node-postgres allows to connect using sockets in a connection string like this:
pg.connect('/some/path', callback)
That is nice, but does not allows to set another database but one with the same user name.

I propose to accept a special protocol named socket to enable the database selection like this:
pg.connect('socket:/some/path?database', callback)
The url.parse('socket:/some/path?database') returns:

{ protocol: 'socket:',
  host: '',
  query: 'database',
  pathname: '/some/path',
  ...}

That data is enough to make the magic, with a small change on function parse at connection-parameters.js. (and also my favorite solution)

That can not change the current behavior for sockets when the first char is "/".

I also try a change on current sockets behavior, splitting the string on spaces. That makes easy to set a database by this way:
pg.connect('/some/path database', callback)

pg/lib/connection-parameters.js
*** 14,20 ****
  var parse = function(str) {
    //unix socket
    if(str.charAt(0) === '/') {
-     return { host: str };
    }
--- 14,21 ----
  var parse = function(str) {
    //unix socket
    if(str.charAt(0) === '/') {
+     var config = str.split(' ');
+     return { host: config[0], database: config[1] };
    }

What you think? node-postgres can use the solution 1, 2, or a mix of both?

I can also code it, if you like.