Re: autoupdating mtime column

If you are really intent on having a way to suppress the mtime update
you could dedicate an additional field to the purpose, eg

UPDATE t SET foo=..., bar=..., keepmtime = true ...

and in the trigger something like

        if new.keepmtime then
                new.keepmtime = false;
        else
                new.mtime = now();

As long as nothing else ever touches keepmtime this would work.
Personally I'm dubious that it's worth the trouble

--- do you
have a real use-case for suppressing mtime updates?