Fix: Return a list instead of a set from get_pk_constraint

This is to fix the Apache Superset bug, that doesn't show the CrateDB
table metadata in the SQL Lab editor.

CHANGES.txt

@@ -5,6 +5,10 @@ Changes for crate
 Unreleased
 ==========
 
+- Improve compatibility with Apache Superset by returning a list instead of a
+  set from ``get_pk_constraint``. Otherwise, Apache Superset would not show
+  CrateDB table metadata in the SQL Lab editor.
+
 
 2022/06/02 0.27.0
 =================

src/crate/client/sqlalchemy/dialect.py

@@ -271,7 +271,7 @@ def get_pk_constraint(self, engine, table_name, schema=None, **kw):
 
             def result_fun(result):
                 rows = result.fetchall()
-                return set(map(lambda el: el[0], rows))
+                return list(set(map(lambda el: el[0], rows)))
         else:
             query = """SELECT constraint_name
                    FROM information_schema.table_constraints

src/crate/client/sqlalchemy/tests/dialect_test.py

@@ -78,7 +78,7 @@ def test_primary_keys(self):
         )
         self.fake_cursor.fetchall = MagicMock(return_value=[["id"], ["id2"], ["id3"]])
 
-        eq_(insp.get_pk_constraint("characters")['constrained_columns'], {"id", "id2", "id3"})
+        eq_(insp.get_pk_constraint("characters")['constrained_columns'], ["id", "id2", "id3"])
         self.fake_cursor.fetchall.assert_called_once_with()
         in_("information_schema.key_column_usage", self.executed_statement)