Oracle Solution for Error ora-29383

ORA-29383: all leaves of top-plan string must be consumer groups

What triggered the Error:

attempt was made to create or modify the specified top-plan but it has some plans as leaves.

What should we do to fix it:

identify which plans and/or plan directives need to be modified, look at all plans that have no plan directives. Then, alter the top-plan so that all its leaves are consumer groups.