ORA-14551: cannot perform a DML operation inside a query
 *Cause:  DML operation like insert, update, delete or select-for-update
          cannot be performed inside a query or under a PDML slave.
 *Action: Ensure that the offending DML operation is not performed or
          use an autonomous transaction to perform the DML operation within
          the query or PDML slave.

create or replace function lock_id(id_input in varchar2) Return varchar2 is
  
  PRAGMA AUTONOMOUS_TRANSACTION;  
  
  res varchar2(32000);

  begin

      update table_id
         set status = '10000'
       where id = id_input;
    
       
     res := '0: Ok';
    
    commit;
    
    return(res);
    
  exception
      
    when others then
      res  := '-2: Error: ' || TO_CHAR(SQLCODE) || ':' || SQLERRM;

      return(res);
         
  end lock_id;  

select lock_id('9.42.33232') from dual

res = OK

select lock_id@dblink_DB1('9.42.33232') from dual

res = ORA-14551

DECLARE
  v_ret   varchar2(32767);
BEGIN
  v_ret := lock_id@dblink_DB1('9.42.33232');


  <other code to actually do something>
END;

select lock_id('9.42.33232') from dual

res = OK

select lock_id('9.42.33232') from dual

a) return one row and call the function once (maybe)
b) restart the query for some reason and thus for one logical invocation call the function more than once
c) remember that it called this function before and for whatever reason reuse the value from that call and thus not call the function at all

1) use of views where the column is not selected
2) column referenced but not necessary to the query because of nesting
3) oracle rewrite that removes the column for some other reason
4) code that gets restarted (thing trigger restart)
5) functions used in the WHERE clause/HAVING clause/GROUP BY clause etc.
6) likely many other situations I don't know about

Patient:  "DOCTOR DOCTOR IT HURTS WHEN I DO THIS..."

Doctor:  "SO DON'T DO THAT"