Index: includes/path.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/path.inc,v
retrieving revision 1.37
diff -u -F^f -r1.37 path.inc
--- includes/path.inc	26 May 2009 09:12:28 -0000	1.37
+++ includes/path.inc	30 May 2009 17:54:44 -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 +361,32 @@ 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() {
+  // For each alias in the database, get the top level component. This is the portion of the 
+  // alias before the first /, if present, otherwise, the whole alias itself. 
+  $whitelist = array();
+  switch(db_driver()) {
+    case 'mysql':
+      // Use SUBSTRING_INDEX on MySQL
+      $sql = "SELECT SUBSTRING_INDEX(src, '/', 1) AS path FROM {url_alias} GROUP BY path";
+      break;
+    case 'pgsql':
+      // USe REGEX_REPLACE on PostgreSQL
+      $sql = "SELECT REGEX_REPLACE(src, '/.*$', '') AS path FROM {url_alias} GROUP BY path";
+      break;
+    case 'sqlite':
+      // TODO Need chx or someone's input here
+      break;
+  }
+  foreach (db_query($sql) as $row) {
+    $whitelist[$row->src] = TRUE;
+  }
+  return $whitelist;
+}