fix: implement set_timezone_to_utc() for BigQuery and ClickHouse

data_diff/databases/_connect.py

@@ -298,7 +298,7 @@ def _connection_created(self, db):
         try:
             db.query(db.dialect.set_timezone_to_utc())
         except NotImplementedError:
-            logging.debug(
+            logging.warning(
                 f"Database '{db}' does not allow setting timezone. We recommend making sure it's set to 'UTC'."
             )
         return db

data_diff/databases/bigquery.py

@@ -152,7 +152,9 @@ def to_comparable(self, value: str, coltype: ColType) -> str:
             return super().to_comparable(value, coltype)
 
     def set_timezone_to_utc(self) -> str:
-        raise NotImplementedError()
+        # BigQuery uses stateless REST jobs; SET @@time_zone only affects a single job,
+        # not subsequent queries. BigQuery stores timestamps in UTC internally.
+        raise NotImplementedError("BigQuery uses stateless jobs; session SET has no cross-query effect.")
 
     def parse_table_name(self, name: str) -> DbPath:
         path = parse_table_name(name)

data_diff/databases/clickhouse.py

@@ -96,7 +96,7 @@ def parse_type(self, table_path: DbPath, info: RawColumnInfo) -> ColType:
     #     return f"'{str(t)[:19]}'"
 
     def set_timezone_to_utc(self) -> str:
-        raise NotImplementedError()
+        return "SET session_timezone = 'UTC'"
 
     def current_timestamp(self) -> str:
         return "now()"