifying sliced QuerySets; refs #22503.

Backport of 327df551e89a505c5756becee97c40198f38aff2 from master