partition and clustering bigquery keys (#6212)

* partition and clustering bigquery keys

* flake8

superset/db_engine_specs.py

@@ -1433,6 +1433,28 @@ class BQEngineSpec(BaseEngineSpec):
                                             '128 characters'.format(mutated_label))
         return mutated_label
 
+    @classmethod
+    def extra_table_metadata(cls, database, table_name, schema_name):
+        indexes = database.get_indexes(table_name, schema_name)
+        if not indexes:
+            return {}
+        partitions_columns = [
+            index.get('column_names', []) for index in indexes
+            if index.get('name') == 'partition'
+        ]
+        cluster_columns = [
+            index.get('column_names', []) for index in indexes
+            if index.get('name') == 'clustering'
+        ]
+        return {
+            'partitions': {
+                'cols': partitions_columns,
+            },
+            'clustering': {
+                'cols': cluster_columns,
+            },
+        }
+
     @classmethod
     def _get_fields(cls, cols):
         """