MySQL generates an unecessary select when updating inherited models

[Anssi Kääriäinen]

The reason is in !SQLUpdateCompiler.pre_sql_setup(): it checks if there are multiple tables in the query, and if the backend can't do a self select on update ("update_can_self_select" db feature) the pre_sql_setup will run a query to fetch the pks to update. However, in inheritance cases this leads to queries like:

SELECT U0.`person_ptr_id` FROM `update_only_fields_employee` U0 WHERE U0.`person_ptr_id` = 2

which is non-necessary to run.

The problem was spotted when running update_only_fields tests on MySQL - the test will be skipped on MySQL, but the test_num_queries_inheritance test shows the problem if the skip is removed.

[Anssi Kääriäinen]

Avoided test failure on MySQL by skipping a failing test

MySQL generates an extra query in inheritance cases when doing an update.
This results in a test failure when checking for number of queries in
update_only_fields tests. Added a skip temporarily to avoid this test
failure. Refs #18304.

Changeset: de79d23ce04193e0bc140991533359002f62ddf9

[Anssi Kääriäinen]

​https://github.com/django/django/pull/61 contains an RFC patch.

[Anssi Kääriäinen <akaariai@…>]

In [d5c7f9efc3702888b556cbf932116421b464e8b8]:

Fixed #18304 -- Optimized save() when update_can_self_select=False

Databases with update_can_self_select = False (MySQL for example)
generated non-necessary queries when saving a multitable inherited
model, and when the save resulted in update.