MySQL Internals Manual  /  Tracing the Optimizer  /  Privilege Checking

8.8 Privilege Checking

In complex scenarios where the query uses SQL SECURITY DEFINER views or stored routines, it may be that a user is denied from seeing the trace of its query because it lacks some extra privileges on those objects. In that case, the trace will be shown as empty and the INSUFFICIENT_PRIVILEGES column will show "1".