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Oracle認(rèn)證：ORACLE綁定變量BINDPEEKING
ORACLE 在9i之后引入了bind peeking,通過bind peeking,oracle可以在硬解析的時(shí)候窺探綁定變量的值，并根據(jù)當(dāng)前綁定變量的值生成執(zhí)行計(jì)劃。在oracle 9i之前的版本中，oracle僅僅通過統(tǒng)計(jì)信息來生成執(zhí)行計(jì)劃。
下面看一下不同版本oracle下綁定變量對執(zhí)行計(jì)劃的影響
SQL> alter system flush shared_pool;
系統(tǒng)已更改。
SQL> alter system set optimizer_features_enable='8.1.7';
系統(tǒng)已更改。
SQL> var v number;
SQL> exec :v := 1;
PL/SQL 過程已成功完成。
SQL> select count(*) from acs_test_tab where record_type = :v;
COUNT(*)
----------
1
SQL> select * from table(dbms_xplan.display_cursor(null,null,'advanced'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
SQL_ID3rg5r8sghcvb3, child number 0
-------------------------------------
select count(*) from acs_test_tab where record_type = :v
Plan hash value: 2956728990
--------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost |
--------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 3 |
| 1 | SORT AGGREGATE | | 1 | 4 | |
|* 2 | INDEX RANGE SCAN| ACS_TEST_TAB_RECORD_TYPE_I | 2 | 8 | 3 |
--------------------------------------------------------------------------------
Predicate Information (identified by operation id)：
---------------------------------------------------
2 - access("RECORD_TYPE"=:V)
已選擇47行。
SQL> alter system flush shared_pool;
系統(tǒng)已更改。
SQL> alter system set optimizer_features_enable='11.2.0.3.1';
系統(tǒng)已更改。
SQL> var v number;
SQL> exec :v := 1;
PL/SQL 過程已成功完成。
SQL> select count(*) from acs_test_tab where record_type = :v;
COUNT(*)
----------
1
SQL> select * from table(dbms_xplan.display_cursor(null,null,'advanced'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
SQL_ID3rg5r8sghcvb3, child number 0
-------------------------------------
select count(*) from acs_test_tab where record_type = :v
Plan hash value: 2956728990
------------------------------------------------------------------------------------------------
| 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 |
------------------------------------------------------------------------------------------------
Peeked Binds (identified by position)：
--------------------------------------
1 - :V (NUMBER)： 1 --綁定變量窺探
Predicate Information (identified by operation id)：
---------------------------------------------------
2 - access("RECORD_TYPE"=:V)
已選擇49行。
SQL> alter system flush shared_pool;
系統(tǒng)已更改。
SQL> exec :v := 2;
PL/SQL 過程已成功完成。
SQL> select count(*) from acs_test_tab where record_type = :v;
COUNT(*)
----------
50000
SQL> select * from table(dbms_xplan.display_cursor(null,null,'advanced'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
SQL_ID3rg5r8sghcvb3, child number 0
-------------------------------------
select count(*) from acs_test_tab where record_type = :v
Plan hash value: 2957754476
----------------------------------------------------------------------------------------------------
| 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 |
----------------------------------------------------------------------------------------------------
Peeked Binds (identified by position)：
--------------------------------------
1 - :V (NUMBER)： 2 --綁定變量窺探，綁定變量會(huì)影響最初硬解析的執(zhí)行計(jì)劃
Predicate Information (identified by operation id)：
---------------------------------------------------
2 - filter("RECORD_TYPE"=:V)
已選擇49行。
使用綁定變量窺測的好處是：可以幫助優(yōu)化器在第一次硬解析時(shí)選擇最優(yōu)的執(zhí)行計(jì)劃。但是同時(shí)這也是其弊端：在第一次硬解析后，后面發(fā)生的所有解析都會(huì)使用第一次硬解析生成的執(zhí)行計(jì)劃，如果數(shù)據(jù)的分布是均勻的，問題不大，如果數(shù)據(jù)分布式傾斜的，那么第一次硬解析生成的執(zhí)行計(jì)劃未必是最優(yōu)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;
系統(tǒng)已更改。
SQL> var v number;
SQL> exec :v := 2;
PL/SQL 過程已成功完成。
SQL> select count(*) from acs_test_tab where record_type = :v;
COUNT(*)
----------
50000
SQL> select * from table(dbms_xplan.display_cursor);
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
SQL_ID3rg5r8sghcvb3, child number 0
-------------------------------------
select count(*) from acs_test_tab where record_type = :v
Plan hash value: 2957754476
----------------------------------------------------------------------------------------------------
| 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 |
----------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id)：
---------------------------------------------------
2 - filter("RECORD_TYPE"=:V)
已選擇19行。
SQL> exec :v := 1
PL/SQL 過程已成功完成。
SQL> select count(*) from acs_test_tab where record_type = :v;
COUNT(*)
----------
1
SQL> select * from table(dbms_xplan.display_cursor);
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
SQL_ID3rg5r8sghcvb3, child number 0
-------------------------------------
select count(*) from acs_test_tab where record_type = :v
Plan hash value: 2957754476
----------------------------------------------------------------------------------------------------
| 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 |
----------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id)：
---------------------------------------------------
2 - filter("RECORD_TYPE"=:V)
已選擇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
----------------------------------------------------------------------------------------------------
SQL_ID1pxm87f6yd0bp, child number 0
-------------------------------------
select count(*) from acs_test_tab where record_type = 1
Plan hash value: 2956728990
------------------------------------------------------------------------------------------------
| 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 |
------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id)：
---------------------------------------------------
2 - access("RECORD_TYPE"=1)
已選擇19行。
對于變量v的取值為1的執(zhí)行計(jì)劃和采用常量1的執(zhí)行計(jì)劃性能差距還是比較大的。
總結(jié)：oracle在9i后引入變量窺測技術(shù)，該技術(shù)對于數(shù)據(jù)分布均勻的數(shù)據(jù)是非常合適的，但是對于分布傾斜的數(shù)據(jù)或者在OLAP系統(tǒng)中是不建議使用的。
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