Index: /trunk/admin/maintenance.php
===================================================================
--- /trunk/admin/maintenance.php	(revision 28431)
+++ /trunk/admin/maintenance.php	(revision 28432)
@@ -147,4 +147,5 @@
     $template->delete_compiled_templates();
     FileCombiner::clear_combined_files();
+    $persistent_cache->purge(true);
     break;
   }
Index: /trunk/admin/plugins_installed.php
===================================================================
--- /trunk/admin/plugins_installed.php	(revision 28431)
+++ /trunk/admin/plugins_installed.php	(revision 28432)
@@ -78,4 +78,5 @@
       {
         $template->delete_compiled_templates();
+        $persistent_cache->purge(true);
       }
       redirect($base_url);
Index: /trunk/include/cache.class.php
===================================================================
--- /trunk/include/cache.class.php	(revision 28432)
+++ /trunk/include/cache.class.php	(revision 28432)
@@ -0,0 +1,143 @@
+<?php
+// +-----------------------------------------------------------------------+
+// | Piwigo - a PHP based photo gallery                                    |
+// +-----------------------------------------------------------------------+
+// | Copyright(C) 2008-2014 Piwigo Team                  http://piwigo.org |
+// | Copyright(C) 2003-2008 PhpWebGallery Team    http://phpwebgallery.net |
+// | Copyright(C) 2002-2003 Pierrick LE GALL   http://le-gall.net/pierrick |
+// +-----------------------------------------------------------------------+
+// | This program is free software; you can redistribute it and/or modify  |
+// | it under the terms of the GNU General Public License as published by  |
+// | the Free Software Foundation                                          |
+// |                                                                       |
+// | This program is distributed in the hope that it will be useful, but   |
+// | WITHOUT ANY WARRANTY; without even the implied warranty of            |
+// | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU      |
+// | General Public License for more details.                              |
+// |                                                                       |
+// | You should have received a copy of the GNU General Public License     |
+// | along with this program; if not, write to the Free Software           |
+// | Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, |
+// | USA.                                                                  |
+// +-----------------------------------------------------------------------+
+
+/**
+  Provides a persistent cache mechanism across multiple page loads/sessions etc...
+*/
+abstract class PersistentCache
+{
+  var $default_lifetime = 86400;
+  protected $instance_key = PHPWG_VERSION;
+
+  /**
+  @return a key that can be safely be used with get/set methods
+  */
+  function make_key($key)
+  {
+    if ( is_array($key) )
+    {
+      $key = implode('&', $key);
+    }
+    $key .= $this->instance_key;
+    return md5($key);
+  }
+
+  /**
+  Searches for a key in the persistent cache and fills corresponding value.
+  @param string $key
+  @param out mixed $value
+  @return false if the $key is not found in cache ($value is not modified in this case)
+  */
+  abstract function get($key, &$value);
+
+  /**
+  Sets a key/value pair in the persistent cache.
+  @param string $key - it should be the return value of make_key function
+  @param mixed $value
+  @param int $lifetime
+  @return false on error
+  */
+  abstract function set($key, $value, $lifetime=null);
+
+  /**
+  Purge the persistent cache.
+  @param boolean $all - if false only expired items will be purged
+  */
+  abstract function purge($all);
+}
+
+
+/**
+  Implementation of a persistent cache using files.
+*/
+class PersistentFileCache extends PersistentCache
+{
+  private $dir;
+
+  function __construct()
+  {
+    global $conf;
+    $this->dir = PHPWG_ROOT_PATH.$conf['data_location'].'cache/';
+  }
+
+  function get($key, &$value)
+  {
+    $loaded = @file_get_contents($this->dir.$key.'.cache');
+    if ($loaded !== false && ($loaded=unserialize($loaded)) !== false)
+    {
+      if ($loaded['expire'] > time())
+      {
+        $value = $loaded['data'];
+        return true;
+      }
+    }
+    return false;
+  }
+
+  function set($key, $value, $lifetime=null)
+  {
+    if ($lifetime === null)
+    {
+      $lifetime = $this->default_lifetime;
+    }
+
+    if (rand() % 199 == 0)
+    {
+      $this->purge(false);
+    }
+
+    $serialized = serialize( array(
+        'expire' => time() + $lifetime,
+        'data' => $value
+      ));
+
+    if (false === @file_put_contents($this->dir.$key.'.cache', $serialized))
+    {
+      mkgetdir($this->dir, MKGETDIR_DEFAULT&~MKGETDIR_DIE_ON_ERROR);
+      if (false === @file_put_contents($this->dir.$key.'.cache', $serialized))
+      {
+        return false;
+      }
+    }
+    return true;
+  }
+
+  function purge($all)
+  {
+    $files = glob($this->dir.'*.cache');
+    if (empty($files))
+    {
+      return;
+    }
+
+    $limit = time() - $this->default_lifetime;
+    foreach ($files as $file)
+    {
+      if ($all || @filemtime($file) < $limit)
+        @unlink($file);
+    }
+  }
+
+}
+
+?>
Index: /trunk/include/common.inc.php
===================================================================
--- /trunk/include/common.inc.php	(revision 28431)
+++ /trunk/include/common.inc.php	(revision 28432)
@@ -104,4 +104,5 @@
 include(PHPWG_ROOT_PATH . 'include/functions.inc.php');
 
+$persistent_cache = new PersistentFileCache();
 
 // Database connection
Index: /trunk/include/functions.inc.php
===================================================================
--- /trunk/include/functions.inc.php	(revision 28431)
+++ /trunk/include/functions.inc.php	(revision 28432)
@@ -38,4 +38,5 @@
 include_once( PHPWG_ROOT_PATH .'include/derivative.inc.php');
 include_once( PHPWG_ROOT_PATH .'include/template.class.php');
+include_once( PHPWG_ROOT_PATH .'include/cache.class.php');
 
 
Index: /trunk/include/section_init.inc.php
===================================================================
--- /trunk/include/section_init.inc.php	(revision 28431)
+++ /trunk/include/section_init.inc.php	(revision 28432)
@@ -292,4 +292,5 @@
       else
       {
+        $cache_key = $persistent_cache->make_key('all_iids'.$user['id'].$user['cache_update_time'].$conf['order_by']);
         unset($page['is_homepage']);
         $where_sql = '1=1';
@@ -302,6 +303,8 @@
     }
 
-    // main query
-    $query = '
+    if ( !isset($cache_key) || !$persistent_cache->get($cache_key, $page['items']))
+    {
+      // main query
+      $query = '
 SELECT DISTINCT(image_id)
   FROM '.IMAGE_CATEGORY_TABLE.'
@@ -313,5 +316,9 @@
 ;';
 
-    $page['items'] = query2array($query,null, 'image_id');
+      $page['items'] = query2array($query,null, 'image_id');
+      
+      if ( isset($cache_key) )
+        $persistent_cache->set($cache_key, $page['items']);
+    }
   }
 }
