id,summary,reporter,owner,description,type,status,priority,milestone,component,version,severity,resolution,keywords,cc
22301,Performance problem with Recent Comments widget,pento,,"When a comment is posted (or the status of a comment changes), the `widget_recent_comments`cache item is invalidated, which the Recent Comments widget uses to populate the widget content. On the next widget display, it will call `get_comments()` to repopulate the cache.

The problem occurs when you have a very large number of comments, the MySQL query will use the `(comment_approved, comment_date_gmt)` index, but if MySQL has to scan too many rows in an index, it'll switch to table scan instead. As the `comment_approved` column is mostly the same value, this will almost always happen. This is compounded by the query occurring on every page load until the cache is re-populated - if the query takes 60 seconds to run, there could potentially be hundreds of instances of the same query running.

So, we need a solution that either hides or eliminates how slow this query can be, and only runs it (at most) once per new comment.

After discussing this with @matt, we have a couple of ideas:

1. Move this query to a `wp_schedule_single_event()` call, which has the bonus of ensuring only one is scheduled at any given time. The downside is that it may cause the cache to be outdated on a low traffic site.

2. Keep a queue of recent comments in cache, and push a new one onto the queue when posted. This avoids the query entirely, but there would be a race condition if two comments were posted at nearly the same time - one of them could be left out of the queue entirely.",defect (bug),new,normal,Awaiting Review,Performance,2.8,normal,,dev-feedback,scribu bronson@… aaroncampbell