sql - Update Duplicate Records -

i have column store table, table_hana below data in hana db:

t_id1   t_id2   t_desc  t_rn 1        1      aaa       ? 1        1      bbb       ? 1        2      ccc       ? 1        3      ddd       ? 1        3      eee       ? 

i need group them t_id1 , t_id2 , update t_rn column of group sequential value shown below:

t_id1   t_id2   t_desc  t_rn 1        1       aaa    1 1        1       bbb    2 1        2       ccc    1 1        3       ddd    1 1        3       eee    2 

since table huge(millions of records), looking solution performance. please suggest.

tia

the first question whether need update. following query returns want:

select h.*,        row_number() on (partition t_id1, t_id2 order t_desc) t_rn table_hana h; 

this may sufficient.

the update expensive:

update table_hana h     set t_rn = (select count(*)                 table_hana h2                 h2.t_id1 = h.t_id1 , h2.t_id2 = h.t_id2 , h2.desc <= h.desc                ); 

an index on table_hana(t_id1, t_id2, t_desc) should both these queries.