MERGE /*+ PARALLEL(8) */
     INTO  table_a a
     USING table_b b
        ON (a.unit = b.unit AND a.group = b.group AND a.loc = b.loc)
WHEN MATCHED
THEN
   UPDATE SET
      a.u_pushonly = b.u_pushonly,
      a.u_seasonind = b.u_seasonind,
      a.eff =
         CASE
            WHEN b.u_season_start_dt <> TO_DATE ('01/01/1970', 'DD/MM/YYYY')
                 OR b.u_season_end_dt <> TO_DATE ('01/01/1970', 'DD/MM/YYYY')
                 OR b.u_pushonly = 1
            THEN
               GREATEST (
                  NVL (b.u_assortment_start_dt,
                       TO_DATE ('01/01/1970', 'DD/MM/YYYY')),
                  b.u_season_start_dt)
            ELSE
               NVL (b.u_assortment_start_dt,
                    TO_DATE ('01/01/1970', 'DD/MM/YYYY'))
         END;