一、本文说明：
操作系统：rhel 5.4 x32
数据库：oracle 11g r2 x32
二、实验内容：
----创建一张jack表，并创建索引jack_ind----
1 SQL> create table jack as select * from dba_objects; 2 3 Table created. 4 5 SQL> create index jack_ind on jack(object_id); 6 7 Index created. 8 ----先做一下统计信息，并连带索引----
9 SQL> exec dbms_stats.gather_table_stats(user,'jack',cascade=>true); 10 11 PL/SQL procedure successfully completed. 12 13 SQL> set autotrace traceonly; 14 SQL> set linesize 120; ----当object_id=100时的值是唯一的，所以查询会走索引----
15 SQL> select * from jack where object_id=100; 16 17 18 Execution Plan 19 ---------------------------------------------------------- 20 Plan hash value: 2860868395 21 22 ---------------------------------------------------------------------------------------- 23 | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | 24 ---------------------------------------------------------------------------------------- 25 | 0 | SELECT STATEMENT | | 1 | 97 | 2 (0)| 00:00:01 | 26 | 1 | TABLE ACCESS BY INDEX ROWID| JACK | 1 | 97 | 2 (0)| 00:00:01 | 27 |* 2 | INDEX RANGE SCAN | JACK_IND | 1 | | 1 (0)| 00:00:01 | 28 ---------------------------------------------------------------------------------------- 29 30 Predicate Information (identified by operation id): 31 --------------------------------------------------- 32 33 2 - access("OBJECT_ID"=100) 34 35 36 Statistics 37 ---------------------------------------------------------- 38 1 recursive calls 39 0 db block gets 40 4 consistent gets 41 42 SQL> set autotrace off; ----再把所有object_id的值修改成100---
43 SQL> update jack set object_id=100; 44 45 72489 rows updated. 46 47 SQL> set autotrace traceonly; 48 SQL> alter session set events '10053 trace name context forever,level 1'; 49 50 Session altered. 51 ----因为现在object_id的值全部都是100了，所以不应该走索引而走全表----
52 SQL> select * from jack where object_id=100; 53 54 72489 rows selected. 55 56 57 Execution Plan 58 ---------------------------------------------------------- 59 Plan hash value: 2860868395 60 61 ---------------------------------------------------------------------------------------- 62 | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | 63 ---------------------------------------------------------------------------------------- 64 | 0 | SELECT STATEMENT | | 1 | 97 | 2 (0)| 00:00:01 | 65 | 1 | TABLE ACCESS BY INDEX ROWID| JACK | 1 | 97 | 2 (0)| 00:00:01 | 66 |* 2 | INDEX RANGE SCAN | JACK_IND | 1 | | 1 (0)| 00:00:01 | 67 ---------------------------------------------------------------------------------------- 68 -----------在这里可以看到执行计划依然走的是索引，而Rows=1，显然执行计划是错误的。 69 Predicate Information (identified by operation id): 70 --------------------------------------------------- 71 72 2 - access("OBJECT_ID"=100) 73 74 75 Statistics 76 ---------------------------------------------------------- 77 0 recursive calls 78 0 db block gets 79 10860 consistent gets 80 0 physical reads 81 SQL> alter session set events '10053 trace name context off'; 82 83 Session altered. 84 85 SQL> set autotrace off; 86 SQL> @/u01/scripts/showtrace 87 88 trace_file_name 89 ------------------------------------------------------------------------------------------------------------------------ 90 /u01/app/oracle/diag/rdbms/yft/yft/trace/yft_ora_4005.trc 91 92 ----退出再进，并对表jack进行一下统计信息----
93 SQL> exec dbms_stats.gather_table_stats(user,'jack',cascade=>true); 94 95 PL/SQL procedure successfully completed. 96 97 SQL> set autotrace trace exp; 98 SQL> alter session set events '10053 trace name context forever,level 1'; 99 100 Session altered. 101 ----查询object_id=100时，执行计划走了全表，显然这次是正确的----
102 SQL> select * from jack where object_id=100; 103 104 Execution Plan 105 ---------------------------------------------------------- 106 Plan hash value: 949574992 107 108 -------------------------------------------------------------------------- 109 | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | 110 -------------------------------------------------------------------------- 111 | 0 | SELECT STATEMENT | | 72482 | 6724K| 290 (1)| 00:00:04 | 112 |* 1 | TABLE ACCESS FULL| JACK | 72482 | 6724K| 290 (1)| 00:00:04 | 113 -------------------------------------------------------------------------- 114 -----在这里Rows=72482已经很接近72489了 115 Predicate Information (identified by operation id): 116 --------------------------------------------------- 117 118 1 - filter("OBJECT_ID"=100) 119 120 SQL> alter session set events '10053 trace name context off'; 121 122 Session altered. 123 124 SQL> set autotrace off; 125 SQL> @/u01/scripts/showtrace 126 127 trace_file_name 128 -------------------------------------------------------------------------------- 129 /u01/app/oracle/diag/rdbms/yft/yft/trace/yft_ora_4032.trc 130 ----先看一下yft_ora_4005.trc中的内容，这个是错误的执行计划的内容-----
131 *************************************** 132 BASE STATISTICAL INFORMATION 133 *********************** 134 Table Stats:: 135 Table: JACK Alias: JACK 136 #Rows: 72489 #Blks: 1058 AvgRowLen: 97.00 137 Index Stats:: 138 Index: JACK_IND Col#: 4 139 LVLS: 1 #LB: 161 #DK: 72489 LB/K: 1.00 DB/K: 1.00 CLUF: 1656.00 140 Access path analysis for JACK ----在10053事件trace文件中我们可以发现#DK的值依然是72489，说明不同值有72489个，才导致执行计划错误。 141 *************************************** 142 SINGLE TABLE ACCESS PATH 143 Single Table Cardinality Estimation for JACK[JACK] 144 Table: JACK Alias: JACK 145 Card: Original: 72489.000000 Rounded: 1 Computed: 1.00 Non Adjusted: 1.00 146 Access Path: TableScan 147 Cost: 289.48 Resp: 289.48 Degree: 0 148 Cost_io: 288.00 Cost_cpu: 26381844 149 Resp_io: 288.00 Resp_cpu: 26381844 150 Access Path: index (AllEqRange) 151 Index: JACK_IND 152 resc_io: 2.00 resc_cpu: 15723 153 ix_sel: 0.000014 ix_sel_with_filters: 0.000014 154 Cost: 2.00 Resp: 2.00 Degree: 1 155 Best:: AccessPath: IndexRange 156 Index: JACK_IND 157 Cost: 2.00 Degree: 1 Resp: 2.00 Card: 1.00 Bytes: 0 158 159 *************************************** 160 ----先看一下yft_ora_4032.trc中的内容，这个是错误的执行计划的内容-----
161 162 *************************************** 163 BASE STATISTICAL INFORMATION 164 *********************** 165 Table Stats:: 166 Table: JACK Alias: JACK 167 #Rows: 72489 #Blks: 1058 AvgRowLen: 95.00 168 Index Stats:: 169 Index: JACK_IND Col#: 4 170 LVLS: 1 #LB: 250 #DK: 1 LB/K: 250.00 DB/K: 1032.00 CLUF: 1032.00 171 Access path analysis for JACK ----当进行统计信息以后#DK的值已经变成1了，而表中object_id的值就只有1个了--100。所以CBO选择了全表 172 *************************************** 173 SINGLE TABLE ACCESS PATH 174 Single Table Cardinality Estimation for JACK[JACK] 175 Table: JACK Alias: JACK 176 Card: Original: 72489.000000 Rounded: 72482 Computed: 72482.35 Non Adjusted: 72482.35 177 Access Path: TableScan 178 Cost: 290.38 Resp: 290.38 Degree: 0 179 Cost_io: 288.00 Cost_cpu: 42327664 180 Resp_io: 288.00 Resp_cpu: 42327664 181 Access Path: index (AllEqRange) 182 Index: JACK_IND 183 resc_io: 1282.00 resc_cpu: 54794826 184 ix_sel: 0.999908 ix_sel_with_filters: 0.999908 185 Cost: 1285.08 Resp: 1285.08 Degree: 1 186 Best:: AccessPath: TableScan 187 Cost: 290.38 Degree: 1 Resp: 290.38 Card: 72482.35 Bytes: 0 188 189 ***************************************