Fixed #36490 -- Avoided unnecessary transaction in bulk_create.

When dealing with an heterogeneous set of object with regards to primary key
assignment that fits in a single batch there's no need to wrap the single
INSERT statement in a transaction.

Author: charettes

django/db/models/query.py

@@ -5,6 +5,7 @@
 import copy
 import operator
 import warnings
+from contextlib import nullcontext
 from functools import reduce
 from itertools import chain, islice
 
@@ -802,7 +803,11 @@ def bulk_create(
         fields = [f for f in opts.concrete_fields if not f.generated]
         objs = list(objs)
         objs_with_pk, objs_without_pk = self._prepare_for_bulk_create(objs)
-        with transaction.atomic(using=self.db, savepoint=False):
+        if objs_with_pk and objs_without_pk:
+            context = transaction.atomic(using=self.db, savepoint=False)
+        else:
+            context = nullcontext()
+        with context:
             self._handle_order_with_respect_to(objs)
             if objs_with_pk:
                 returned_columns = self._batched_insert(
@@ -1919,30 +1924,36 @@ def _batched_insert(
         batch_size = min(batch_size, max_batch_size) if batch_size else max_batch_size
         inserted_rows = []
         bulk_return = connection.features.can_return_rows_from_bulk_insert
-        for item in [objs[i : i + batch_size] for i in range(0, len(objs), batch_size)]:
-            if bulk_return and (
-                on_conflict is None or on_conflict == OnConflict.UPDATE
-            ):
-                inserted_rows.extend(
+        batches = [objs[i : i + batch_size] for i in range(0, len(objs), batch_size)]
+        if len(batches) > 1:
+            context = transaction.atomic(using=self.db, savepoint=False)
+        else:
+            context = nullcontext()
+        with context:
+            for item in batches:
+                if bulk_return and (
+                    on_conflict is None or on_conflict == OnConflict.UPDATE
+                ):
+                    inserted_rows.extend(
+                        self._insert(
+                            item,
+                            fields=fields,
+                            using=self.db,
+                            on_conflict=on_conflict,
+                            update_fields=update_fields,
+                            unique_fields=unique_fields,
+                            returning_fields=self.model._meta.db_returning_fields,
+                        )
+                    )
+                else:
                     self._insert(
                         item,
                         fields=fields,
                         using=self.db,
                         on_conflict=on_conflict,
                         update_fields=update_fields,
                         unique_fields=unique_fields,
-                        returning_fields=self.model._meta.db_returning_fields,
                     )
-                )
-            else:
-                self._insert(
-                    item,
-                    fields=fields,
-                    using=self.db,
-                    on_conflict=on_conflict,
-                    update_fields=update_fields,
-                    unique_fields=unique_fields,
-                )
         return inserted_rows
 
     def _chain(self):

tests/bulk_create/tests.py

@@ -14,6 +14,7 @@
 from django.db.models.functions import Lower, Now
 from django.test import (
     TestCase,
+    TransactionTestCase,
     override_settings,
     skipIfDBFeature,
     skipUnlessDBFeature,
@@ -884,3 +885,35 @@ def test_db_default_field_excluded(self):
     def test_db_default_primary_key(self):
         (obj,) = DbDefaultPrimaryKey.objects.bulk_create([DbDefaultPrimaryKey()])
         self.assertIsInstance(obj.id, datetime)
+
+
+@skipUnlessDBFeature("supports_transactions", "has_bulk_insert")
+class BulkCreateTransactionTests(TransactionTestCase):
+    available_apps = ["bulk_create"]
+
+    def test_no_unnecessary_transaction(self):
+        with self.assertNumQueries(1):
+            Country.objects.bulk_create(
+                [Country(id=1, name="France", iso_two_letter="FR")]
+            )
+        with self.assertNumQueries(1):
+            Country.objects.bulk_create([Country(name="Canada", iso_two_letter="CA")])
+
+    def test_objs_with_and_without_pk(self):
+        with self.assertNumQueries(4):
+            Country.objects.bulk_create(
+                [
+                    Country(id=1, name="France", iso_two_letter="FR"),
+                    Country(name="Canada", iso_two_letter="CA"),
+                ]
+            )
+
+    def test_multiple_batches(self):
+        with self.assertNumQueries(4):
+            Country.objects.bulk_create(
+                [
+                    Country(name="France", iso_two_letter="FR"),
+                    Country(name="Canada", iso_two_letter="CA"),
+                ],
+                batch_size=1,
+            )