Fixed #36088 -- Avoided unnecessary DEFAULT usage on bulk_create().

When all values of a field with a db_default are DatabaseDefault, which
is the case most of the time, there is no point in specifying explicit
DEFAULT for all INSERT VALUES as that's what the database will do anyway
if not specified.

In the case of PostgreSQL doing so can even be harmful as it prevents
the usage of the UNNEST strategy and in the case of Oracle, which
doesn't support the usage of the DEFAULT keyword, it unnecessarily
requires providing literal db defaults.

Thanks Lily Foote for the review.

2 files changed, 7 insertions, 1 deletions

diff --git a/tests/backends/models.py b/tests/backends/models.py
index 1ed108c2b8..afb6ebe303 100644
--- a/tests/backends/models.py
+++ b/tests/backends/models.py
@@ -5,7 +5,7 @@ from django.db import models
 
 class Square(models.Model):
     root = models.IntegerField()
-    square = models.PositiveIntegerField()
+    square = models.PositiveIntegerField(db_default=9)
 
     def __str__(self):
         return "%s ** 2 == %s" % (self.root, self.square)
diff --git a/tests/backends/postgresql/test_compilation.py b/tests/backends/postgresql/test_compilation.py
index 67fe893e35..5a86a427ff 100644
--- a/tests/backends/postgresql/test_compilation.py
+++ b/tests/backends/postgresql/test_compilation.py
@@ -27,3 +27,9 @@ class BulkCreateUnnestTests(TestCase):
                 [Square(root=2, square=4), Square(root=3, square=9)]
             )
         self.assertIn("UNNEST", ctx[0]["sql"])
+
+    def test_unnest_eligible_db_default(self):
+        with self.assertNumQueries(1) as ctx:
+            squares = Square.objects.bulk_create([Square(root=3), Square(root=3)])
+        self.assertIn("UNNEST", ctx[0]["sql"])
+        self.assertEqual([square.square for square in squares], [9, 9])