W-Shadow.com

WordPress transients are very similar to DB options but they also support expiration times. The Transients API documentation states:

Our transient will die naturally of old age once $expiration seconds have passed since we last ran set_transient()

What you might not know if you haven’t explored the source code of the transients API is that transients stored in the database* will not actually “die of old age”. Instead, a stale transient will stick around until someone tries to access it. Only then will WordPress notice that the transient has expired and delete it. If no-one ever attempts to read a stale transient – e.g. because the plugin that created it has been uninstalled – it will stay in your database forever.

* If you have a suitable caching plugin installed, transients can be stored in a fast in-memory cache like APC or Memcached instead of the database.

Fortunately, since the expiration time of each transient is also stored in the database, we can easily clean up stale transients with a little bit of SQL magic. Run the two queries below to delete all stale transients. You can use either phpMyAdmin or a plugin like WP-DBManager to execute SQL queries.

DELETE a, b
FROM
	wp_options a, wp_options b

WHERE
 a.option_name LIKE "_transient_%" AND
 a.option_name NOT LIKE "_transient_timeout_%" AND
 b.option_name = CONCAT(
		"_transient_timeout_",
		SUBSTRING(
			a.option_name,
			CHAR_LENGTH("_transient_") + 1
		)
	)
 AND b.option_value < UNIX_TIMESTAMP()

DELETE a, b
FROM
	wp_options a, wp_options b

WHERE
 a.option_name LIKE "_site_transient_%" AND
 a.option_name NOT LIKE "_site_transient_timeout_%" AND
 b.option_name = CONCAT(
		"_site_transient_timeout_",
		SUBSTRING(
			a.option_name,
			CHAR_LENGTH("_site_transient_") + 1
		)
	)
 AND b.option_value < UNIX_TIMESTAMP()