Refs #27090 -- Added real database sequence introspection.

Thanks Mariusz Felisiak for the Oracle part and Tim Graham for the
review.

tests/backends/oracle/test_introspection.py

@@ -0,0 +1,29 @@
+import unittest
+
+from django.db import connection
+from django.test import TransactionTestCase
+
+from ..models import Person
+
+
+@unittest.skipUnless(connection.vendor == 'oracle', 'Oracle tests')
+class DatabaseSequenceTests(TransactionTestCase):
+    available_apps = []
+
+    def test_get_sequences(self):
+        with connection.cursor() as cursor:
+            seqs = connection.introspection.get_sequences(cursor, Person._meta.db_table, Person._meta.local_fields)
+            self.assertEqual(len(seqs), 1)
+            self.assertIsNotNone(seqs[0]['name'])
+            self.assertEqual(seqs[0]['table'], Person._meta.db_table)
+            self.assertEqual(seqs[0]['column'], 'id')
+
+    def test_get_sequences_manually_created_index(self):
+        with connection.cursor() as cursor:
+            with connection.schema_editor() as editor:
+                editor._drop_identity(Person._meta.db_table, 'id')
+                seqs = connection.introspection.get_sequences(cursor, Person._meta.db_table, Person._meta.local_fields)
+                self.assertEqual(seqs, [{'table': Person._meta.db_table, 'column': 'id'}])
+                # Recreate model, because adding identity is impossible.
+                editor.delete_model(Person)
+                editor.create_model(Person)