wp_list_authors() performance issue. Queries for user data on authors with no posts

[billerickson]

On a client site with a single author, 4 posts, and thousands of subscribers (a membership site), we were getting over 17,000 queries on a 404 page that included the wp_list_authors() function: ​https://cl.ly/aa7ed29b1815

To reproduce this issue:

1. Install Query Monitor + Debug Bar
2. Add wp_list_authors() to a page and note the number of queries
3. Generate new users (wp user generate) and notice the increase in queries

The key issue is the check for empty posts runs after get_userdata() is called, so a query for that user's metadata happens regardless of their post count.

Moving the post count check above get_userdata() fixes the issue.

[pento]

Moving to 5.2, as this isn't a trunk regression.

[billerickson]

This issue affects almost every Genesis-based website. The default Genesis 404 page builds a sitemap listing all posts, pages and authors: ​https://gist.github.com/billerickson/858ad34efdea9c75417b4217e33f8c3b#file-archive-php-L37

Any Genesis site with a sufficiently large registered user list (ie: membership sites) can be taken down by hitting random 404s since they’re uncached in most instances and trigger thousands of queries.

This patch is an easy win for performance.

[ianbelanger]

I tested the patch and it does limit the number queries as intended, however it was throwing 2 PHP notices for each user due to the fact that $author = get_userdata( $author_id ); was moved after $posts = isset( $author_count[ $author->ID ] ) ? $author_count[ $author->ID ] : 0; in the loop, so $author was not yet defined.

45105.1.diff changes $posts = isset( $author_count[ $author->ID ] ) ? $author_count[ $author->ID ] : 0; to $posts = isset( $author_count[ $author_id ] ) ? $author_count[ $author_id ] : 0; to avoid throwing the PHP notices, while still limiting the number of queries.

[dswebsme]

Patch 45105.1.diff tested and confirmed. This patch correctly addresses the reported issue without generating any PHP notices (as mentioned as a side effect of the original patch).

Ready for commit.

[SergeyBiryukov]

In 45235:

Users: In wp_list_authors(), check for author's post count before getting author's metadata.

This significantly reduces the number of SQL queries when wp_list_authors() is called on a site where the majority of users don't have any posts, e.g. a membership site.

Props billerickson, ianbelanger, dswebsme.
Fixes #45105.