documentation should warn of cost of ordering

[outofculture@…]

In ​https://docs.djangoproject.com/en/dev/ref/models/options/#ordering it makes no mention of the database cost of ordering, especially for ordering on non-indexed fields, or the implicit joins of chained orderings. (It doesn't even mention that it chains orderings!) ​https://docs.djangoproject.com/en/dev/ref/models/querysets/#order-by is another location where the cost might be mentioned. It could additionally go in ​https://docs.djangoproject.com/en/dev/topics/db/optimization/#don-t-retrieve-things-you-don-t-need as a hint for things to look for when optimizing.

Recommend for model options and querysets pages: "Warning: Ordering is not a free operation. Each field you add to the ordering will incur a cost from your database. Each foreign key you add will include all of its default orderings implicitly."

Recommend for optimization page: "==Don't order if you don't care== Ordering is not free; each field added is an operation the database has to perform."

[Preston Holmes]

I think the mention in the optimization docs has priority - but there should be some caution about the risk of having too many performance warnings in the API docs

[Ivan Kolodyazhny]

Here is pull request with patch ​https://github.com/django/django/pull/1603

[Tim Graham <timograham@…>]

In cbf08c6b0ca39406cada4bd1ffc6d334a79822e8:

Fixed #16895 -- Warned about cost of QuerySet ordering

Thanks outofculture at gmail.com for the suggestion.

[Tim Graham <timograham@…>]

In 276e053803c847ce645b1859a162a997a4ee3789:

[1.6.x] Fixed #16895 -- Warned about cost of QuerySet ordering

Thanks outofculture at gmail.com for the suggestion.

Backport of cbf08c6b0c from master

[Tim Graham <timograham@…>]

In fb51c9a0f262eae0ebb36ba1067dc0a4db314401:

Fixed spelling; refs #16895.

Thanks Panagiotis Issaris for the report.

[Tim Graham <timograham@…>]

In 01ad5085148bf7dcfc3af4d05cc8c262680f85d9:

[1.6.x] Fixed spelling; refs #16895.

Thanks Panagiotis Issaris for the report.

Backport of fb51c9a0f2 from master