Don't update the post_thumbnail_cache if the thumbnail is already cached

[prettyboymp]

There is no reason to run the query in update_post_thumbnail_cache() if all of the thumbnail posts are already cached. Before adding a post_id to the thumb_ids array, cache should be checked.

[nacin]

Nice.

[prettyboymp]

The attachment also fixes a bug where the update_post_thumbnail_cache() actually doesn't help anything when running object cache since the wp_query param cache_results defaults to false if you're running object cache.

[duck_]

[19918] should do the trick.