I don't know if MySQL had the equivalent to this, but with PostgreSQL, you can connect to a unix domain socket instead of TCP/localhost. This has the advantage of being faster because it doesn't have the overhead of TCP.
It doesn't seem as if Drupal's connection scheme is capable of supporting this. Using raw pg_connect() in php, the connection would be established thusly:
pg_connect('user=drupal dbname=drupal host=/tmp');
Optionally, the host= could be omitted entirely, and the PG drivers will default to connecting to the local socket in /tmp.
if I tweak the following: $db_url = 'pgsql://drupal@/tmp/drupal';
It doesn't work. I assume the url decoding is barfing on that. Leaving out the host name causes a similar error.
Is this possible? I suppose it's not the end of the world if it's not, but I thought I'd ask.