oracle中使用group by优化distinct

今天mentor给了一个sql语句优化的任务。（环境是sql developer）有一个语句执行很慢，查询出来的结果有17544条记录，但需970秒，速度很慢。语句是这样的：

select distinct  'amend_new', 
       reporttitle, 
       reportsubtitle, 
       cab_cab_transactions.branchcode, 
       cab_cab_transactions.prtfo_cd, 
       cab_cab_transactions.sstm_scrty_id, 
       cab_cab_transactions.sstm_trx_id, 
       cab_cab_transactions.trde_dttm, 
       cab_cab_transactions.efcte_dttm, 
       cab_cab_transactions.due_stlmnt_dt, 
       cab_cab_transactions.cncl_efcte_dttm, 
       cab_cab_transactions.trde_sstm_id, 
       cab_cab_transactions.trx_type_cd, 
       cab_cab_transactions.trx_type_dscrn, 
       cab_cab_transactions.trx_subtype_cd, 
       cab_cab_transactions.trde_stat_flg, 
       cab_cab_transactions.csh_cr_dr_indcr, 
       cab_cab_transactions.long_shrt_indcr, 
       cab_cab_transactions.lcl_crncy, 
       cab_cab_transactions.stlmt_crncy, 
       cab_cab_transactions.nomin_qty, 
       cab_cab_transactions.price, 
       cab_cab_transactions.lcl_cst, 
       cab_cab_transactions.prtfo_cst, 
       cab_cab_transactions.lcl_book_cst, 
       cab_cab_transactions.prtfo_book_cst, 
       cab_cab_transactions.lcl_sell_prcds, 
       cab_cab_transactions.prtfo_sell_prcds, 
       cab_cab_transactions.lcl_gnls, 
       cab_cab_transactions.prtfo_gnls, 
       cab_cab_transactions.lcl_acrd_intrt, 
       cab_cab_transactions.prtfo_acrd_intrt, 
       cab_cab_transactions.stlmt_crncy_stlmt_amt, 
       cab_cab_transactions.lcl_net_amt, 
       cab_cab_transactions.prtfo_net_amt, 
       cab_cab_transactions.fx_bght_amt, 
       cab_cab_transactions.fx_sold_amt, 
       cab_cab_transactions.prtfo_crncy_stlmt_amt, 
       cab_cab_transactions.prtfo_net_incme, 
       cab_cab_transactions.dvnd_crncy_net_incme, 
       cab_cab_transactions.dvnd_type_cd, 
       cab_cab_transactions.lcl_intrt_pd_rec, 
       cab_cab_transactions.prtfo_intrt_pd_rec, 
       cab_cab_transactions.lcl_dvdnd_pd_rec, 
       cab_cab_transactions.prtfo_dvdnd_pd_rec, 
       cab_cab_transactions.lcl_sundry_inc_pd_rec, 
       cab_cab_transactions.prtfo_sundry_inc_pd_rec, 
       cab_cab_transactions.bnk_csh_cptl_secid, 
       cab_cab_transactions.bnk_csh_inc_secid, 
       cab_cab_transactions.reportdate, 
       cab_cab_transactions.filename, 
        sysdate, 
       'e483448' 
   from cab_cfg_trx_type_mapping right join(cab_cab_tran_adjustments 
      inner join cab_cab_transactions on(cab_cab_transactions.branchcode = cab_cab_tran_adjustments.branchcode ) 
       and(cab_cab_tran_adjustments.sstm_trx_id = cab_cab_transactions.sstm_trx_id)) on(cab_cfg_trx_type_mapping.cab_trx_type_cd = cab_cab_transactions.trx_type_cd) 
       and(nvl(cab_cfg_trx_type_mapping.cab_trx_subtype_cd,' ') = nvl(cab_cab_transactions.trx_subtype_cd,' ') 
       and (cab_cfg_trx_type_mapping.branchcode=cab_cab_transactions.branchcode)) 
      where cab_cab_transactions.prtfo_cd in 
       (select distinct prtfo_cd 
        from cab_cab_valuations_working 
        where created_by = 'e483448' 
          and branchcode='isa') 
       and cab_cab_tran_adjustments.efcte_dttm > '2011-07-31' 
       and cab_cab_tran_adjustments.efcte_dttm <= '2011-08-31' 
       and eff_trde_stat_flg <> 'x' 
       and cab_cab_transactions.branchcode = 'isa' 
       and cab_cab_tran_adjustments.branchcode = 'isa' 
       and(cab_cfg_trx_type_mapping.cab_reportgroup = 'cabvaluation' or cab_cfg_trx_type_mapping.cab_reportgroup is null) 

问题在distinct上面，它会导致对全表扫描，而且会导致排序，然后删除重复的记录，所以速度很慢，因此需要优化distinct。查了不少资料，并逐一尝试，最后发现了一个非常可观的优化结果，用group by。语句如下：

select   'amend_new', 
       reporttitle, 
       reportsubtitle, 
       cab_cab_transactions.branchcode, 
       cab_cab_transactions.prtfo_cd, 
       cab_cab_transactions.sstm_scrty_id, 
       cab_cab_transactions.sstm_trx_id, 
       cab_cab_transactions.trde_dttm, 
       cab_cab_transactions.efcte_dttm, 
       cab_cab_transactions.due_stlmnt_dt, 
       cab_cab_transactions.cncl_efcte_dttm, 
       cab_cab_transactions.trde_sstm_id, 
       cab_cab_transactions.trx_type_cd, 
       cab_cab_transactions.trx_type_dscrn, 
       cab_cab_transactions.trx_subtype_cd, 
       cab_cab_transactions.trde_stat_flg, 
       cab_cab_transactions.csh_cr_dr_indcr, 
       cab_cab_transactions.long_shrt_indcr, 
       cab_cab_transactions.lcl_crncy, 
       cab_cab_transactions.stlmt_crncy, 
       cab_cab_transactions.nomin_qty, 
       cab_cab_transactions.price, 
       cab_cab_transactions.lcl_cst, 
       cab_cab_transactions.prtfo_cst, 
       cab_cab_transactions.lcl_book_cst, 
       cab_cab_transactions.prtfo_book_cst, 
       cab_cab_transactions.lcl_sell_prcds, 
       cab_cab_transactions.prtfo_sell_prcds, 
       cab_cab_transactions.lcl_gnls, 
       cab_cab_transactions.prtfo_gnls, 
       cab_cab_transactions.lcl_acrd_intrt, 
       cab_cab_transactions.prtfo_acrd_intrt, 
       cab_cab_transactions.stlmt_crncy_stlmt_amt, 
       cab_cab_transactions.lcl_net_amt, 
       cab_cab_transactions.prtfo_net_amt, 
       cab_cab_transactions.fx_bght_amt, 
       cab_cab_transactions.fx_sold_amt, 
       cab_cab_transactions.prtfo_crncy_stlmt_amt, 
       cab_cab_transactions.prtfo_net_incme, 
       cab_cab_transactions.dvnd_crncy_net_incme, 
       cab_cab_transactions.dvnd_type_cd, 
       cab_cab_transactions.lcl_intrt_pd_rec, 
       cab_cab_transactions.prtfo_intrt_pd_rec, 
       cab_cab_transactions.lcl_dvdnd_pd_rec, 
       cab_cab_transactions.prtfo_dvdnd_pd_rec, 
       cab_cab_transactions.lcl_sundry_inc_pd_rec, 
       cab_cab_transactions.prtfo_sundry_inc_pd_rec, 
       cab_cab_transactions.bnk_csh_cptl_secid, 
       cab_cab_transactions.bnk_csh_inc_secid, 
       cab_cab_transactions.reportdate, 
       cab_cab_transactions.filename, 
        sysdate, 
       'e483448' 
   from cab_cfg_trx_type_mapping right join(cab_cab_tran_adjustments 
      inner join cab_cab_transactions on(cab_cab_transactions.branchcode = cab_cab_tran_adjustments.branchcode ) 
       and(cab_cab_tran_adjustments.sstm_trx_id = cab_cab_transactions.sstm_trx_id)) on(cab_cfg_trx_type_mapping.cab_trx_type_cd = cab_cab_transactions.trx_type_cd) 
       and(nvl(cab_cfg_trx_type_mapping.cab_trx_subtype_cd,' ') = nvl(cab_cab_transactions.trx_subtype_cd,' ') 
       and (cab_cfg_trx_type_mapping.branchcode=cab_cab_transactions.branchcode)) 
      where cab_cab_transactions.prtfo_cd in 
       (select distinct prtfo_cd 
        from cab_cab_valuations_working 
        where created_by = 'e483448' 
          and branchcode='isa') 
       and cab_cab_tran_adjustments.efcte_dttm > '2011-07-31' 
       and cab_cab_tran_adjustments.efcte_dttm <= '2011-08-31' 
       and eff_trde_stat_flg <> 'x' 
       and cab_cab_transactions.branchcode = 'isa' 
       and cab_cab_tran_adjustments.branchcode = 'isa' 
       and(cab_cfg_trx_type_mapping.cab_reportgroup = 'cabvaluation' or cab_cfg_trx_type_mapping.cab_reportgroup is null) 
       group by  reporttitle, 
       reportsubtitle, 
       cab_cab_transactions.branchcode, 
       cab_cab_transactions.prtfo_cd, 
       cab_cab_transactions.sstm_scrty_id, 
       cab_cab_transactions.sstm_trx_id, 
       cab_cab_transactions.trde_dttm, 
       cab_cab_transactions.efcte_dttm, 
       cab_cab_transactions.due_stlmnt_dt, 
       cab_cab_transactions.cncl_efcte_dttm, 
       cab_cab_transactions.trde_sstm_id, 
       cab_cab_transactions.trx_type_cd, 
       cab_cab_transactions.trx_type_dscrn, 
       cab_cab_transactions.trx_subtype_cd, 
       cab_cab_transactions.trde_stat_flg, 
       cab_cab_transactions.csh_cr_dr_indcr, 
       cab_cab_transactions.long_shrt_indcr, 
       cab_cab_transactions.lcl_crncy, 
       cab_cab_transactions.stlmt_crncy, 
       cab_cab_transactions.nomin_qty, 
       cab_cab_transactions.price, 
       cab_cab_transactions.lcl_cst, 
       cab_cab_transactions.prtfo_cst, 
       cab_cab_transactions.lcl_book_cst, 
       cab_cab_transactions.prtfo_book_cst, 
       cab_cab_transactions.lcl_sell_prcds, 
       cab_cab_transactions.prtfo_sell_prcds, 
       cab_cab_transactions.lcl_gnls, 
       cab_cab_transactions.prtfo_gnls, 
       cab_cab_transactions.lcl_acrd_intrt, 
       cab_cab_transactions.prtfo_acrd_intrt, 
       cab_cab_transactions.stlmt_crncy_stlmt_amt, 
       cab_cab_transactions.lcl_net_amt, 
       cab_cab_transactions.prtfo_net_amt, 
       cab_cab_transactions.fx_bght_amt, 
       cab_cab_transactions.fx_sold_amt, 
       cab_cab_transactions.prtfo_crncy_stlmt_amt, 
       cab_cab_transactions.prtfo_net_incme, 
       cab_cab_transactions.dvnd_crncy_net_incme, 
       cab_cab_transactions.dvnd_type_cd, 
       cab_cab_transactions.lcl_intrt_pd_rec, 
       cab_cab_transactions.prtfo_intrt_pd_rec, 
       cab_cab_transactions.lcl_dvdnd_pd_rec, 
       cab_cab_transactions.prtfo_dvdnd_pd_rec, 
       cab_cab_transactions.lcl_sundry_inc_pd_rec, 
       cab_cab_transactions.prtfo_sundry_inc_pd_rec, 
       cab_cab_transactions.bnk_csh_cptl_secid, 
       cab_cab_transactions.bnk_csh_inc_secid, 
       cab_cab_transactions.reportdate, 
       cab_cab_transactions.filename 

最后执行时间只有15.1秒，快了60多倍，不得不说这优化效果还是很可观的。不过查了很多资料，仍然没有发现合理地解释：为什么distinct 和group by的效率会有这么大差别。查的很多资料，讲的基本都是两者相差不大，实现也差不多。有待解决。

distinct和group by这两者本质上应该没有可比性，distinct 取出唯一列，group by 是分组，但有时候在优化的时候，在没有聚合函数的时候，他们查出来的结果也一样。