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在升级到 Oracle 11g 后，我在优化 Oracle 查询时遇到了问题，这个问题开始让我有点抓狂。

请注意，这个问题现在已经完全编辑，因为我在创建一个简单的测试用例后获得了更多信息。原始问题可在此处获得：https ://stackoverflow.com/revisions/12304320/1 。

这个问题是，当连接两个表时，其中一个表between在日期列上有条件，如果查询连接到远程表，则不会发生绑定窥视。

这是一个帮助重现问题的测试用例。首先设置两个源表。第一个是日期列表，是本月的第一天，可以追溯到三十年前

create table mike_temp_etl_control
as 
select
  add_months(trunc(sysdate, 'MM'), 1-row_count) as reporting_date
from (
  select level as row_count
  from dual
  connect by level < 360
);

然后一些数据来自dba_objects：

create table mike_temp_dba_objects as
select owner, object_name, subobject_name, object_id, created
from dba_objects
union all
select owner, object_name, subobject_name, object_id, created
from dba_objects;

然后创建一个空表来运行数据：

create table mike_temp_1
as
select 
  a.OWNER,
  a.OBJECT_NAME,
  a.SUBOBJECT_NAME,
  a.OBJECT_ID,
  a.CREATED,
  b.REPORTING_DATE
from 
  mike_temp_dba_objects a
  join mike_temp_etl_control b on (
      b.reporting_date between add_months(a.created, -24) and a.created)
  where 1=2;

然后运行代码。您可能需要创建一个更大的版本 mike_temp_dba_objects 以减慢查询速度（或使用其他方法来获取执行计划）。在查询运行时，我通过从不同的会话运行从会话中获取执行计划select * from table(dbms_xplan.display_cursor(sql_id => 'xxxxxxxxxxx'))。

declare
  pv_report_start_date date := date '2002-01-01';
  v_report_end_date date := date '2012-07-01';

begin

  INSERT /*+ APPEND */
  INTO mike_temp_5
  select 
    a.OWNER,
    a.OBJECT_NAME,
    a.SUBOBJECT_NAME,
    a.OBJECT_ID,
    a.CREATED,
    b.REPORTING_DATE
from 
  mike_temp_dba_objects a
  join mike_temp_etl_control b on (
    b.reporting_date between add_months(a.created, -24) and a.created)
  cross join dual@emirrl -- This line causes problems...
where 
  b.reporting_date between add_months(pv_report_start_date, -12) and v_report_end_date;

  rollback;  
end;

通过在查询中有一个远程表，mike_temp_etl_control 表的基数估计是完全错误的，并且绑定偷看似乎没有发生。

上面查询的执行计划如下所示：

---------------------------------------------------------------------------------------
| Id  | Operation                | Name                  | Rows  | Bytes | Cost (%CPU)|
---------------------------------------------------------------------------------------
|   0 | INSERT STATEMENT         |                       |       |       |   373 (100)|
|   1 |  LOAD AS SELECT          |                       |       |       |            |
|*  2 |   FILTER                 |                       |       |       |            |
|   3 |    MERGE JOIN            |                       |     5 |   655 |   373  (21)|
|   4 |     SORT JOIN            |                       |  1096 |   130K|   370  (20)|
|   5 |      MERGE JOIN CARTESIAN|                       |  1096 |   130K|   369  (20)|
|   6 |       REMOTE             | DUAL                  |     1 |       |     2   (0)|
|   7 |       BUFFER SORT        |                       |  1096 |   130K|   367  (20)|
|*  8 |        TABLE ACCESS FULL | MIKE_TEMP_DBA_OBJECTS |  1096 |   130K|   367  (20)|
|*  9 |     FILTER               |                       |       |       |            |
|* 10 |      SORT JOIN           |                       |     2 |    18 |     3  (34)|
|* 11 |       TABLE ACCESS FULL  | MIKE_TEMP_ETL_CONTROL |     2 |    18 |     2   (0)|
---------------------------------------------------------------------------------------

如果我然后用dual本地版本替换遥控器，我会得到正确的基数（139 而不是 2）：

-------------------------------------------------------------------------------------
| Id  | Operation              | Name                  | Rows  | Bytes | Cost (%CPU)|
-------------------------------------------------------------------------------------
|   0 | INSERT STATEMENT       |                       |       |       | 10682 (100)|
|   1 |  LOAD AS SELECT        |                       |       |       |            |
|*  2 |   FILTER               |                       |       |       |            |
|   3 |    MERGE JOIN          |                       |   152K|    19M| 10682   (3)|
|   4 |     SORT JOIN          |                       |   438K|    51M| 10632   (2)|
|   5 |      NESTED LOOPS      |                       |   438K|    51M|   369  (20)|
|   6 |       FAST DUAL        |                       |     1 |       |     2   (0)|
|*  7 |       TABLE ACCESS FULL| MIKE_TEMP_DBA_OBJECTS |   438K|    51M|   367  (20)|
|*  8 |     FILTER             |                       |       |       |            |
|*  9 |      SORT JOIN         |                       |   139 |  1251 |     3  (34)|
|* 10 |       TABLE ACCESS FULL| MIKE_TEMP_ETL_CONTROL |   139 |  1251 |     2   (0)|
-------------------------------------------------------------------------------------

所以，我想问题是我怎样才能得到正确的基数来估计？这是 Oracle 错误还是预期的行为？

4

1 回答 1

1

我认为你应该搞砸动态采样。它在 11g 中的工作方式不同，因此可能是您遇到麻烦的原因。

于 2012-09-07T12:18:34.893 回答