fixes #1830 faster way to calculate number of orphans

* store count result in config table, as a cache
* resets this cache on invalidate_user_cache
* instead of a costly SQL query with a LEFT JOIN, use 2 simple COUNT in the 2 tables, and compare

admin/include/functions.php

@@ -2206,6 +2206,7 @@ UPDATE '.USER_CACHE_TABLE.'
   SET need_update = \'true\';';
     pwg_query($query);
   }
+  conf_delete_param('count_orphans');
   trigger_notify('invalidate_user_cache', $full);
 }
 
@@ -3269,6 +3270,33 @@ SELECT path
   return count($ids);
 }
 
+function count_orphans()
+{
+  if (is_null(conf_get_param('count_orphans')))
+  {
+    // we don't care about the list of image_ids, we only care about the number
+    // of orphans, so let's use a faster method than calling count(get_orphans())
+    $query = '
+SELECT
+    COUNT(*)
+  FROM '.IMAGES_TABLE.'
+;';
+    list($image_counter_all) = pwg_db_fetch_row(pwg_query($query));
+
+    $query = '
+SELECT
+    COUNT(DISTINCT(image_id))
+  FROM '.IMAGE_CATEGORY_TABLE.'
+;';
+    list($image_counter_in_categories) = pwg_db_fetch_row(pwg_query($query));
+
+    $counter = $image_counter_all - $image_counter_in_categories;
+    conf_update_param('count_orphans', $counter, true);
+  }
+
+  return conf_get_param('count_orphans');
+}
+
 /**
  * Return the list of image ids associated to no album
  *