Reflect table can reflect table with no columns

The :meth:`_engine.Inspector.reflect_table` method now supports reflecting
tables that do not have user defined columns. This allows
:meth:`_schema.MetaData.reflect` to properly complete reflection on
databases that contain such tables.   Currently, only PostgreSQL is known
to support such a construct among the common database backends.

Fixes: #3247
Closes: #7118
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/7118
Pull-request-sha: cb8ce01957e9a1453290a7c2728af8c60ef55fa1

Change-Id: I906cebe17d13554d79086b92f3e1e51ffba3e818

2 files changed, 64 insertions, 0 deletions

diff --git a/lib/sqlalchemy/testing/requirements.py b/lib/sqlalchemy/testing/requirements.py
index a546e1feb..f8b5dd606 100644
--- a/lib/sqlalchemy/testing/requirements.py
+++ b/lib/sqlalchemy/testing/requirements.py
@@ -589,9 +589,17 @@ class SuiteRequirements(Requirements):
 
     @property
     def table_reflection(self):
+        """target database has general support for table reflection"""
         return exclusions.open()
 
     @property
+    def reflect_tables_no_columns(self):
+        """target database supports creation and reflection of tables with no
+        columns, or at least tables that seem to have no columns."""
+
+        return exclusions.closed()
+
+    @property
     def comment_reflection(self):
         return exclusions.closed()
 
diff --git a/lib/sqlalchemy/testing/suite/test_reflection.py b/lib/sqlalchemy/testing/suite/test_reflection.py
index 916d74db3..88189c2d9 100644
--- a/lib/sqlalchemy/testing/suite/test_reflection.py
+++ b/lib/sqlalchemy/testing/suite/test_reflection.py
@@ -1063,6 +1063,61 @@ class ComponentReflectionTest(fixtures.TablesTest):
             assert id_.get("autoincrement", True)
 
 
+class TableNoColumnsTest(fixtures.TestBase):
+    __requires__ = ("reflect_tables_no_columns",)
+    __backend__ = True
+
+    @testing.fixture
+    def table_no_columns(self, connection, metadata):
+        Table("empty", metadata)
+        metadata.create_all(connection)
+
+    @testing.fixture
+    def view_no_columns(self, connection, metadata):
+        Table("empty", metadata)
+        metadata.create_all(connection)
+
+        Table("empty", metadata)
+        event.listen(
+            metadata,
+            "after_create",
+            DDL("CREATE VIEW empty_v AS SELECT * FROM empty"),
+        )
+
+        # for transactional DDL the transaction is rolled back before this
+        # drop statement is invoked
+        event.listen(
+            metadata, "before_drop", DDL("DROP VIEW IF EXISTS empty_v")
+        )
+        metadata.create_all(connection)
+
+    @testing.requires.reflect_tables_no_columns
+    def test_reflect_table_no_columns(self, connection, table_no_columns):
+        t2 = Table("empty", MetaData(), autoload_with=connection)
+        eq_(list(t2.c), [])
+
+    @testing.requires.reflect_tables_no_columns
+    def test_get_columns_table_no_columns(self, connection, table_no_columns):
+        eq_(inspect(connection).get_columns("empty"), [])
+
+    @testing.requires.reflect_tables_no_columns
+    def test_reflect_incl_table_no_columns(self, connection, table_no_columns):
+        m = MetaData()
+        m.reflect(connection)
+        assert set(m.tables).intersection(["empty"])
+
+    @testing.requires.views
+    @testing.requires.reflect_tables_no_columns
+    def test_reflect_view_no_columns(self, connection, view_no_columns):
+        t2 = Table("empty_v", MetaData(), autoload_with=connection)
+        eq_(list(t2.c), [])
+
+    @testing.requires.views
+    @testing.requires.reflect_tables_no_columns
+    def test_get_columns_view_no_columns(self, connection, view_no_columns):
+        eq_(inspect(connection).get_columns("empty_v"), [])
+
+
 class ComponentReflectionTestExtra(fixtures.TestBase):
 
     __backend__ = True
@@ -1641,6 +1696,7 @@ class CompositeKeyReflectionTest(fixtures.TablesTest):
 __all__ = (
     "ComponentReflectionTest",
     "ComponentReflectionTestExtra",
+    "TableNoColumnsTest",
     "QuotedNameArgumentTest",
     "HasTableTest",
     "HasIndexTest",