Add caching to `count_many_users_posts()`

Trac ticket: https://core.trac.wordpress.org/ticket/63045

Add proper caching support to count_many_users_posts() function

Enhances the performance of count_many_users_posts() function with working cache key generation.

Trac ticket: https://core.trac.wordpress.org/ticket/63045

I'm not a fan of md5() the query statement so I pushed a new patch version in ​https://github.com/WordPress/wordpress-develop/pull/9106/ that:

• More transparent cache keys – avoids md5() and instead uses readable, parameter-based keys (user IDs, post type, $public_only), which makes it easier to debug and trace.
• Proper separation of cached data – different input combinations get unique keys, reducing the risk of stale or incorrect cache results.
• Better cache grouping – avoids using a static group, so it's safer and more aligned with how the query behaves.
• Easier to maintain or extend – the logic is clearer and more flexible for future changes.

Overall, it's a more accurate, debuggable, and scalable approach to caching in count_many_users_posts().

My questions:

1. ❓ Should we have the OC an expiration? If yes, how long? I have set to 1 hour but it's open for discussions.
2. ❓ We probably need cache clearing function, but on what actions it should hook into?

During the bi-weekly coffee-hours, this needs more discussion regarding the caching approach, either using readable cache key or using md5.

Props @flixos90

I strongly believe we should work on #59592 first, as that will effect this ticket's approach.

Add object caching to count_many_users_posts() to avoid repeated database queries, similar to count_user_posts().

I checked the PR ​https://github.com/WordPress/wordpress-develop/pull/9106/files and 63045-cache-count_many_users_posts.diff​

The issue with both approaches is different cache_key would be generated if order of array $users is different

$userlist    = implode( ',', array_map( 'absint', $users ) );
$cache_key   = "count_many_users_posts_{$post_type}_{$public_only}_" . str_replace( ',', '_', $userlist ) . '_' . get_current_user_id();

For above code,

$users = [ 1, 2, 3 ] → $userlist = "1,2,3" → _1_2_3_ in the cache key.
$users = [ 3, 2, 1 ] → $userlist = "3,2,1" → _3_2_1_ in the cache key.

To fix this, we should normalize the $users array before building the key:

• Ensure all IDs are integers.
• Remove duplicates.
• Sort the array to guarantee consistent order.

Here is the suggested fix for this,

$users      = array_map( 'absint', (array) $users );
$users      = array_filter( $users );   // remove 0/invalids
$users      = array_unique( $users );   // remove duplicates
sort( $users );                         // enforce consistent order
$userlist   = implode( '_', $users );
$cache_key  = "count_many_users_posts_{$post_type}_{$public_only}_{$userlist}_" . get_current_user_id();

Hey @kalpeshh, for your information, needs-refresh means, that the patch is not applying correctly. What I think you were trying to add is changes-requested. I'm adding this for you.

Hi @SirLouen yes my bad. Thanks for updating to changes-requested

Thanks for the feedback @kalpeshh! Updated ​https://github.com/WordPress/wordpress-develop/pull/9106.

I just noticed, however, that @spacedmonkey has also requested feedback on ​https://github.com/WordPress/wordpress-develop/pull/8444 , which I only saw after I have updated my branch.

By the way that PR has unit test(still need some polishing) so I' think we should use that instead of mine.

Keeping the existing keywords for ​https://github.com/WordPress/wordpress-develop/pull/8444

Update: @peterwilsoncc and @westonruter have reviewed PR ​https://github.com/WordPress/wordpress-develop/pull/9106, and I’ve applied their feedback. The changes include:

1. Switching to md5 for the cache key to handle Memcached’s 250-character limit
2. Replacing with wp_cache_get_salted()
3. Adding a dedicated unit test function specifically for this ticket

Marking the ticket as needs testing.