=== modified file 'includes/path.inc'
--- includes/path.inc	2009-05-26 09:12:28 +0000
+++ includes/path.inc	2009-05-31 00:29:03 +0000
@@ -66,8 +66,16 @@ function drupal_lookup_path($action, $pa
     $count = NULL;
     $system_paths = array();
     $no_aliases = array();
+
+    $whitelist = drupal_path_alias_whitelist_rebuild();
+    variable_set('path_alias_whitelist', $whitelist);
   }
   elseif ($count > 0 && $path != '') {
+    // Retrieve the path alias whitelist
+    $whitelist = variable_get('path_alias_whitelist', array());
+    // And derive the top level component of the path
+    $top_level = strtok($path, '/');
+
     if ($action == 'alias') {
       // During the first call to drupal_lookup_path() per language, load the
       // expected system paths for the page from cache.
@@ -93,6 +101,12 @@ function drupal_lookup_path($action, $pa
       if (isset($map[$path_language][$path])) {
         return $map[$path_language][$path];
       }
+      elseif (!isset($whitelist[$top_level])) {
+        // Check the path whitelist, if the top_level part before the first /
+        // is not in the list, then there is no need to do anything further,
+        // it is not in the database
+        return FALSE;
+      }
       // For system paths which were not cached, query aliases individually.
       else if (!isset($no_aliases[$path_language][$path])) {
         // Get the most fitting result falling back with alias without language
@@ -347,3 +361,23 @@ function drupal_match_path($path, $patte
 function current_path() {
   return $_GET['q'];
 }
+
+/**
+ * Rebuild the path alias white list.
+ *
+ * @return
+ *   An array containing a white list of path aliases.
+ */
+function drupal_path_alias_whitelist_rebuild() {
+  $whitelist = array();
+  $query = db_select('url_alias', 'u');
+  $query->addField('u', 'src');
+  $query->range(0, 1);
+  while ($result = $query->execute()->fetchField()) {
+    $part = strtok($result, '/');
+    $whitelist[$part] = TRUE;
+    $query->condition('src', "$part/%", 'NOT LIKE');
+    $query->condition('src', $part, '<>');
+  }
+  return $whitelist;
+}

=== modified file 'modules/path/path.test'
--- modules/path/path.test	2009-05-16 19:07:02 +0000
+++ modules/path/path.test	2009-05-31 00:45:40 +0000
@@ -235,3 +235,32 @@ class PathLanguageTestCase extends Drupa
   }
 }
 
+class PathWhiteListTestCase extends DrupalWebTestCase {
+  public static function getInfo() {
+    return array(
+      'name' => t('Path aliases whitelist'),
+      'description' => t('Confirm that the whitelist is built correctly.'),
+      'group' => t('Path'),
+    );
+  }
+
+  function setUp() {
+    parent::setUp('path');    
+  }
+  
+  function testWhiteList() {
+    // Lets create a bunch of aliases.
+    for ($i = 0; $i < 3; $i++) {
+      $prefix = $this->randomName();
+      $prefixes[] = $prefix;
+      path_set_alias($prefix, $this->randomName());
+      for ($j = 0; $j < 3; $j++) {
+        path_set_alias($prefix ."/$j", $this->randomName());
+      }
+    }
+    sort($prefixes);
+    $whitelist = array_keys(drupal_path_alias_whitelist_rebuild());
+    sort($whitelist);
+    $this->assertIdentical($prefixes, $whitelist, t('Whitelist functionality OK'));
+  }
+}
\ No newline at end of file