Hello Heikki,
The use case for transform_null_equals='warn' that I see is to wean off an application that uses transform_null_equals='on', to find all the queries that rely on it. But I'm not too excited about that either. The documented case for using transform_null_equals='on' is compatibility with Microsoft Access, and this wouldn't help with that. Besides, it's easy enough to grep the application or the server log for " = NULL", to find all such queries.
I'm in favor in having this feature, even if off by default, because I could enable it and it would help my students when learning SQL in interactive sessions, where "grep '= *NULL'" cannot be issued.
-- Fabien.
