feat: add separate endpoint to fetch function names for autocomplete (#12840)

* WIP

* Add unit test for API

* Add spec

* Fix unit test

* Fix unit test

* Fix test

* Fix test

* Add period to error message

tests/databases/api_tests.py

@@ -137,7 +137,6 @@ class TestDatabaseApi(SupersetTestCase):
             "explore_database_id",
             "expose_in_sqllab",
             "force_ctas_schema",
-            "function_names",
             "id",
         ]
         self.assertGreater(response["count"], 0)
@@ -589,7 +588,8 @@ class TestDatabaseApi(SupersetTestCase):
         assert rv.status_code == 200
         assert "can_read" in data["permissions"]
         assert "can_write" in data["permissions"]
-        assert len(data["permissions"]) == 2
+        assert "can_function_names" in data["permissions"]
+        assert len(data["permissions"]) == 3
 
     def test_get_invalid_database_table_metadata(self):
         """
@@ -1125,3 +1125,20 @@ class TestDatabaseApi(SupersetTestCase):
 
         db.session.delete(database)
         db.session.commit()
+
+    @mock.patch("superset.db_engine_specs.base.BaseEngineSpec.get_function_names",)
+    def test_function_names(self, mock_get_function_names):
+        example_db = get_example_database()
+        if example_db.backend in {"hive", "presto"}:
+            return
+
+        mock_get_function_names.return_value = ["AVG", "MAX", "SUM"]
+
+        self.login(username="admin")
+        uri = "api/v1/database/1/function_names/"
+
+        rv = self.client.get(uri)
+        response = json.loads(rv.data.decode("utf-8"))
+
+        assert rv.status_code == 200
+        assert response == {"function_names": ["AVG", "MAX", "SUM"]}