Don't output the adjacent posts rel link by default

[johnbillion]

The query performed by get_adjacent_post() is an expensive one, and quickly becomes a performance bottleneck on sites with a large number of posts.

This function is called twice when viewing a single post in order to output the link rel='prev' and link rel='next' links, and is often one of the first things disabled by WordPress-specific hosts in order to increase site performance.

The usefulness of these tags is questionable. In addition, the links don't account for paginated posts (using <!--nextpage-->) and are therefore possibly incorrect in this situation.

/cc @joostdevalk for SEO review :-)

[joostdevalk]

+1 from me, it's actually a wrong use of rel=next and rel=prev that we correct in Yoast SEO.

[swissspidy]

@joostdevalk How would correct usage look like? Only use it for paginated posts?

[joostdevalk]

Replying to swissspidy:

@joostdevalk How would correct usage look like? Only use it for paginated posts?

Yes, and for archives.

[johnbillion]

I'm sure I saw an existing ticket for adding rel links to archives but I can't find it now.

[joostdevalk]

That would be #18672

[johnbillion]

@joostdevalk Do you want to get one of your folks to knock up a patch for this?

[johnbillion]

39111.diff​ removes the rel links but notably the number of database queries on the page doesn't get reduced if you use one of the default themes because they all include previous/next post links which uses the same query. If you use a theme which doesn't display previous/next post links on the page, then this provides the performance benefit as mentioned in the summary in addition to removing these tags which are unnecessary from an SEO perspective.

[johnbillion]

In 48966:

Posts, Post Types: Don't output the adjacent posts rel link by default.

This has a considerable performance impact on sites which don't otherwise show previous and next post links, and has no SEO benefit.

Props joostdevalk, swissspidy, johnbillion

Fixes #39111

[jnylen0]

Should the adjacent_posts_rel_link_wp_head function be marked as deprecated also? I don't see that it's used anywhere else.

Also just a note for others following this ticket, enabling memcached or another object caching solution will help this performance issue significantly, since the get_adjacent_post function uses wp_cache_set and wp_cache_get appropriately.

[johnbillion]

I don't think there's a need to deprecate the function. It still functions as expected, and plugins may wish to use it, it's just no longer used in core.

[SergeyBiryukov]

In 48995:

Docs: Add a @since note about adjacent_posts_rel_link_wp_head() no longer being used in core.

Follow-up to [48966].

Props jnylen0.
See #39111.