diff --git a/src/rdb/rdb/rdb.cc b/src/rdb/rdb/rdb.cc
index 64f8192f4..8e624f78c 100644
--- a/src/rdb/rdb/rdb.cc
+++ b/src/rdb/rdb/rdb.cc
@@ -417,18 +417,18 @@ ValueBase::create_from_shape (const db::Shape &shape, const db::CplxTrans &trans
 //  ValueWrapper implementation
 
 std::string 
-ValueWrapper::to_string (const Database *rdb) const
+ValueWrapper::to_string () const
 {
   std::string r;
   r.reserve (200);
 
-  if (tag_id () > 0 && rdb) {
+  if (tag_id () > 0) {
     r += "[";
-    const Tag &tag = rdb->tags ().tag (tag_id ());
-    if (tag.is_user_tag ()) {
+    auto np = rdb::Tags::name_for_id (tag_id ());
+    if (np.second) {
       r += "#";
     }
-    r += tl::to_word_or_quoted_string (tag.name ());
+    r += tl::to_word_or_quoted_string (np.first);
     r += "] ";
   }
 
@@ -438,14 +438,14 @@ ValueWrapper::to_string (const Database *rdb) const
 }
 
 void 
-ValueWrapper::from_string (Database *rdb, const std::string &s)
+ValueWrapper::from_string (const std::string &s)
 {
   tl::Extractor ex (s.c_str ());
-  from_string (rdb, ex);
+  from_string (ex);
 }
 
 void 
-ValueWrapper::from_string (Database *rdb, tl::Extractor &ex)
+ValueWrapper::from_string (tl::Extractor &ex)
 {
   id_type tag_id = 0;
 
@@ -456,7 +456,7 @@ ValueWrapper::from_string (Database *rdb, tl::Extractor &ex)
     std::string tn;  
     ex.read_word_or_quoted (tn);
 
-    tag_id = rdb->tags ().tag (tn, user_tag).id ();
+    tag_id = rdb::Tags::id_for_name (tn, user_tag);
 
     ex.test ("]");
 
@@ -477,16 +477,16 @@ Values::operator= (const Values &d)
 }
 
 bool
-Values::compare (const Values &other, const std::map<id_type, id_type> &tag_map, const std::map<id_type, id_type> &rev_tag_map) const
+Values::compare (const Values &other, const std::set<id_type> &common_tags) const
 {
   Values::const_iterator a = begin (), b = other.begin ();
   while (a != end () && b != other.end ()) {
 
     id_type t12 = 0;
     while (a != end () && a->tag_id () != 0) {
-      auto j = tag_map.find (a->tag_id ());
-      if (j != tag_map.end ()) {
-        t12 = j->second;
+      auto j = common_tags.find (a->tag_id ());
+      if (j != common_tags.end ()) {
+        t12 = a->tag_id ();
         break;
       }
       ++a;
@@ -494,9 +494,9 @@ Values::compare (const Values &other, const std::map<id_type, id_type> &tag_map,
 
     id_type t2 = 0;
     while (b != other.end () && b->tag_id () != 0) {
-      auto j = rev_tag_map.find (b->tag_id ());
-      if (j != rev_tag_map.end ()) {
-        t2 = j->first;
+      auto j = common_tags.find (b->tag_id ());
+      if (j != common_tags.end ()) {
+        t2 = b->tag_id ();
         break;
       }
       ++b;
@@ -529,7 +529,7 @@ Values::compare (const Values &other, const std::map<id_type, id_type> &tag_map,
 }
 
 std::string 
-Values::to_string (const Database *rdb) const
+Values::to_string () const
 {
   std::string r;
   r.reserve (200);
@@ -540,7 +540,7 @@ Values::to_string (const Database *rdb) const
       r += ";";
     }
 
-    r += v->to_string (rdb);
+    r += v->to_string ();
 
   }
 
@@ -548,14 +548,14 @@ Values::to_string (const Database *rdb) const
 }
 
 void 
-Values::from_string (Database *rdb, const std::string &s) 
+Values::from_string (const std::string &s)
 {
   tl::Extractor ex (s.c_str ());
 
   while (! ex.at_end ()) {
 
     ValueWrapper v;
-    v.from_string (rdb, ex);
+    v.from_string (ex);
 
     add (v);
 
@@ -905,6 +905,41 @@ Categories::import_category (Category *category)
 // ------------------------------------------------------------------------------------------
 //  Tags implementation
 
+static tl::Mutex s_tag_namespace_mutex;
+static std::map<std::pair<std::string, bool>, id_type> s_tag_namespace;
+static std::vector<std::pair<std::string, bool> > s_tags;
+
+const std::pair<std::string, bool> &
+Tags::name_for_id (id_type tag_id)
+{
+  tl::MutexLocker locker (&s_tag_namespace_mutex);
+  tl_assert (tag_id > 0 && tag_id <= s_tags.size ());
+  return s_tags [tag_id - 1];
+}
+
+id_type
+Tags::id_for_name (const std::string &name, bool user_flag)
+{
+  tl::MutexLocker locker (&s_tag_namespace_mutex);
+
+  auto k = std::make_pair (name, user_flag);
+  auto t = s_tag_namespace.find (k);
+  if (t != s_tag_namespace.end ()) {
+    return t->second;
+  }
+
+  id_type id = s_tags.size () + 1;
+  s_tag_namespace [k] = id;
+  s_tags.push_back (k);
+  return id;
+}
+
+Tag
+Tags::tag_for_name (const std::string &name, bool user_flag)
+{
+  return Tag (id_for_name (name, user_flag), name, user_flag);
+}
+
 Tags::Tags ()
 {
   //  .. nothing yet ..
@@ -913,7 +948,7 @@ Tags::Tags ()
 void 
 Tags::clear ()
 {
-  m_ids_for_names.clear ();
+  m_tags_per_id.clear ();
   m_tags.clear ();
 }
 
@@ -926,26 +961,31 @@ Tags::tag (const std::string &name, bool user_tag) const
 Tag &
 Tags::tag (const std::string &name, bool user_tag)
 {
-  std::map <std::pair<std::string, bool>, id_type>::const_iterator i = m_ids_for_names.find (std::make_pair (name, user_tag));
-  if (i == m_ids_for_names.end ()) {
-    i = m_ids_for_names.insert (std::make_pair (std::make_pair (name, user_tag), m_tags.size () + 1)).first;
-    m_tags.push_back (Tag (i->second, name, user_tag));
+  id_type id = id_for_name (name, user_tag);
+  auto i = m_tags_per_id.find (id);
+  if (i == m_tags_per_id.end ()) {
+    m_tags.push_back (Tag (id, name, user_tag));
+    m_tags_per_id [id] = m_tags.size () - 1;
+    return m_tags.back ();
+  } else {
+    return m_tags [i->second];
   }
-  return m_tags [i->second - 1];
 }
 
 const Tag &
 Tags::tag (id_type id) const
 {
-  tl_assert (id < m_tags.size () + 1 && id > 0);
-  return m_tags [id - 1];
+  auto i = m_tags_per_id.find (id);
+  tl_assert (i != m_tags_per_id.end ());
+  return m_tags [i->second];
 }
 
 Tag &
 Tags::tag (id_type id)
 {
-  tl_assert (id < m_tags.size () + 1 && id > 0);
-  return m_tags [id - 1];
+  auto i = m_tags_per_id.find (id);
+  tl_assert (i != m_tags_per_id.end ());
+  return m_tags [i->second];
 }
 
 void
@@ -958,7 +998,7 @@ Tags::import_tag (const Tag &t)
 bool 
 Tags::has_tag (const std::string &name, bool user_tag) const
 {
-  return m_ids_for_names.find (std::make_pair (name, user_tag)) != m_ids_for_names.end ();
+  return m_tags_per_id.find (id_for_name (name, user_tag)) != m_tags_per_id.end ();
 }
 
 // ------------------------------------------------------------------------------------------
@@ -1006,30 +1046,25 @@ Item &Item::operator= (const Item &d)
 void 
 Item::add_tag (id_type tag_id)
 {
-  if (m_tag_ids.size () <= tag_id) {
-    m_tag_ids.resize (tag_id + 1, false);
-  }
-  m_tag_ids [tag_id] = true;
+  m_tag_ids.insert (tag_id);
 }
 
 void 
 Item::remove_tag (id_type tag_id)
 {
-  if (m_tag_ids.size () > tag_id) {
-    m_tag_ids [tag_id] = false;
-  }
+  m_tag_ids.erase (tag_id);
 }
 
 void 
 Item::remove_tags ()
 {
-  m_tag_ids = std::vector <bool> ();
+  m_tag_ids.clear ();
 }
 
 bool 
 Item::has_tag (id_type tag_id) const
 {
-  return m_tag_ids.size () > tag_id && m_tag_ids [tag_id];
+  return m_tag_ids.find (tag_id) != m_tag_ids.end ();
 }
 
 std::string 
@@ -1082,13 +1117,12 @@ Item::tag_str () const
 
   if (! m_tag_ids.empty ()) {
 
-    id_type tag_id = 0;
-    for (std::vector<bool>::const_iterator t = m_tag_ids.begin (); t != m_tag_ids.end (); ++t, ++tag_id) {
+    for (std::set<id_type>::const_iterator t = m_tag_ids.begin (); t != m_tag_ids.end (); ++t) {
       if (*t) {
         if (! r.empty ()) {
           r += ",";
         }
-        const Tag &tag = mp_database->tags ().tag (tag_id);
+        const Tag &tag = mp_database->tags ().tag (*t);
         if (tag.is_user_tag ()) {
           r += "#";
         }
@@ -1801,33 +1835,30 @@ namespace
   class ValueMapEntryCompare
   {
   public:
-    ValueMapEntryCompare (const std::map<id_type, id_type> &tag2tag, const std::map<id_type, id_type> &rev_tag2tag)
+    ValueMapEntryCompare (const std::set<id_type> &common_tags)
     {
-      mp_tag2tag = &tag2tag;
-      mp_rev_tag2tag = &rev_tag2tag;
+      mp_common_tags = &common_tags;
     }
 
     bool operator() (const Item *a, const Item *b) const
     {
-      return a->values ().compare (b->values (), *mp_tag2tag, *mp_rev_tag2tag);
+      return a->values ().compare (b->values (), *mp_common_tags);
     }
 
   private:
-    const std::map<id_type, id_type> *mp_tag2tag;
-    const std::map<id_type, id_type> *mp_rev_tag2tag;
+    const std::set<id_type> *mp_common_tags;
   };
 
   class ValueMapEntry
   {
   public:
     ValueMapEntry ()
-      : mp_tag2tag (0), mp_rev_tag2tag (0)
+      : mp_common_tags (0)
     { }
 
-    void build (const rdb::Database &rdb, id_type cell_id, id_type cat_id, const std::map<id_type, id_type> &tag2tag, const std::map<id_type, id_type> &rev_tag2tag)
+    void build (const rdb::Database &rdb, id_type cell_id, id_type cat_id, const std::set<id_type> &common_tags)
     {
-      mp_tag2tag = &tag2tag;
-      mp_rev_tag2tag = &rev_tag2tag;
+      mp_common_tags = &common_tags;
 
       auto i2i = rdb.items_by_cell_and_category (cell_id, cat_id);
 
@@ -1841,13 +1872,13 @@ namespace
         m_items.push_back ((*i).operator-> ());
       }
 
-      ValueMapEntryCompare cmp (*mp_tag2tag, *mp_rev_tag2tag);
+      ValueMapEntryCompare cmp (*mp_common_tags);
       std::sort (m_items.begin (), m_items.end (), cmp);
     }
 
     const Item *find (const rdb::Item &item) const
     {
-      ValueMapEntryCompare cmp (*mp_tag2tag, *mp_rev_tag2tag);
+      ValueMapEntryCompare cmp (*mp_common_tags);
 
       auto i = std::lower_bound (m_items.begin (), m_items.end (), &item, cmp);
       if (i == m_items.end ()) {
@@ -1863,8 +1894,7 @@ namespace
 
   public:
     std::vector<const Item *> m_items;
-    const std::map<id_type, id_type> *mp_tag2tag;
-    const std::map<id_type, id_type> *mp_rev_tag2tag;
+    const std::set<id_type> *mp_common_tags;
   };
 }
 
@@ -1901,16 +1931,15 @@ Database::apply (const rdb::Database &other)
     map_category (*c, *this, cat2cat);
   }
 
-  std::map<std::string, id_type> tags_by_name;
+  std::set<id_type> t1;
   for (auto c = tags ().begin_tags (); c != tags ().end_tags (); ++c) {
-    tags_by_name.insert (std::make_pair (c->name (), c->id ()));
+    t1.insert (c->id ());
   }
 
+  std::set<id_type> common_tags;
   for (auto c = other.tags ().begin_tags (); c != other.tags ().end_tags (); ++c) {
-    auto t = tags_by_name.find (c->name ());
-    if (t != tags_by_name.end ()) {
-      tag2tag.insert (std::make_pair (t->second, c->id ()));
-      rev_tag2tag.insert (std::make_pair (c->id (), t->second));
+    if (t1.find (c->id ()) != t1.end ()) {
+      common_tags.insert (c->id ());
     }
   }
 
@@ -1932,7 +1961,7 @@ Database::apply (const rdb::Database &other)
     auto vmap = value_map.find (std::make_pair (icell->second, icat->second));
     if (vmap == value_map.end ()) {
       vmap = value_map.insert (std::make_pair (std::make_pair (icell->second, icat->second), ValueMapEntry ())).first;
-      vmap->second.build (other, icell->second, icat->second, tag2tag, rev_tag2tag);
+      vmap->second.build (other, icell->second, icat->second, common_tags);
     }
 
     //  find a value in the reference DB
diff --git a/src/rdb/rdb/rdb.h b/src/rdb/rdb/rdb.h
index 808af53ba..7f0e2e339 100644
--- a/src/rdb/rdb/rdb.h
+++ b/src/rdb/rdb/rdb.h
@@ -629,17 +629,17 @@ public:
   /**
    *  @brief Convert the values collection to a string 
    */
-  std::string to_string (const Database *rdb = 0) const;
+  std::string to_string () const;
 
   /**
    *  @brief Fill the values collection from the string
    */
-  void from_string (Database *rdb, const std::string &s);  
+  void from_string (const std::string &s);
 
   /**
    *  @brief Fill the values collection from an extractor
    */
-  void from_string (Database *rdb, tl::Extractor &ex);  
+  void from_string (tl::Extractor &ex);
 
 private:
   ValueBase *mp_ptr;
@@ -687,7 +687,7 @@ public:
    *
    *  The order of the values matters.
    */
-  bool compare (const Values &other, const std::map<id_type, id_type> &tag_map, const std::map<id_type, id_type> &rev_tag_map) const;
+  bool compare (const Values &other, const std::set<id_type> &common_tags) const;
 
   /**
    *  @brief The const iterator (begin)
@@ -760,12 +760,12 @@ public:
   /**
    *  @brief Convert the values collection to a string 
    */
-  std::string to_string (const Database *rdb) const;
+  std::string to_string () const;
 
   /**
    *  @brief Fill the values collection from the string
    */
-  void from_string (Database *rdb, const std::string &s);  
+  void from_string (const std::string &s);
 
 private:
   std::list <ValueWrapper> m_values;
@@ -1057,7 +1057,7 @@ private:
   size_t m_multiplicity;
   std::string m_comment;
   bool m_visited;
-  std::vector <bool> m_tag_ids;
+  std::set <id_type> m_tag_ids;
   Database *mp_database;
   std::string m_image_str;
 
@@ -1982,11 +1982,32 @@ public:
    */
   void clear ();
 
+  /**
+   *  @brief Gets the name and user flag for a tag ID
+   *
+   *  This method will assert if the tag ID is not valid in the global namespace.
+   */
+  static const std::pair<std::string, bool> &name_for_id (id_type tag_id);
+
+  /**
+   *  @brief Gets the id for a given name and user flag
+   *
+   *  This will pull the ID from the global namespace.
+   */
+  static id_type id_for_name (const std::string &name, bool user_flag);
+
+  /**
+   *  @brief Gets a tag object for a given name and user flag
+   *
+   *  This will create the tag from the global namespace.
+   */
+  static Tag tag_for_name (const std::string &name, bool user_flag);
+
 private:
   friend class Database;
 
-  mutable std::map <std::pair<std::string, bool>, id_type> m_ids_for_names;
-  mutable std::vector <Tag> m_tags;
+  mutable std::map<id_type, size_t> m_tags_per_id;
+  mutable std::vector<Tag> m_tags;
 };
 
 /**
diff --git a/src/rdb/rdb/rdbFile.cc b/src/rdb/rdb/rdbFile.cc
index dc533bf7f..1544ffcaf 100644
--- a/src/rdb/rdb/rdbFile.cc
+++ b/src/rdb/rdb/rdbFile.cc
@@ -40,51 +40,47 @@ struct ValueConverter
 {
   typedef std::string pb_type;
 
-  ValueConverter (rdb::Database *rdb)
-    : mp_rdb (rdb)
+  ValueConverter ()
   {
   }
 
   std::string to_string (const ValueWrapper &value) const
   {
-    return value.to_string (mp_rdb);
+    return value.to_string ();
   }
 
   void from_string (const std::string &s, ValueWrapper &value) const
   {
-    value.from_string (mp_rdb, s);
+    value.from_string (s);
   }
 
   std::string pb_encode (const ValueWrapper &value) const
   {
-    return value.to_string (mp_rdb);
+    return value.to_string ();
   }
 
   void pb_decode (const std::string &s, ValueWrapper &value) const
   {
-    value.from_string (mp_rdb, s);
+    value.from_string (s);
   }
-
-private:
-  rdb::Database *mp_rdb;
 };
 
-static 
-tl::XMLElementList categories_format = 
-  tl::make_element_with_parent_ref<rdb::Category, rdb::Categories::const_iterator, rdb::Categories> (&rdb::Categories::begin, &rdb::Categories::end, &rdb::Categories::import_category, "category", 
-    tl::make_member<std::string, rdb::Category> (&rdb::Category::name, &rdb::Category::set_name, "name") + 
-    tl::make_member<std::string, rdb::Category> (&rdb::Category::description, &rdb::Category::set_description, "description") +
-    tl::make_element_with_parent_ref<rdb::Categories, rdb::Category> (&rdb::Category::sub_categories, &rdb::Category::import_sub_categories, "categories",
-      &categories_format
-    ) 
-  ) 
-;
-
 //  generation of the RDB file XML structure
 static tl::XMLStruct <rdb::Database> 
-make_rdb_structure (rdb::Database *rdb)
+make_rdb_structure ()
 {
-  return tl::XMLStruct <rdb::Database>("report-database", 
+  static
+  tl::XMLElementList categories_format =
+    tl::make_element_with_parent_ref<rdb::Category, rdb::Categories::const_iterator, rdb::Categories> (&rdb::Categories::begin, &rdb::Categories::end, &rdb::Categories::import_category, "category",
+      tl::make_member<std::string, rdb::Category> (&rdb::Category::name, &rdb::Category::set_name, "name") +
+      tl::make_member<std::string, rdb::Category> (&rdb::Category::description, &rdb::Category::set_description, "description") +
+      tl::make_element_with_parent_ref<rdb::Categories, rdb::Category> (&rdb::Category::sub_categories, &rdb::Category::import_sub_categories, "categories",
+        &categories_format
+      )
+    )
+  ;
+
+  return tl::XMLStruct <rdb::Database>("report-database",
     tl::make_member<std::string, rdb::Database> (&rdb::Database::description, &rdb::Database::set_description, "description") +
     tl::make_member<std::string, rdb::Database> (&rdb::Database::original_file, &rdb::Database::set_original_file, "original-file") +
     tl::make_member<std::string, rdb::Database> (&rdb::Database::generator, &rdb::Database::set_generator, "generator") +
@@ -122,13 +118,15 @@ make_rdb_structure (rdb::Database *rdb)
         tl::make_member<std::string, rdb::Item> (&rdb::Item::comment, &rdb::Item::set_comment, "comment") +
         tl::make_member<std::string, rdb::Item> (&rdb::Item::image_str, &rdb::Item::set_image_str, "image") +
         tl::make_element<rdb::Values, rdb::Item> (&rdb::Item::values, &rdb::Item::set_values, "values", 
-          tl::make_member<rdb::ValueWrapper, rdb::Values::const_iterator, rdb::Values> (&rdb::Values::begin, &rdb::Values::end, &rdb::Values::add, "value", ValueConverter (rdb))
+          tl::make_member<rdb::ValueWrapper, rdb::Values::const_iterator, rdb::Values> (&rdb::Values::begin, &rdb::Values::end, &rdb::Values::add, "value", ValueConverter ())
         )
       )
     )
   );
 }
 
+static tl::XMLStruct <rdb::Database> s_rdb_struct = make_rdb_structure ();
+
 // -------------------------------------------------------------
 //  Implementation of rdb::Database::save and write
 //  TODO: move this somewhere else - with generalized functionality
@@ -144,7 +142,7 @@ void
 rdb::Database::write (const std::string &fn)
 {
   tl::OutputStream os (fn, tl::OutputStream::OM_Auto);
-  make_rdb_structure (this).write (os, *this);
+  s_rdb_struct.write (os, *this);
 
   if (tl::verbosity () >= 10) {
     tl::log << "Saved RDB to " << fn;
@@ -168,7 +166,7 @@ public:
   {
     tl::SelfTimer timer (tl::verbosity () >= 11, "Reading marker database file");
     tl::XMLStreamSource in (m_input_stream, tl::to_string (tr ("Reading RDB")));
-    make_rdb_structure (&db).parse (in, db); 
+    s_rdb_struct.parse (in, db);
   }
 
   virtual const char *format () const 
diff --git a/testdata/ruby/rdbTest.rb b/testdata/ruby/rdbTest.rb
index eb99bd859..26d37f38a 100644
--- a/testdata/ruby/rdbTest.rb
+++ b/testdata/ruby/rdbTest.rb
@@ -595,7 +595,7 @@ class RDB_TestClass < TestBase
     assert_equal(db.is_modified?, false)
 
     tag_id = db.tag_id("x")
-    assert_equal(tag_id, 1)
+    assert_equal(db.tag_name(tag_id), "x")
     db.set_tag_description(tag_id, "xdesc")
     assert_equal(db.tag_description(tag_id), "xdesc")