feat: add support for JOIN in Druid (apache#16770)

superset/config.py

@@ -334,6 +334,8 @@ def _try_json_readsha(filepath: str, length: int) -> Optional[str]:
     # Experimental feature introducing a client (browser) cache
     "CLIENT_CACHE": False,
     "DISABLE_DATASET_SOURCE_EDIT": False,
+    # When using a recent version of Druid that supports JOINs turn this on
+    "DRUID_JOINS": False,
     "DYNAMIC_PLUGINS": False,
     # For some security concerns, you may need to enforce CSRF protection on
     # all query request to explore_json endpoint. In Superset, we use

superset/db_engine_specs/druid.py

@@ -19,6 +19,7 @@
 from datetime import datetime
 from typing import Any, Dict, Optional, TYPE_CHECKING
 
+from superset import is_feature_enabled
 from superset.db_engine_specs.base import BaseEngineSpec
 from superset.exceptions import SupersetException
 from superset.utils import core as utils
@@ -35,7 +36,7 @@ class DruidEngineSpec(BaseEngineSpec):  # pylint: disable=abstract-method
 
     engine = "druid"
     engine_name = "Apache Druid"
-    allows_joins = False
+    allows_joins = is_feature_enabled("DRUID_JOINS")
     allows_subqueries = True
 
     _time_grain_expressions = {