优化模式区别(all_rows & first_rows_n)

FIRST_ROWS优化模式以最快速度地检索出结果 集中的一行为其指导目标。当系统用户正在使用OLTP系统检索单条记录时，该 优化模式最为有效。但是该模式对于批处理密集型(batch)作业环境来说并不是最理想 的选择，在这种环境中一个查询通常需要检索许多行。FIRST_ROWS提示 一般会强制使用某些索引，而在默认环境（ALL_ROWS）中可能不采用这些索引。在使 用UPDATE和DELETE语句时FIRST_ROWS模式会被忽略，因这些DML操 作中所查询到的所有记录都会被更新或删除。另当使用以下分组语句(如GROUP BY,DISTINCT,INTERSECT,MINUS和UNION)时FIRST_ROWS模式均被ALL_ROWS模式取代，因为这些语句进行分组时必须检索所有行。当语句中有ORDER BY子句时，如果索引扫描可以进行实际的排序工作，则优化器将避免额外的排 序。当索引扫描可用并且索引处于内部表(inner table)时，优化器将更倾向于NESTED LOOPS即嵌套循环而非SORT MERGE排 序连接。 另10g中现有的FIRST_ROWS模式的变体FIRST_ROWS_N来 指定以多少行数最快返回。这个值介于10~1000之间，这个使用FIRST_ROWS_N的新方法是完全基于成本的方法，它对于N的取值较敏感，若N甚小，优化器就会产生包 括嵌套循环以及索引查找的计划。如果N值较大，优化器也可能生成由散列连接和全表扫描组 成的计划(类似于ALL_ROWS)。 又FIRST_ROW与FIRST_ROWS_N存 在不同，FIRST_ROW模式中保量了部分基于规则的代码，而FIRST_ROWS_N模式则是完完全全基于统计信息计算相应成本，如Oracle文档所述： ALL_ROWS优化模式指导查询以最快速度检索出所 有行(最佳吞吐量)。当系统用户 处于需要大量批处理报告的环境中，该模式较理想。 在实际的SQL硬解析过程中，FIRST_ROWS_N模式将首先以ALL_ROWS模 式的方式计算一次各执行计划的具体代价，之后将我们需要的N条记录代入成本计算中代替实 际全部的候选行(CARD)以得出FIRST_ROWS_N中 的计划成本。 create table test as select  * from dba_objects; create table testa as select * from test; alter session set events'10053 trace name context forever,level 1';    --使用10053事 件获取成本计算过程trace alter session set optimizer_mode=all_rows; select test.owner from test,testa where test.object_id=testa.object_id alter session set events'10053 trace name context off'; 下为ALL_ROWS模式中，最佳连接方式的选 取： NL Join Outer table: Card: 9622.00  Cost: 35.37  Resp: 35.37  Degree: 1  Bytes: 7 Inner table: TESTA  Alias: TESTA Access Path: TableScan NL Join:  Cost: 318924.52  Resp: 318924.52  Degree: 0 Cost_io: 315358.00  Cost_cpu: 27736509932 Resp_io: 315358.00  Resp_cpu: 27736509932 Access Path: index (index (FFS)) Index: INDA_ID resc_io: 5.69  resc_cpu: 1304190 ix_sel: 0.0000e+00  ix_sel_with_filters: 1 Inner table: TESTA  Alias: TESTA Access Path: index (FFS) NL Join:  Cost: 56375.98  Resp: 56375.98  Degree: 0 Cost_io: 54762.00  Cost_cpu: 12551800804 Resp_io: 54762.00  Resp_cpu: 12551800804 Access Path: index (AllEqJoinGuess) Index: INDA_ID resc_io: 1.00  resc_cpu: 8171 ix_sel: 1.0393e-04  ix_sel_with_filters: 1.0393e-04 NL Join: Cost: 9667.48  Resp: 9667.48  Degree: 1 Cost_io: 9657.00  Cost_cpu: 81507910 Resp_io: 9657.00  Resp_cpu: 81507910 Best NL cost: 9667.48 resc: 9667.48 resc_io: 9657.00 resc_cpu: 81507910 resp: 9667.48 resp_io: 9657.00 resp_cpu: 81507910 Join Card:  9622.00 = outer (9622.00) * inner (9622.00) * sel (1.0393e-04) Join Card - Rounded: 9622 Computed: 9622.00 SM Join Outer table: resc: 35.37  card 9622.00  bytes: 7  deg: 1  resp: 35.37 Inner table: TESTA  Alias: TESTA resc: 7.17  card: 9622.00  bytes: 3  deg: 1  resp: 7.17 using dmeth: 2  #groups: 1 SORT resource      Sort statistics Sort           70 Area size:      131072 Max Area size:    12582912 Degree:               1 Blocks to Sort:      17 Row size:           14 Total Rows:           9622 Initial runs:         2 Merge passes:        1 IO Cost / pass:         10 Total IO sort cost: 27      Total CPU sort cost: 13931876 Total Temp space used: 254000 SM join: Resc: 203.62  Resp: 203.62  [multiMatchCost=0.00] HA Join Outer table: resc: 35.37  card 9622.00  bytes: 7  deg: 1  resp: 35.37 Inner table: TESTA  Alias: TESTA resc: 7.17  card: 9622.00  bytes: 3  deg: 1  resp: 7.17 using dmeth: 2  #groups: 1 Cost per ptn: 0.81  #ptns: 1 hash_area: 124 (max=3072)   Hash join: Resc: 43.35  Resp: 43.35  [multiMatchCost=0.00] HA Join (swap) Outer table: resc: 7.17  card 9622.00  bytes: 3  deg: 1  resp: 7.17 Inner table: TEST  Alias: TEST resc: 35.37  card: 9622.00  bytes: 7  deg: 1  resp: 35.37 using dmeth: 2  #groups: 1 Cost per ptn: 0.81  #ptns: 1 hash_area: 124 (max=3072)   Hash join: Resc: 43.35  Resp: 43.35  [multiMatchCost=0.00] HA cost: 43.35 resc: 43.35 resc_io: 42.00 resc_cpu: 10480460 resp: 43.35 resp_io: 42.00 resp_cpu: 10480460 Best:: JoinMethod: Hash Cost: 43.35  Degree: 1  Resp: 43.35  Card: 9622.00  Bytes: 10 *********************** Best so far: Table#: 0  cost: 35.3706  card: 9622.0000  bytes: 67354 Table#: 1  cost: 43.3476  card: 9622.0000  bytes: 96220 可以看到连接中二表上的候选行都是9622条，实际结果集也是9622条。 我们来看FIRST_ROWS_10情况下的trace: alter session set events'10053 trace name context forever,level 1'; alter session set optimizer_mode=first_rows_10; select test.owner from test,testa where test.object_id=testa.object_id; alter session set events'10053 trace name context off'; Now joining: TEST[TEST]#0 *************** NL Join Outer table: Card: 11.00  Cost: 2.00  Resp: 2.00  Degree: 1  Bytes: 3 Inner table: TEST  Alias: TEST Access Path: TableScan NL Join:  Cost: 368.08  Resp: 368.08  Degree: 0 Cost_io: 364.00  Cost_cpu: 31713898 Resp_io: 364.00  Resp_cpu: 31713898 Access Path: index (AllEqJoinGuess) Index: IND_ID resc_io: 2.00  resc_cpu: 15503 ix_sel: 1.0393e-04  ix_sel_with_filters: 1.0393e-04 NL Join (ordered): Cost: 24.02  Resp: 24.02  Degree: 1 Cost_io: 24.00  Cost_cpu: 178973 Resp_io: 24.00  Resp_cpu: 178973 Best NL cost: 24.02 resc: 24.02 resc_io: 24.00 resc_cpu: 178973 resp: 24.02 resp_io: 24.00 resp_cpu: 178973 Join Card:  11.00 = outer (11.00) * inner (9622.00) * sel (1.0393e-04) Join Card - Rounded: 11 Computed: 11.00 SM Join Outer table: resc: 7.17  card 9622.00  bytes: 3  deg: 1  resp: 7.17 Inner table: TEST  Alias: TEST resc: 35.37  card: 9622.00  bytes: 7  deg: 1  resp: 35.37 using dmeth: 2  #groups: 1 SORT resource      Sort statistics Sort           70 Area size:      131072 Max Area size:    12582912 Degree:               1 Blocks to Sort:      22 Row size:           18 Total Rows:           9622 Initial runs:         2 Merge passes:        1 IO Cost / pass:         14 Total IO sort cost: 36      Total CPU sort cost: 14055006 Total Temp space used: 320000 SORT resource      Sort statistics Sort           70 Area size:      131072 Max Area size:    12582912 Degree:               1 Blocks to Sort:      17 Row size:           14 Total Rows:           9622 Initial runs:         2 Merge passes:        1 IO Cost / pass:         10 Total IO sort cost: 27      Total CPU sort cost: 13931876 Total Temp space used: 254000 SM join: Resc: 109.14  Resp: 109.14  [multiMatchCost=0.00] SM cost: 109.14 resc: 109.14 resc_io: 105.00 resc_cpu: 32173386 resp: 109.14 resp_io: 105.00 resp_cpu: 32173386 SM Join (with index on outer) Access Path: index (FullScan) Index: IND_ID resc_io: 167.00  resc_cpu: 5134300 ix_sel: 1  ix_sel_with_filters: 1 Cost: 167.66  Resp: 167.66  Degree: 1 Outer table: resc: 167.66  card 11.00  bytes: 7  deg: 1  resp: 167.66 Inner table: TESTA  Alias: TESTA resc: 7.17  card: 9622.00  bytes: 3  deg: 1  resp: 7.17 using dmeth: 2  #groups: 1 SORT resource      Sort statistics Sort           70 Area size:      131072 Max Area size:    12582912 Degree:               1 Blocks to Sort:      17 Row size:           14 Total Rows:           9622 Initial runs:         2 Merge passes:        1 IO Cost / pass:         10 Total IO sort cost: 27      Total CPU sort cost: 13931876 Total Temp space used: 254000 SM join: Resc: 203.62  Resp: 203.62  [multiMatchCost=0.00] HA Join Outer table: resc: 35.37  card 9622.00  bytes: 7  deg: 1  resp: 35.37 Inner table: TESTA  Alias: TESTA resc: 7.17  card: 9622.00  bytes: 3  deg: 1  resp: 7.17 using dmeth: 2  #groups: 1 Cost per ptn: 0.81  #ptns: 1 hash_area: 124 (max=3072)   Hash join: Resc: 43.35  Resp: 43.35  [multiMatchCost=0.00] HA Join (swap) Outer table: resc: 7.17  card 9622.00  bytes: 3  deg: 1  resp: 7.17 Inner table: TEST  Alias: TEST resc: 2.00  card: 11.00  bytes: 7  deg: 1  resp: 2.00 using dmeth: 2  #groups: 1 Cost per ptn: 0.69  #ptns: 1 hash_area: 124 (max=3072)   Hash join: Resc: 9.85  Resp: 9.85  [multiMatchCost=0.00] HA cost: 9.85 resc: 9.85 resc_io: 9.00 resc_cpu: 6646477 resp: 9.85 resp_io: 9.00 resp_cpu: 6646477 First K Rows: copy A one plan, tab=TESTA Best:: JoinMethod: Hash Cost: 9.85  Degree: 1  Resp: 9.85  Card: 9622.00  Bytes: 17 *********************** Best so far: Table#: 0  cost: 2.0012  card: 11.0000  bytes: 77 Table#: 1  cost: 9.8546  card: 9622.0000  bytes: 163574 可以看到此次计算中代入了用户希望最先返回的结果 条数11(为10+1),通过设 置连接对象的候选结果集(Card)以到达相关优化目的，相应的COST均有所下降。 下为FIRST_ROWS_1000的情况： alter session set events'10053 trace name context forever,level 1'; alter session set optimizer_mode=first_rows_1000; select test.owner from test,testa where test.object_id=testa.object_id; alter session set events'10053 trace name context off'; NL Join Outer table: Card: 1000.00  Cost: 5.04  Resp: 5.04  Degree: 1  Bytes: 7 Inner table: TESTA  Alias: TESTA Access Path: TableScan NL Join:  Cost: 33147.66  Resp: 33147.66  Degree: 0 Cost_io: 32777.00  Cost_cpu: 2882616819 Resp_io: 32777.00  Resp_cpu: 2882616819 Access Path: index (index (FFS)) Index: INDA_ID resc_io: 5.69  resc_cpu: 1304190 ix_sel: 0.0000e+00  ix_sel_with_filters: 1 Inner table: TESTA  Alias: TESTA Access Path: index (FFS) NL Join:  Cost: 5861.74  Resp: 5861.74  Degree: 0 Cost_io: 5694.00  Cost_cpu: 1304492819 Resp_io: 5694.00  Resp_cpu: 1304492819 Access Path: index (AllEqJoinGuess) Index: INDA_ID resc_io: 1.00  resc_cpu: 8171 ix_sel: 1.0393e-04  ix_sel_with_filters: 1.0393e-04 NL Join: Cost: 1006.09  Resp: 1006.09  Degree: 1 Cost_io: 1005.00  Cost_cpu: 8474019 Resp_io: 1005.00  Resp_cpu: 8474019 Best NL cost: 1006.09 resc: 1006.09 resc_io: 1005.00 resc_cpu: 8474019 resp: 1006.09 resp_io: 1005.00 resp_cpu: 8474019 Join Card:  1000.00 = outer (1000.00) * inner (9622.00) * sel (1.0393e-04) Join Card - Rounded: 1000 Computed: 1000.00 SM Join Outer table: resc: 35.37  card 9622.00  bytes: 7  deg: 1  resp: 35.37 Inner table: TESTA  Alias: TESTA resc: 7.17  card: 9622.00  bytes: 3  deg: 1  resp: 7.17 using dmeth: 2  #groups: 1 SORT resource      Sort statistics Sort           70 Area size:      131072 Max Area size:    12582912 Degree:               1 Blocks to Sort:      22 Row size:           18 Total Rows:           9622 Initial runs:         2 Merge passes:        1 IO Cost / pass:         14 Total IO sort cost: 36      Total CPU sort cost: 14055006 Total Temp space used: 320000 SORT resource      Sort statistics Sort           70 Area size:      131072 Max Area size:    12582912 Degree:               1 Blocks to Sort:      17 Row size:           14 Total Rows:           9622 Initial runs:         2 Merge passes:        1 IO Cost / pass:         10 Total IO sort cost: 27      Total CPU sort cost: 13931876 Total Temp space used: 254000 SM join: Resc: 109.14  Resp: 109.14  [multiMatchCost=0.00] SM cost: 109.14 resc: 109.14 resc_io: 105.00 resc_cpu: 32173386 resp: 109.14 resp_io: 105.00 resp_cpu: 32173386 SM Join (with index on outer) Access Path: index (FullScan) Index: IND_ID resc_io: 167.00  resc_cpu: 5134300 ix_sel: 1  ix_sel_with_filters: 1 Cost: 167.66  Resp: 167.66  Degree: 1 Outer table: resc: 167.66  card 1000.00  bytes: 7  deg: 1  resp: 167.66 Inner table: TESTA  Alias: TESTA resc: 7.17  card: 9622.00  bytes: 3  deg: 1  resp: 7.17 using dmeth: 2  #groups: 1 SORT resource      Sort statistics Sort           70 Area size:      131072 Max Area size:    12582912 Degree:               1 Blocks to Sort:      17 Row size:           14 Total Rows:           9622 Initial runs:         2 Merge passes:        1 IO Cost / pass:         10 Total IO sort cost: 27      Total CPU sort cost: 13931876 Total Temp space used: 254000 SM join: Resc: 203.62  Resp: 203.62  [multiMatchCost=0.00] HA Join Outer table: resc: 35.37  card 9622.00  bytes: 7  deg: 1  resp: 35.37 Inner table: TESTA  Alias: TESTA resc: 7.17  card: 9622.00  bytes: 3  deg: 1  resp: 7.17 using dmeth: 2  #groups: 1 Cost per ptn: 0.81  #ptns: 1 hash_area: 124 (max=3072)   Hash join: Resc: 43.35  Resp: 43.35  [multiMatchCost=0.00] HA Join (swap) Outer table: resc: 7.17  card 9622.00  bytes: 3  deg: 1  resp: 7.17 Inner table: TEST  Alias: TEST resc: 5.04  card: 1000.00  bytes: 7  deg: 1  resp: 5.04 using dmeth: 2  #groups: 1 Cost per ptn: 0.70  #ptns: 1 hash_area: 124 (max=3072)   Hash join: Resc: 12.91  Resp: 12.91  [multiMatchCost=0.00] HA cost: 12.91 resc: 12.91 resc_io: 12.00 resc_cpu: 7038524 resp: 12.91 resp_io: 12.00 resp_cpu: 7038524 First K Rows: copy A one plan, tab=TESTA Best:: JoinMethod: Hash Cost: 12.91  Degree: 1  Resp: 12.91  Card: 9622.00  Bytes: 17 *********************** Best so far: Table#: 0  cost: 5.0389  card: 1000.0000  bytes: 7000 Table#: 1  cost: 12.9051  card: 9622.0000  bytes: 163574 可以看到此处代入了1000为某一连接对象的候选行数。 MOS上有一个著名的《MIGRATING TO THE COST-BASED OPTIMIZER》教材，详细介绍了RBO和CBO的区别： [gview file="http://youyus.com/wp-content/uploads/resource/40178_rbo_rip.doc"]