oracle中使用group by优化distinct

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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：

1. SELECT DISTINCT 'AMEND_NEW',  
2. reporttitle,  
3. reportsubtitle,  
4. cab_cab_transactions.branchcode,  
5. cab_cab_transactions.prtfo_cd,  
6. cab_cab_transactions.sstm_scrty_id,  
7. cab_cab_transactions.sstm_trx_id,  
8. cab_cab_transactions.trde_dttm,  
9. cab_cab_transactions.efcte_dttm,  
10. cab_cab_transactions.due_stlmnt_dt,  
11. cab_cab_transactions.cncl_efcte_dttm,  
12. cab_cab_transactions.trde_sstm_id,  
13. cab_cab_transactions.trx_type_cd,  
14. cab_cab_transactions.trx_type_dscrn,  
15. cab_cab_transactions.trx_subtype_cd,  
16. cab_cab_transactions.trde_stat_flg,  
17. cab_cab_transactions.csh_cr_dr_indcr,  
18. cab_cab_transactions.long_shrt_indcr,  
19. cab_cab_transactions.lcl_crncy,  
20. cab_cab_transactions.stlmt_crncy,  
21. cab_cab_transactions.nomin_qty,  
22. cab_cab_transactions.price,  
23. cab_cab_transactions.lcl_cst,  
24. cab_cab_transactions.prtfo_cst,  
25. cab_cab_transactions.lcl_book_cst,  
26. cab_cab_transactions.prtfo_book_cst,  
27. cab_cab_transactions.lcl_sell_prcds,  
28. cab_cab_transactions.prtfo_sell_prcds,  
29. cab_cab_transactions.lcl_gnls,  
30. cab_cab_transactions.prtfo_gnls,  
31. cab_cab_transactions.lcl_acrd_intrt,  
32. cab_cab_transactions.prtfo_acrd_intrt,  
33. cab_cab_transactions.stlmt_crncy_stlmt_amt,  
34. cab_cab_transactions.lcl_net_amt,  
35. cab_cab_transactions.prtfo_net_amt,  
36. cab_cab_transactions.fx_bght_amt,  
37. cab_cab_transactions.fx_sold_amt,  
38. cab_cab_transactions.prtfo_crncy_stlmt_amt,  
39. cab_cab_transactions.prtfo_net_incme,  
40. cab_cab_transactions.dvnd_crncy_net_incme,  
41. cab_cab_transactions.dvnd_type_cd,  
42. cab_cab_transactions.lcl_intrt_pd_rec,  
43. cab_cab_transactions.prtfo_intrt_pd_rec,  
44. cab_cab_transactions.lcl_dvdnd_pd_rec,  
45. cab_cab_transactions.prtfo_dvdnd_pd_rec,  
46. cab_cab_transactions.lcl_sundry_inc_pd_rec,  
47. cab_cab_transactions.prtfo_sundry_inc_pd_rec,  
48. cab_cab_transactions.bnk_csh_cptl_secid,  
49. cab_cab_transactions.bnk_csh_inc_secid,  
50. cab_cab_transactions.reportdate,  
51. cab_cab_transactions.filename,  
52. sysdate,  
53. 'e483448' 
54. FROM cab_cfg_trx_type_mapping RIGHT JOIN(cab_cab_tran_adjustments  
55. INNER JOIN cab_cab_transactions ON(cab_cab_transactions.branchcode = cab_cab_tran_adjustments.branchcode )  
56. 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)  
57. AND(nvl(cab_cfg_trx_type_mapping.cab_trx_subtype_cd,' ') = nvl(cab_cab_transactions.trx_subtype_cd,' ')  
58. AND (cab_cfg_trx_type_mapping.branchcode=cab_cab_transactions.branchcode))  
59. WHERE cab_cab_transactions.prtfo_cd IN 
60. (SELECT DISTINCT prtfo_cd  
61. FROM cab_cab_valuations_working  
62. WHERE created_by = 'e483448' 
63. AND branchcode='ISA')  
64. AND cab_cab_tran_adjustments.efcte_dttm > '2011-07-31' 
65. AND cab_cab_tran_adjustments.efcte_dttm <= '2011-08-31' 
66. AND eff_trde_stat_flg <> 'X' 
67. AND cab_cab_transactions.branchcode = 'ISA' 
68. AND cab_cab_tran_adjustments.branchcode = 'ISA' 
69. 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：

1. SELECT 'AMEND_NEW',  
2. reporttitle,  
3. reportsubtitle,  
4. cab_cab_transactions.branchcode,  
5. cab_cab_transactions.prtfo_cd,  
6. cab_cab_transactions.sstm_scrty_id,  
7. cab_cab_transactions.sstm_trx_id,  
8. cab_cab_transactions.trde_dttm,  
9. cab_cab_transactions.efcte_dttm,  
10. cab_cab_transactions.due_stlmnt_dt,  
11. cab_cab_transactions.cncl_efcte_dttm,  
12. cab_cab_transactions.trde_sstm_id,  
13. cab_cab_transactions.trx_type_cd,  
14. cab_cab_transactions.trx_type_dscrn,  
15. cab_cab_transactions.trx_subtype_cd,  
16. cab_cab_transactions.trde_stat_flg,  
17. cab_cab_transactions.csh_cr_dr_indcr,  
18. cab_cab_transactions.long_shrt_indcr,  
19. cab_cab_transactions.lcl_crncy,  
20. cab_cab_transactions.stlmt_crncy,  
21. cab_cab_transactions.nomin_qty,  
22. cab_cab_transactions.price,  
23. cab_cab_transactions.lcl_cst,  
24. cab_cab_transactions.prtfo_cst,  
25. cab_cab_transactions.lcl_book_cst,  
26. cab_cab_transactions.prtfo_book_cst,  
27. cab_cab_transactions.lcl_sell_prcds,  
28. cab_cab_transactions.prtfo_sell_prcds,  
29. cab_cab_transactions.lcl_gnls,  
30. cab_cab_transactions.prtfo_gnls,  
31. cab_cab_transactions.lcl_acrd_intrt,  
32. cab_cab_transactions.prtfo_acrd_intrt,  
33. cab_cab_transactions.stlmt_crncy_stlmt_amt,  
34. cab_cab_transactions.lcl_net_amt,  
35. cab_cab_transactions.prtfo_net_amt,  
36. cab_cab_transactions.fx_bght_amt,  
37. cab_cab_transactions.fx_sold_amt,  
38. cab_cab_transactions.prtfo_crncy_stlmt_amt,  
39. cab_cab_transactions.prtfo_net_incme,  
40. cab_cab_transactions.dvnd_crncy_net_incme,  
41. cab_cab_transactions.dvnd_type_cd,  
42. cab_cab_transactions.lcl_intrt_pd_rec,  
43. cab_cab_transactions.prtfo_intrt_pd_rec,  
44. cab_cab_transactions.lcl_dvdnd_pd_rec,  
45. cab_cab_transactions.prtfo_dvdnd_pd_rec,  
46. cab_cab_transactions.lcl_sundry_inc_pd_rec,  
47. cab_cab_transactions.prtfo_sundry_inc_pd_rec,  
48. cab_cab_transactions.bnk_csh_cptl_secid,  
49. cab_cab_transactions.bnk_csh_inc_secid,  
50. cab_cab_transactions.reportdate,  
51. cab_cab_transactions.filename,  
52. sysdate,  
53. 'e483448' 
54. FROM cab_cfg_trx_type_mapping RIGHT JOIN(cab_cab_tran_adjustments  
55. INNER JOIN cab_cab_transactions ON(cab_cab_transactions.branchcode = cab_cab_tran_adjustments.branchcode )  
56. 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)  
57. AND(nvl(cab_cfg_trx_type_mapping.cab_trx_subtype_cd,' ') = nvl(cab_cab_transactions.trx_subtype_cd,' ')  
58. AND (cab_cfg_trx_type_mapping.branchcode=cab_cab_transactions.branchcode))  
59. WHERE cab_cab_transactions.prtfo_cd IN 
60. (SELECT DISTINCT prtfo_cd  
61. FROM cab_cab_valuations_working  
62. WHERE created_by = 'e483448' 
63. AND branchcode='ISA')  
64. AND cab_cab_tran_adjustments.efcte_dttm > '2011-07-31' 
65. AND cab_cab_tran_adjustments.efcte_dttm <= '2011-08-31' 
66. AND eff_trde_stat_flg <> 'X' 
67. AND cab_cab_transactions.branchcode = 'ISA' 
68. AND cab_cab_tran_adjustments.branchcode = 'ISA' 
69. AND(cab_cfg_trx_type_mapping.cab_reportgroup = 'CABValuation' OR cab_cfg_trx_type_mapping.cab_reportgroup IS NULL)  
70. GROUP BY reporttitle,  
71. reportsubtitle,  
72. cab_cab_transactions.branchcode,  
73. cab_cab_transactions.prtfo_cd,  
74. cab_cab_transactions.sstm_scrty_id,  
75. cab_cab_transactions.sstm_trx_id,  
76. cab_cab_transactions.trde_dttm,  
77. cab_cab_transactions.efcte_dttm,  
78. cab_cab_transactions.due_stlmnt_dt,  
79. cab_cab_transactions.cncl_efcte_dttm,  
80. cab_cab_transactions.trde_sstm_id,  
81. cab_cab_transactions.trx_type_cd,  
82. cab_cab_transactions.trx_type_dscrn,  
83. cab_cab_transactions.trx_subtype_cd,  
84. cab_cab_transactions.trde_stat_flg,  
85. cab_cab_transactions.csh_cr_dr_indcr,  
86. cab_cab_transactions.long_shrt_indcr,  
87. cab_cab_transactions.lcl_crncy,  
88. cab_cab_transactions.stlmt_crncy,  
89. cab_cab_transactions.nomin_qty,  
90. cab_cab_transactions.price,  
91. cab_cab_transactions.lcl_cst,  
92. cab_cab_transactions.prtfo_cst,  
93. cab_cab_transactions.lcl_book_cst,  
94. cab_cab_transactions.prtfo_book_cst,  
95. cab_cab_transactions.lcl_sell_prcds,  
96. cab_cab_transactions.prtfo_sell_prcds,  
97. cab_cab_transactions.lcl_gnls,  
98. cab_cab_transactions.prtfo_gnls,  
99. cab_cab_transactions.lcl_acrd_intrt,  
100. cab_cab_transactions.prtfo_acrd_intrt,  
101. cab_cab_transactions.stlmt_crncy_stlmt_amt,  
102. cab_cab_transactions.lcl_net_amt,  
103. cab_cab_transactions.prtfo_net_amt,  
104. cab_cab_transactions.fx_bght_amt,  
105. cab_cab_transactions.fx_sold_amt,  
106. cab_cab_transactions.prtfo_crncy_stlmt_amt,  
107. cab_cab_transactions.prtfo_net_incme,  
108. cab_cab_transactions.dvnd_crncy_net_incme,  
109. cab_cab_transactions.dvnd_type_cd,  
110. cab_cab_transactions.lcl_intrt_pd_rec,  
111. cab_cab_transactions.prtfo_intrt_pd_rec,  
112. cab_cab_transactions.lcl_dvdnd_pd_rec,  
113. cab_cab_transactions.prtfo_dvdnd_pd_rec,  
114. cab_cab_transactions.lcl_sundry_inc_pd_rec,  
115. cab_cab_transactions.prtfo_sundry_inc_pd_rec,  
116. cab_cab_transactions.bnk_csh_cptl_secid,  
117. cab_cab_transactions.bnk_csh_inc_secid,  
118. cab_cab_transactions.reportdate,  
119. 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。