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。
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