document index created for a ForeignKey

[jbauer@…]

It would be helpful if the Model field docs mentioned that Django automatically creates an index for ForeignKey columns.

​https://docs.djangoproject.com/en/dev/ref/models/fields/#foreignkey

The docs could also mention how to disable this behavior (db_index=False) and why the user may want to do this:

• FK created for consistency, not joins
• alternative special indexes (partial, multiple column, etc) will be created instead

Relevant links:

• https://code.djangoproject.com/ticket/13730
• ​http://stackoverflow.com/questions/4836327/do-i-need-to-add-a-db-index-to-this-django-model
• ​http://stackoverflow.com/questions/5984842/django-does-it-automatically-generate-indexes-for-foreign-keys-columns

[Tim Graham <timograham@…>]

In db598dd8a053fe17c3308f89cd8f40676e2c479e:

Fixed #18046 - Documented than an index is created by default for ForeignKeys; thanks jbauer for the suggestion.

[Tim Graham <timograham@…>]

In fd90a906333b569340e5507fec3f3a4d4c1d0f47:

[1.4.X] Fixed #18046 - Documented that an index is created by default for ForeignKeys; thanks jbauer for the suggestion.

Backport of db598dd8a0 from master