Oracle - Crie um índice somente se não existir

declare 
  already_exists  exception; 
  columns_indexed exception;
  pragma exception_init( already_exists, -955 ); 
  pragma exception_init(columns_indexed, -1408);
begin 
  execute immediate 'create index ord_customer_ix on orders (customer_id)'; 
  dbms_output.put_line( 'created' ); 
exception 
  when already_exists or columns_indexed then 
  dbms_output.put_line( 'skipped' );  
end;