Nearly 2 years ago I wrote a one liner for someone in the WordPress IRC channel to change the author permalink structure. At the time I had not taken the time to really understand WP_Rewrite and as such didn’t understand the implications of flushing the rewrite rules on each page load.

It is sufficient to say that since then, I have taken the time to understand it better and I am fully aware of the negative implications of performing flushes every time someone hits your site. For one thing the default behavior of flush_rules() is to also update the .htaccess file as well as updating the serialized array in your wp_options table that contain the internal WordPress rewrites. Assuming you are using a nasty permalink structure that starts with something like %category% or %postname% that serialized array can grow exponentially with the number of pages you have ¹.

To make a long story sort, I have known that I should have changed the code for almost as long as the post has been published, but I was too lazy to do anything about it. It took a few carefully placed pokes and prods to get me moving, and as such I have updated the post to reflect the removal of using flush_rules().