总结：oracle在9i后引入变量窥测技术，该技术对于数据分布均匀的数据是非常合适的，但是对于分布倾斜的数据或者在OLAP系统中是不建议使用的。
ORACLE绑定变量BINDPEEKING(三)
| Name | Rows | Bytes | Cost (%CPU)| Time |
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| 0 | SELECT STATEMENT | | | | 136 (100)| |
| 1 | SORT AGGREGATE | | 1 | 4 | | |
|* 2 | INDEX FAST FULL SCAN| ACS_TEST_TAB_RECORD_TYPE_I | 48031 | 187K| 136 (1)| 00:00:02 |
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Predicate Information (identified by operation id):
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2 - filter("RECORD_TYPE"=:V)
已选择19行。
SQL> select count(*) from acs_test_tab where record_type = 1;
COUNT(*)
----------
1
SQL> select * from table(dbms_xplan.display_cursor);
PLAN_TABLE_OUTPUT
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SQL_ID 1pxm87f6yd0bp, child number 0
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select count(*) from acs_test_tab where record_type = 1
Plan hash value: 2956728990
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| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 3 (100)| |
| 1 | SORT AGGREGATE | | 1 | 4 | | |
|* 2 | INDEX RANGE SCAN| ACS_TEST_TAB_RECORD_TYPE_I | 1 | 4 | 3 (0)| 00:00:01 |
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Predicate Information (identified by operation id):
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2 - access("RECORD_TYPE"=1)
已选择19行。
对于变量v的取值为1的执行计划和采用常量1的执行计划性能差距还是比较大的。