ORA-00604 ORA-01555 ORA-06512

Error While Converting Dictionary Managed Tablespace to Locally Managed Tablespace

SQL> EXECUTE DBMS_SPACE_ADMIN.TABLESPACE_MIGRATE_TO_LOCAL('TABLESPACE_NAME');

BEGIN DBMS_SPACE_ADMIN.TABLESPACE_MIGRATE_TO_LOCAL('TABLESPACE_NAME'); END;
*
ERROR at line 1:
ORA-00604: error occurred at recursive SQL level 1
ORA-01555: snapshot too old: rollback segment number 0 with name "SYSTEM" too
small
ORA-06512: at "SYS.DBMS_SPACE_ADMIN", line 227
ORA-06512: at line 1

Solution:
ORA-604 & ORA-1555 Rollback Segment 0 With Name "System" Too small (Doc ID 862469.1)
The SYSTEM rollback segment has no undo retention policy, and is usually not configured to be very large,it is possible to encounter a situation where we could run out of space and hence raise an ORA-1555 error.

set echo on
set feedback on
alter system set "_smu_debug_mode"=4;
drop table helper_for_1555;
create table helper_for_1555 (col1 varchar2(4000));

insert into helper_for_1555 values (rpad('a', 3999));
declare
a number;
begin
for a in 1..18 loop
insert /*+ APPEND +*/ into helper_for_1555 select * from helper_for_1555;
commit;
end loop;
end;
/

rem alter SYSTEM rbseg's storage params
alter rollback segment system storage (next 256M);
alter rollback segment system storage (optimal 1024M);
select segment_name, blocks, bytes, extents from  dba_segments where segment_type='ROLLBACK';

rem bloat the segment
alter system set "_in_memory_undo"=false;
set transaction use rollback segment SYSTEM;
delete from helper_for_1555;
rollback;

select segment_name, blocks, bytes, extents from dba_segments where segment_type='ROLLBACK';

alter system set "_smu_debug_mode"=0;
alter system set "_in_memory_undo"=true;

Now again try to convert the tablespace
SQL> EXECUTE DBMS_SPACE_ADMIN.TABLESPACE_MIGRATE_TO_LOCAL('TABLESPACE_NAME');