Unnecessary queries performed when updating a post without providing categories or tags

[johnbillion]

Given an existing post with at least some categories or tags present, updating the post via wp_update_post() (which calls wp_insert_post()) with data that does not include categories or tags, many unnecessary taxonomy-related database queries are performed. This slows down the saving significantly.

• If $postarr doesn't contain a post_category element, there's no point in calling wp_set_post_categories().
• If $postarr doesn't contain a tags_input element, there's no point in calling wp_set_post_tags()

Todo

• Need tests to verify the above is correct
• Need a list of queries that are performed before and after the change

To reproduce

1. Publish a post with at least one category
2. Update the post via:

   <?php
   wp_insert_post( [
     'ID' => $id,
     'post_content' => 'Hello, World!',
   ] );
   

3. Observe that a significant number of unnecessary taxonomy queries are performed

Trac ticket: https://core.trac.wordpress.org/ticket/59354

[LeonidasMilossis]

If $postarr doesn't contain a tags_input element, there's no point in calling wp_set_post_tags().

We already do that in wp_insert_post() and indeed there's no extra queries unless we add 'tags_input' => $tag_id, in the wp_insert_post() params.

So, that part can probably be removed from the scope of this ticket.

The submitted PR adds a unit test that attests to that.