When you grant privileges, by default, you are the one who can revoke those privileges. The AS grantor clause lets you establish another user as the source of the privileges you are granting. When you use this clause, the login provided in the AS grantor clause replaces your login in the appropriate system catalog table. You can use this clause only if you have the DBA privilege on the database.

After you use this clause, only the specified grantor can REVOKE the effects of the current GRANT. Even a DBA cannot revoke a privilege unless that DBA is listed in the system catalog table as the source who granted the privilege.

You receive an error because the database server record shows the original grantor as user tom, and you cannot revoke the privilege. Although you are the DBA, you cannot revoke a privilege that another user granted.

The AS grantor clause is not valid in the GRANT DEFAULT ROLE statement.