If you have a ton of posts in your WordPress blog (we have over 35K in one site at work), it turns out that the Previous and Next links on each post may be running a tough query on your database.
I wanted to know why MySQL was using so much CPU and wrongly assumed it was due to a bad tuning effort (it usually is). I googled “SELECT p.ID FROM wp_posts AS p INNER JOIN wp_term_relationships AS tr ON p.ID = tr.object_id INNER JOIN wp_term_taxonomy tt ON tr.term_taxonomy_id = tt.term_taxonomy_id” which was in my output of MySQL’s
show full processlist command. It led me to this StackExchange page which showed an alternative, more efficient version of the WP function calls that produce those previous and next links.
In our case, we just didn’t need those links and our theme let us turn them off from the admin. An instant and dramatic drop in CPU by MySQL ensued.