The goal of this patch is to prevent cache storms. When serving cached pages, logic checks the timestamp of the cached page -- if older than the global timestamp we try to grab a lock. Only one user can grab the lock for a given page at a time. If the user fails to grab the lock, a stale version of the cached page is returned. Once the page is rebuilt, the new version is returned on subsequent requests.
The drupal_lookup_path function unnecessarily counts all rows of the url_alias table to determine if any aliases are defined. This can be expensive on a transactional database such as MySQL with the InnoDB Storage Engine or PostgreSQL when a website has a lot of aliases. This patch modifies the query to always only return one row, possible because any existing pid will be greater than 0. If no aliases are defined, it will return 0.