游标共享之cursor_sharing=similar
cursor_sharing=similar ，exact和force折中的参数。当探测到谓词会导致执行计划的改变，就会重新解析，否则不会。
适用场景：在无法将应用的代码修改为绑定变量情况下，oracle提供的又一种解决方法，similar的好处是最大限度地避免CBO在绑定变量的情况下做出的错误的判断，但它的代价是分析的次数变大。
参数还有两种选择：
游标共享之cursor_sharing=force
游标共享之cursor_sharing=EXACT
下面来做个试验：
SQL> Drop table t purge;
SQL> show parameter cursor_sharing;
NAME TYPE VALUE
------------------------------------ -------------------------------- ------------------------------
_optimizer_extended_cursor_sharing string UDO
cursor_sharing string FORCE
SQL> create table t as select * from dba_objects;
SQL> Create index ind_t_object_id on t(object_id) nologging;
SQL> exec dbms_stats.gather_table_stats(user,'t',cascade => true);
SQL> alter session set cursor_sharing=similar;
SQL> Set autotrace traceonly
SQL> select /*+test_similar*/* from t where object_id<10;
已选择8行。
执行计划
----------------------------------------------------------
Plan hash value: 1630734149
----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 7 | 651 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 7 | 651 | 3 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IND_T_OBJECT_ID | 7 | | 2 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("OBJECT_ID"<10)
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
5 consistent gets
0 physical reads
0 redo size
1788 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
8 rows processed
SQL> select /*+test_similar*/* from t where object_id<100000;
已选择50675行。
执行计划
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 50675 | 4602K| 149 (1)| 00:00:02 |
|* 1 | TABLE ACCESS FULL| T | 50675 | 4602K| 149 (1)| 00:00:02 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OBJECT_ID"<100000)
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
4042 consistent gets
0 physical reads
0 redo size
2460611 bytes sent via SQL*Net to client
37543 bytes received via SQL*Net from client
3380 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
50675 rows processed
----当探测到执行计划会改变时，则会重新解析
SQL> Set autotrace off
SQL> alter system flush shared_pool;
SQL> Set autotrace traceonly
SQL> select /*+test_similar*/* from t where object_id<10;
已选择8行。
执行计划
----------------------------------------------------------
Plan hash value: 1630734149
-----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 7 | 651 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 7 | 651 | 3 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IND_T_OBJECT_ID | 7 | | 2 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("OBJECT_ID"<10)
统计信息
----------------------------------------------------------
941 recursive calls
0 db block gets
149 consistent gets
0 physical reads
0 redo size
1788 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
10 sorts (memory)
0 sorts (disk)
8 rows processed
SQL>
select /*+test_similar*/* from t where object_id<100000;
已选择50675行。
执行计划
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 50675 | 4602K| 149 (1)| 00:00:02 |
|* 1 | TABLE ACCESS FULL| T | 50675 | 4602K| 149 (1)| 00:00:02 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OBJECT_ID"<100000)
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
4042 consistent gets
0 physical reads
0 redo size
2460611 bytes sent via SQL*Net to client
37543 bytes received via SQL*Net from client
3380 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
50675 rows processed
SQL> Set autotrace off
SQL> col sql_text format a80;
SQL> select sql_id,child_number,sql_text from v$sql where sql_text like '%/*+test_similar*/%'
2 and sql_text not like '%sql_text%'
3 And sql_text not like '%EXPLAIN PLAN%';
SQL_ID CHILD_NUMBER SQL_TEXT
------------- ------------ ---------------------------------------------------------------------------
d5101w24qr4hk 0 select /*+test_similar*/* from t where object_id<:"SYS_B_0"
d5101w24qr4hk 1 select /*+test_similar*/* from t where object_id<:"SYS_B_0"
----当探测到执行计划不会改变时，则不会重新解析
SQL> alter system flush shared_pool;
SQL> Set autotrace traceonly
SQL> select /*+test_similar*/* from t where object_name='aaa';
执行计划
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2 | 186 | 149 (1)| 00:00:02 |
|* 1 | TABLE ACCESS FULL| T | 2 | 186 | 149 (1)| 00:00:02 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OBJECT_NAME"='aaa')
统计信息
----------------------------------------------------------
941 recursive calls
0 db block gets
846 consistent gets
0 physical reads
0 redo size
992 bytes sent via SQL*Net to client
374 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
10 sorts (memory)
0 sorts (disk)
0 rows processed
SQL> select /*+test_similar*/* from t where object_name='bbb';
执行计划
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2 | 186 | 149 (1)| 00:00:02 |
|* 1 | TABLE ACCESS FULL| T | 2 | 186 | 149 (1)| 00:00:02 |
--------------------------------------------------------------------------
Predicate Information (identified by op