wp_dashboard_recent_comments executes an unnecessary SQL query

[kalessil]

The function fetches comments from the DB in a loop that breaks when the target number of comments is reached, or the query returns no results.

If the very first query returns, e.g., just a single comment, this logic causes an unnecessary DB query.

While it's not an issue for smaller instances, for WooCommerce stores with many orders/products, even a few comments, avoiding unnecessary DB queries (expensive in the mentioned setups) improves dashboard performance.

To reproduce, you'll need a testing site with fewer than 25 comments (e.g., just one) and the Query Monitor plugin (to explore DB queries): log in to the admin area, and after you get automatically redirected to the dashboard, note two queries fetching comments (with limit 0, 25, 25, 50).

I'll create a GitHub PR shortly - for reviews and better highlighting the technical side of the issue.

Tune wp_dashboard_recent_comments performance:

• further reduce the number of count calls (related to history of changes in this functions)
• harden while-loop condition to avoid SQLs when the last query fetched less that aimed (next query will fetch nothing)

Trac ticket: https://core.trac.wordpress.org/ticket/64506