Index: trunk/core/install/upgrades.sql
===================================================================
diff -u -N -r8397 -r8555
--- trunk/core/install/upgrades.sql	(.../upgrades.sql)	(revision 8397)
+++ trunk/core/install/upgrades.sql	(.../upgrades.sql)	(revision 8555)
@@ -104,3 +104,11 @@
 );
 
 ALTER TABLE Events ADD ReplacementTags TEXT AFTER Event;
+
+
+# ====== v 4.2.0 ====
+ALTER TABLE Category ADD `TreeLeft` BIGINT NOT NULL AFTER `ParentPath` ,
+ADD `TreeRight` BIGINT NOT NULL AFTER `TreeLeft` ;
+
+ALTER TABLE Category ADD INDEX ( `TreeLeft` , `TreeRight` ) ;
+
Index: trunk/core/units/categories/cache_updater.php
===================================================================
diff -u -N -r7884 -r8555
--- trunk/core/units/categories/cache_updater.php	(.../cache_updater.php)	(revision 7884)
+++ trunk/core/units/categories/cache_updater.php	(.../cache_updater.php)	(revision 8555)
@@ -239,6 +239,9 @@
 			$data['item_template']		=	'';
 			$data['children_count'] = 0;
 			$data['system'] = 0;
+			$data['left'] = 0;
+			$data['right'] = 2;
+			$data['debug_title'] = 'ROOT';
 			$this->Stack->Push($data);
 		}
 
@@ -248,6 +251,7 @@
 			$data['children_count'] = count($data['children']);
 			$this->QueryPermissions($data);
 			$data['queried'] = 1;
+			$data['right'] = $data['left']+1;
 
 			if ($sql = $data['perms']->GetInsertSQL()) {
 				$this->Conn->Query($sql);
@@ -259,7 +263,7 @@
 		// start with first child if we haven't started yet
 		if (!isset($data['current_child'])) $data['current_child'] = 0;
 
-		// if we have more children
+		// if we have more children on CURRENT LEVEL
 		if (isset($data['children'][$data['current_child']])) {
 			if ($this->StrictPath) {
 				while ( isset($data['children'][ $data['current_child'] ]) && !in_array($data['children'][ $data['current_child'] ], $this->StrictPath) ) {
@@ -277,20 +281,24 @@
 			$next_data['current_id']		=	$data['children'][ $data['current_child'] ]; //next iteration should process child
 			$next_data['perms']				=	$data['perms']; //we should copy our permissions to child - inheritance
 			$next_data['perms']->SetCatId($next_data['current_id']);
+			$next_data['left'] = $data['right'];
 			$data['current_child']++;
 			$this->Stack->Update($data); //we need to update ourself for the iteration after the next (or further) return to next child
 			$this->Stack->Push($next_data); //next iteration should process this child
 			return true;
 		}
 		else {
-			$this->UpdateCachedPath($data);
+			$this->Stack->Update($data);
 			$prev_data = $this->Stack->Pop(); //remove ourself from stack if we have finished all the childs (or there are none)
+			$data['right'] = $prev_data['right'];
+			$this->UpdateCachedPath($data);
 			// we are getting here if we finished with current level, so check if it's first level - then bail out.
 
 			$this->doneCats++; // moved by Kostja from above, seems to fix the prob
 			$has_more = $this->Stack->Count() > 0;
 			if ($has_more) {
 				$next_data = $this->Stack->Get();
+				$next_data['right'] = $data['right']+1;
 				$next_data['children_count'] += $data['children_count'];
 				$this->Stack->Update($next_data);
 			}
@@ -305,6 +313,8 @@
 								'NamedParentPath'			=>	$data['system'] ? $data['file_name'] : implode('/', $data['named_path'] ),
 								'CachedCategoryTemplate'	=>	$data['category_template'],
 								'CachedDescendantCatsQty'	=>	$data['children_count'],
+								'TreeLeft' => $data['left'],
+								'TreeRight' => $data['right'],
 						);
 
 		$i = 1;
@@ -330,6 +340,7 @@
 				$data['titles'][$i][] = $record['l'.$i.'_Name'] ? $record['l'.$i.'_Name'] : $record['l'.$this->primaryLanguageId.'_Name'];
 				$i++;
 			}
+			$data['debug_title'] = $record['l1_Name'];
 
 			$data['parent_path'][] = $category_id;
 			$data['named_path'][] = preg_replace('/^Content\\//', '', $record['Filename']);