Fixed #27928 -- Avoided SET/DROP DEFAULT unless a field changes from null to non-null.

Thanks Christophe Pettus, Matteo Pietro Russo for reports and Tim for review.

django/db/backends/base/schema.py

@@ -578,6 +578,8 @@ class BaseDatabaseSchemaEditor:
         old_default = self.effective_default(old_field)
         new_default = self.effective_default(new_field)
         needs_database_default = (
+            old_field.null and
+            not new_field.null and
             old_default != new_default and
             new_default is not None and
             not self.skip_default(new_field)

tests/schema/tests.py

@@ -1956,6 +1956,21 @@ class SchemaTests(TransactionTestCase):
             if connection.features.can_introspect_default:
                 self.assertIsNone(field.default)
 
+    @unittest.skipIf(connection.vendor == 'sqlite', 'SQLite naively remakes the table on field alteration.')
+    def test_alter_field_default_doesnt_perfom_queries(self):
+        """
+        No queries are performed if a field default changes and the field's
+        not changing from null to non-null.
+        """
+        with connection.schema_editor() as editor:
+            editor.create_model(AuthorWithDefaultHeight)
+        old_field = AuthorWithDefaultHeight._meta.get_field('height')
+        new_default = old_field.default * 2
+        new_field = PositiveIntegerField(null=True, blank=True, default=new_default)
+        new_field.set_attributes_from_name('height')
+        with connection.schema_editor() as editor, self.assertNumQueries(0):
+            editor.alter_field(AuthorWithDefaultHeight, old_field, new_field, strict=True)
+
     def test_add_textfield_unhashable_default(self):
         # Create the table
         with connection.schema_editor() as editor: