Taxonomy: Unregistering taxonomy with default term endlessly regenerates options

[dlh]

[48480] added logic to unregister_taxonomy() to delete the default term option for the taxonomy if it had a default term. This logic causes the default term option for the taxonomy to be created and then deleted on every request: created when the taxonomy is registered, deleted when the taxonomy is unregistered, created again in the next request because it was deleted in the previous request, and so on.

Since taxonomies are registered at runtime and can't be unregistered unless they're already registered, the option will necessarily be created and deleted within the same request and again on every subsequent request, as opposed to a one-time operation like plugin deactivation.

I can sympathize with the desire to "clean up" the database as part of unregistering the taxonomy, but the cost to performance seems unnecessarily high in this case. There's also precedent: the {$taxonomy}_children option isn't deleted when the taxonomy is unregistered, either.

Accordingly, the linked Pull Request would remove the delete_option() call from unregister_taxonomy().

Trac ticket: https://core.trac.wordpress.org/ticket/54472

[audrasjb]

Moving this ticket to 6.0 as we're releasing WP 5.9 beta 1.

[costdev]

With 6.0 RC1 starting soon, I'm moving this to the 6.1 milestone.

[peterwilsoncc]

I've tested this with a ​mini plugin that registers and then unregisters a taxonomy with a default term.

Trunk

Each and every page load includes the following database queries:

INSERT INTO `wp_options` (`option_name`, `option_value`, `autoload`)
VALUES ('default_term_pwcc_test_taxonomy', '19', 'yes')
ON DUPLICATE KEY
UPDATE `option_name` = VALUES(`option_name`), `option_value` = VALUES(`option_value`), `autoload` = VALUES(`autoload`);

DELETE
FROM `wp_options`
WHERE `option_name` = 'default_term_pwcc_test_taxonomy';

With the patch applied

The FIRST request adds the default option to the database as the taxonomy is registered:

INSERT INTO `wp_options` (`option_name`, `option_value`, `autoload`)
VALUES ('default_term_pwcc_test_taxonomy', '19', 'yes')
ON DUPLICATE KEY
UPDATE `option_name` = VALUES(`option_name`), `option_value` = VALUES(`option_value`), `autoload` = VALUES(`autoload`);

The SECOND and subsequent requests make no changes to the database.

[peterwilsoncc]

In 53669:

Taxonomy: Retain default term option when unregistering taxos.

No longer delete the default term option in unregister_taxonomy() to improve database performance.

Since taxonomies are registered at runtime and can't be unregistered unless they're already registered, prior to this
change the option was created and deleted on each request.

Deleting the option should occur on a one-time opperation such as plugin deactivation.

Follow up to [48480].

Props dlh.
Fixes #54472.

Committed in 5b5c74b904757afb9fe0a03b0a37d2e8bd127db4 (canonically https://core.trac.wordpress.org/changeset/53669)