Implement totalRows() for StorageMemory

dbms/src/Storages/StorageMemory.cpp

@@ -136,6 +136,15 @@ void StorageMemory::truncate(const ASTPtr &, const Context &, TableStructureWrit
     data.clear();
 }
 
+std::optional<UInt64> StorageMemory::totalRows() const
+{
+    UInt64 rows = 0;
+    std::lock_guard lock(mutex);
+    for (auto & buffer : data)
+        rows += buffer.rows();
+    return rows;
+}
+
 
 void registerStorageMemory(StorageFactory & factory)
 {

dbms/src/Storages/StorageMemory.h

@@ -42,11 +42,13 @@ public:
 
     void truncate(const ASTPtr &, const Context &, TableStructureWriteLockHolder &) override;
 
+    std::optional<UInt64> totalRows() const override;
+
 private:
     /// The data itself. `list` - so that when inserted to the end, the existing iterators are not invalidated.
     BlocksList data;
 
-    std::mutex mutex;
+    mutable std::mutex mutex;
 
 protected:
     StorageMemory(const StorageID & table_id_, ColumnsDescription columns_description_, ConstraintsDescription constraints_);