Oracle 11.2.0.1.0下ACS测试令人失望

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   在版本11.2.0.1.0，ACS（adaptiver cursor sharing）很令我失望，测试的结果是：

    1. 在绑定窥探有效的情况下，使用绑定变量，在直方图准确的情况下，CBO总是以执行的第一条SQL的执行计划为准。

    2. 在绑定窥探失效的情况下，使用绑定变量，CBO完全忽略直方图。

   在这个版本下的SQL，如果是偏于OLAP的SQL，还是不用使用绑定变量为佳，跟10g一样的处理方式。

SQL> select * from v$version;

--由于在11.2.0.1.0版本下用dbms_stats收集直方图不准确，所以用analyze

--在Oracle 11.2.0.1.0下dbms_stats.gather_table_stats收集直方图不准

SQL> set autotrace off

--使用绑定窥探，先使用查大部分数据的条件，可以看到两个都是全表扫描

--先看到绑定窥探是有效的

SQL> show parameter _optim_peek_user_binds;
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
_optim_peek_user_binds               boolean     TRUE
SQL> alter system flush shared_pool;
SQL> alter system flush buffer_cache;
SQL> var ccc varchar2(10);
SQL> exec :ccc:=‘100‘;
SQL> set autotrace trace stat
SQL> select * from test where name=:ccc;
已选择99999行。
统计信息
----------------------------------------------------------
        354  recursive calls
          0  db block gets
      13565  consistent gets
       6853  physical reads
          0  redo size
    1564422  bytes sent via SQL*Net to client
      73664  bytes received via SQL*Net from client
       6668  SQL*Net roundtrips to/from client
          7  sorts (memory)
          0  sorts (disk)
      99999  rows processed
SQL> set autotrace off
SQL> select hash_value, child_number from v$sql
    where sql_text = ‘select * from test where name=:ccc‘;
HASH_VALUE CHILD_NUMBER
---------- ------------
1301684711            0
SQL> select * from table(dbms_xplan.display_cursor(1301684711,0));
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
HASH_VALUE  1301684711, child number 0
--------------------------------------
select * from test where name=:ccc
Plan hash value: 1357081020
--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |       |       |  2664 (100)|          |
|*  1 |  TABLE ACCESS FULL| TEST | 99999 |    86M|  2664   (1)| 00:00:38 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("NAME"=:CCC)
SQL> exec :ccc:=‘200‘;
SQL> set autotrace trace stat
SQL> select * from test where name=:ccc;
统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
       6849  consistent gets
          0  physical reads
          0  redo size
       1342  bytes sent via SQL*Net to client
        338  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
SQL> set autotrace off

--可以看到执行两条SQL，都是一个执行计划
SQL> select hash_value, child_number from v$sql
    where sql_text = ‘select * from test where name=:ccc‘;
HASH_VALUE CHILD_NUMBER
---------- ------------
1301684711            0

--使用绑定窥探，先使用查小部分数据的条件

SQL> alter system flush shared_pool;
SQL> alter system flush buffer_cache;
SQL> select hash_value, child_number from v$sql
    where sql_text = ‘select * from test where name=:ccc‘;
未选定行
SQL> exec :ccc:=‘200‘;
SQL> set autotrace trace stat
SQL> select * from test where name=:ccc;
统计信息
----------------------------------------------------------
        354  recursive calls
          0  db block gets
         53  consistent gets
         27  physical reads
          0  redo size
       1342  bytes sent via SQL*Net to client
        338  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          7  sorts (memory)
          0  sorts (disk)
          1  rows processed
SQL> set autotrace off
SQL> select hash_value, child_number from v$sql
    where sql_text = ‘select * from test where name=:ccc‘;
HASH_VALUE CHILD_NUMBER
---------- ------------
1301684711            0
SQL> select * from table(dbms_xplan.display_cursor(1301684711,0));
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------
HASH_VALUE  1301684711, child number 0
--------------------------------------
select * from test where name=:ccc
Plan hash value: 688048857
----------------------------------------------------------------------------------------
| Id  | Operation                   | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |          |       |       |     2 (100)|          |
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST     |     1 |   909 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | IND_NAME |     1 |       |     1   (0)| 00:00:01 |
----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("NAME"=:CCC)
SQL> exec :ccc:=‘100‘;
SQL> set autotrace trace stat
SQL> select * from test where name=:ccc;
已选择99999行。
统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
      20106  consistent gets
       6884  physical reads
          0  redo size
    1564422  bytes sent via SQL*Net to client
      73664  bytes received via SQL*Net from client
       6668  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
      99999  rows processed
SQL> set autotrace off

--可以看到执行两条SQL，都是一个执行计划
SQL> select hash_value, child_number from v$sql
    where sql_text = ‘select * from test where name=:ccc‘;
HASH_VALUE CHILD_NUMBER
---------- ------------
1301684711            0

--不使用绑定窥探，先使用查小部分数据的条件

SQL> alter session set "_optim_peek_user_binds"=false;
SQL> alter system flush shared_pool;
SQL> alter system flush buffer_cache;
SQL> select hash_value, child_number from v$sql
    where sql_text = ‘select * from test where name=:ccc‘;
SQL> exec :ccc:=‘200‘;
SQL> set autotrace trace stat
SQL> select * from test where name=:ccc;
统计信息
----------------------------------------------------------
        354  recursive calls
          0  db block gets
       6899  consistent gets
       6853  physical reads
          0  redo size
       1342  bytes sent via SQL*Net to client
        338  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          7  sorts (memory)
          0  sorts (disk)
          1  rows processed
SQL> set autotrace off
SQL> select hash_value, child_number from v$sql
    where sql_text = ‘select * from test where name=:ccc‘;
HASH_VALUE CHILD_NUMBER
---------- ------------
1301684711            0
--看到是全表扫描，正确的是索引扫描
SQL> select * from table(dbms_xplan.display_cursor(1301684711,0));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------
HASH_VALUE  1301684711, child number 0
--------------------------------------
select * from test where name=:ccc
Plan hash value: 1357081020
--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |       |       |  2663 (100)|          |
|*  1 |  TABLE ACCESS FULL| TEST | 50000 |    43M|  2663   (1)| 00:00:38 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("NAME"=:CCC)
SQL> exec :ccc:=‘100‘;
SQL> set autotrace trace stat
SQL> select * from test where name=:ccc;
已选择99999行。
统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
      13515  consistent gets
          0  physical reads
          0  redo size
    1564422  bytes sent via SQL*Net to client
      73664  bytes received via SQL*Net from client
       6668  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
      99999  rows processed
SQL> set autotrace off
SQL> select hash_value, child_number from v$sql
    where sql_text = ‘select * from test where name=:ccc‘;
HASH_VALUE CHILD_NUMBER
---------- ------------
1301684711            0

Oracle 11.2.0.1.0下ACS测试令人失望

标签：des   style   blog   http   io   ar   color   os   使用   

原文地址：http://blog.csdn.net/stevendbaguo/article/details/41847829