Prime users in cache in WP_Query

[spacedmonkey]

In WP_Query, when a post is setup using setup_postdata / generate_postdata methods, the function get_userdata is called. If there are posts that are by multiple authors, this can result in a query per user / author.

The function cache_users can be called and caches primed for all users in a single request.

Trac ticket: https://core.trac.wordpress.org/ticket/55716

[spacedmonkey]

I have created a patch for this, including unit tests.

Author data for not need to be primed until we get to loop around posts using the the_post method. Add a new _prime_user_caches function to ensure that user meta is correctly primed.

[spacedmonkey]

In 53482:

Prime users cache in WP_Query and post REST API controller.

For a call to WP_Query or a post REST API request that contains posts from multiple authors, call the cache_users function, to ensure that all user data for post authors is primed in
a single database query. This results in far fewer database queries on multiple author sites.

Props spacedmonkey, timothyblynjacobs, peterwilsoncc.
Fixes #55716.

Committed

[SergeyBiryukov]

In 53483:

Query: Some documentation and test improvements for update_post_author_caches():

• Make the descriptions for update_post_author_caches() and update_post_caches() more specific.
• Move the unit test into its own file, for consistency with update_post_cache() tests. This also allows for using shared fixtures in case more tests are added in the future.

Follow-up to [53482].

See #55716.

[spacedmonkey]

Follow on ticket #56100

[desrosj]

There are a lot of caching related changes in 6.1 and a dev note should be written collecting them all. Marking this for inclusion.

[spacedmonkey]

Made a start of dev notes ​here.

[milana_cap]

Dev note: ​https://make.wordpress.org/core/2022/10/07/improvements-to-wp_query-performance-in-6-1/