Oracle认证：ORACLE绑定变量BINDPEEKING
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Peeked Binds (identified by position)：
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1 - :V (NUMBER)： 2 --绑定变量窥探，绑定变量会影响最初硬解析的执行计划
Predicate Information (identified by operation id)：
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2 - filter("RECORD_TYPE"=:V)
已选择49行。
使用绑定变量窥测的好处是：可以帮助优化器在第一次硬解析时选择最优的执行计划。但是同时这也是其弊端：在第一次硬解析后，后面发生的所有解析都会使用第一次硬解析生成的执行计划，如果数据的分布是均匀的，问题不大，如果数据分布式倾斜的.，那么第一次硬解析生成的执行计划未必是最优的，甚至可能是非常糟糕的。例如：
SQL> show parameter optimizer_feat
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
optimizer_features_enable string 11.2.0.3.1
SQL> alter system flush shared_pool;
系统已更改。
SQL> var v number;
SQL> exec :v := 2;
PL/SQL 过程已成功完成。
SQL> select count(*) from acs_test_tab where record_type = :v;
COUNT(*)
----------
50000
SQL> select * from table(dbms_xplan.display_cursor);
PLAN_TABLE_OUTPUT
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SQL_ID3rg5r8sghcvb3, child number 0
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select count(*) from acs_test_tab where record_type = :v
Plan hash value: 2957754476
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| Id | Operation | 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> exec :v := 1
PL/SQL 过程已成功完成。
SQL> select count(*) from acs_test_tab where record_type = :v;
COUNT(*)
----------
1
SQL> select * from table(dbms_xplan.display_cursor);
PLAN_TABLE_OUTPUT
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SQL_ID3rg5r8sghcvb3, child number 0
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select count(*) from acs_test_tab where record_type = :v
Plan hash value: 2957754476
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| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------------------
| 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)：
---------------------------------------------------
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_ID1pxm87f6yd0bp, 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 |
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