It looks like wordpress is not going to release a fix when they dropped post2cat table, starting from the 2.3 release. Or, I just haven’t found their fix on this. I also noticed this when the client I’m working for at Staff Off Shore decided to upgrade his blog site. What I did is modify that block of code and use the term_relationships instead to come up with this SQL and return similar results.

mysql > SELECT DISTINCT p.ID, p.post_title FROM blog_posts p
      > LEFT JOIN blog_term_relationships tr ON p.ID = tr.object_id
      > WHERE p.post_status = 'publish' AND p.ID <> 55
      > AND p.post_type = 'post' AND tr.term_taxonomy_id IN (4)
      > ORDER BY p.post_date DESC LIMIT 5;

I have uploaded a version of cattag_related_posts function and it needs to be patch to any existing theme functions file (functions.php) or the “category-tagging” plugin that still queries on “wp_post2cat” table. This is what I am using, example of which are the items under “Possibly Related” header below this post (not the google of course).

Use it in your template like so, though I think I disabled the $exclude_ids parameter for now, but it should work without it.

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cattag_related_posts(
	$limit = 5,		# number of posts to return
	$display_posts = true,	# swap values with display_page
	$display_pages = false,	# to get post or page
	$before = '<li>',	# html wrapper
	$after = '</li>',
	$notfound = false,
	$order = 'DATE_DESC',	# DATE_DESC or RANDOM
	$exclude_ids = ''
);

Download: cattag_related_posts.txt