Improve performance of aggregate operations on empty dictionaries

CHANGELOG.md

@@ -3,6 +3,7 @@
 ### Enhancements
 * Add support to synchronize collections embedded in Mixed properties and other collections (except sets) ([PR #7353](https://github.com/realm/realm-core/pull/7353)).
 * Improve performance of change notifications on nested collections somewhat ([PR #7402](https://github.com/realm/realm-core/pull/7402)).
+* Improve performance of aggregate operations on Dictionaries of objects, particularly when the dictionaries are empty ([PR #7418](https://github.com/realm/realm-core/pull/7418))
 
 ### Fixed
 * Fixed conflict resolution bug which may result in an crash when the AddInteger instruction on Mixed properties is merged against updates to a non-integer type ([PR #7353](https://github.com/realm/realm-core/pull/7353)).

src/realm/collection.hpp

@@ -175,6 +175,12 @@ class CollectionBase : public Collection {
         return PathElement(ndx);
     }
 
+    // Clone this collection if it contains objects, and return nullptr otherwise
+    virtual LinkCollectionPtr clone_as_obj_list() const
+    {
+        return nullptr;
+    }
+
     /// Return true if the collection has changed since the last call to
     /// `has_changed()`. Note that this function is not idempotent and updates
     /// the internal state of the accessor if it has changed.
@@ -202,7 +208,7 @@ class CollectionBase : public Collection {
     }
 
     /// If this is a collection of links, get the target table.
-    virtual TableRef get_target_table() const final
+    TableRef get_target_table() const
     {
         return get_obj().get_target_table(get_col_key());
     }
@@ -815,24 +821,12 @@ void update_unresolved(std::vector<size_t>& vec, const BPlusTree<ObjKey>* tree);
 
 /// Clear the context flag on the tree if there are no more unresolved links.
 void check_for_last_unresolved(BPlusTree<ObjKey>* tree);
-
-/// Proxy class needed because the ObjList interface clobbers method names from
-/// CollectionBase.
-struct ObjListProxy : ObjList {
-    virtual TableRef proxy_get_target_table() const = 0;
-
-    TableRef get_target_table() const final
-    {
-        return proxy_get_target_table();
-    }
-};
-
 } // namespace _impl
 
 /// Base class for collections of objects, where unresolved links (tombstones)
 /// can occur.
 template <class Interface>
-class ObjCollectionBase : public Interface, public _impl::ObjListProxy {
+class ObjCollectionBase : public Interface, public ObjList {
 public:
     static_assert(std::is_base_of_v<CollectionBase, Interface>);
 
@@ -868,7 +862,15 @@ class ObjCollectionBase : public Interface, public _impl::ObjListProxy {
         return m_unresolved.size() != 0;
     }
 
-    using Interface::get_target_table;
+    LinkCollectionPtr clone_as_obj_list() const final
+    {
+        return clone_obj_list();
+    }
+
+    TableRef get_target_table() const final
+    {
+        return Interface::get_target_table();
+    }
 
 protected:
     ObjCollectionBase() = default;
@@ -956,10 +958,6 @@ class ObjCollectionBase : public Interface, public _impl::ObjListProxy {
     // Sorted set of indices containing unresolved links.
     mutable std::vector<size_t> m_unresolved;
 
-    TableRef proxy_get_target_table() const final
-    {
-        return Interface::get_target_table();
-    }
     bool matches(const ObjList& other) const final
     {
         return get_owning_obj().get_key() == other.get_owning_obj().get_key() &&

src/realm/dictionary.cpp

@@ -1190,6 +1190,14 @@ void Dictionary::set_collection_ref(Index index, ref_type ref, CollectionType ty
     m_values->set(ndx, Mixed(ref, type));
 }
 
+LinkCollectionPtr Dictionary::clone_as_obj_list() const
+{
+    if (get_value_data_type() == type_Link) {
+        return std::make_unique<DictionaryLinkValues>(*this);
+    }
+    return nullptr;
+}
+
 /************************* DictionaryLinkValues *************************/
 
 DictionaryLinkValues::DictionaryLinkValues(const Obj& obj, ColKey col_key)

src/realm/dictionary.hpp

@@ -226,6 +226,8 @@ class Dictionary final : public CollectionBaseImpl<DictionaryBase>, public Colle
 
     void to_json(std::ostream&, JSONOutputMode, util::FunctionRef<void(const Mixed&)>) const override;
 
+    LinkCollectionPtr clone_as_obj_list() const final;
+
 private:
     using Base::set_collection;
 

src/realm/object-store/results.cpp

@@ -810,13 +810,8 @@ Query Results::do_get_query() const
             return Query(m_table, std::make_unique<TableView>(m_table_view));
         }
         case Mode::Collection:
-            if (auto list = dynamic_cast<ObjList*>(m_collection.get())) {
-                return m_table->where(*list);
-            }
-            if (auto dict = dynamic_cast<Dictionary*>(m_collection.get())) {
-                if (dict->get_value_data_type() == type_Link) {
-                    return m_table->where(*dict);
-                }
+            if (auto objlist = m_collection->clone_as_obj_list()) {
+                return m_table->where(std::move(objlist));
             }
             return m_query;
         case Mode::Table:

src/realm/table.cpp

@@ -1082,9 +1082,9 @@ void Table::do_erase_root_column(ColKey col_key)
     bump_storage_version();
 }
 
-Query Table::where(const DictionaryLinkValues& dictionary_of_links) const
+Query Table::where(const Dictionary& dict) const
 {
-    return Query(m_own_ref, dictionary_of_links);
+    return Query(m_own_ref, dict.clone_as_obj_list());
 }
 
 void Table::set_table_type(Type table_type, bool handle_backlinks)

src/realm/table.hpp

@@ -591,7 +591,11 @@ class Table {
     {
         return Query(m_own_ref, list);
     }
-    Query where(const DictionaryLinkValues& dictionary_of_links) const;
+    Query where(const Dictionary& dict) const;
+    Query where(LinkCollectionPtr&& list) const
+    {
+        return Query(m_own_ref, std::move(list));
+    }
 
     Query query(const std::string& query_string,
                 const std::vector<mpark::variant<Mixed, std::vector<Mixed>>>& arguments = {}) const;