diff --git a/haystack/database/base.py b/haystack/database/base.py
index 5a5ada63c..4ad69ca6b 100644
--- a/haystack/database/base.py
+++ b/haystack/database/base.py
@@ -18,4 +18,8 @@ class BaseDocumentStore:
     def get_document_ids_by_tag(self, tag):
         pass
 
+    @abstractmethod
+    def get_document_count(self):
+        pass
+
 
diff --git a/haystack/database/sql.py b/haystack/database/sql.py
index a00538e25..4101e447f 100644
--- a/haystack/database/sql.py
+++ b/haystack/database/sql.py
@@ -98,3 +98,7 @@ class SQLDocumentStore(BaseDocumentStore):
             row = Document(name=doc["name"], text=doc["text"])
             self.session.add(row)
         self.session.commit()
+
+    def get_document_count(self):
+        return self.session.query(Document).count()
+
diff --git a/haystack/indexing/io.py b/haystack/indexing/io.py
index e474bfad3..cae24a0c3 100644
--- a/haystack/indexing/io.py
+++ b/haystack/indexing/io.py
@@ -19,11 +19,10 @@ def write_documents_to_db(datastore, document_dir, clean_func=None, only_empty_d
     :return: None
     """
     file_paths = Path(document_dir).glob("**/*.txt")
-    n_docs = 0
 
     # check if db has already docs
     if only_empty_db:
-        n_docs = len(datastore.get_all_documents())
+        n_docs = datastore.get_document_count()
         if n_docs > 0:
             logger.info(f"Skip writing documents since DB already contains {n_docs} docs ...  "
                         "(Disable `only_empty_db`, if you want to add docs anyway.)")