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.

django/db/models/query.py

@@ -5,6 +5,7 @@ The main QuerySet implementation. This provides the public API for the ORM.
 import copy
 import operator
 import warnings
+from contextlib import nullcontext
 from functools import reduce
 from itertools import chain, islice
 
@@ -802,7 +803,11 @@ class QuerySet(AltersData):
         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(
@@ -1919,7 +1924,13 @@ class QuerySet(AltersData):
         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)]:
+        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
                 ):

tests/bulk_create/tests.py

@@ -14,6 +14,7 @@ from django.db.models import FileField, Value
 from django.db.models.functions import Lower, Now
 from django.test import (
     TestCase,
+    TransactionTestCase,
     override_settings,
     skipIfDBFeature,
     skipUnlessDBFeature,
@@ -884,3 +885,35 @@ class BulkCreateTests(TestCase):
     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,
+            )