ORACLE谓词推入初解

oracle谓词推入:所谓推入,是把库表中判断的条件推入进视图中进行判断。后续不再对库表进行筛选。

创建视图

create or replace view haoview as
select hao1.* from hao1,hao2
where hao1.object_id=hao2.object_id;
那么对于这样一个简单的查询,可见谓词hao3.object_name=haoview.object_name被merge到了view中:
select hao3.object_name
from hao3,haoview
where hao3.object_name=haoview.object_name
and hao3.object_id=999;

-----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 44 | 129 (3)| 00:00:02 |
| 1 | NESTED LOOPS | | 1 | 44 | 129 (3)| 00:00:02 |
|* 2 | HASH JOIN | | 1 | 40 | 128 (3)| 00:00:02 |
| 3 | TABLE ACCESS BY INDEX ROWID| HAO3 | 1 | 20 | 2 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | HAO3IDX | 1 | | 1 (0)| 00:00:01 |
| 5 | TABLE ACCESS FULL | HAO1 | 36311 | 709K| 125 (2)| 00:00:02 |
|* 6 | INDEX RANGE SCAN | HAO2IDX | 1 | 4 | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("HAO3"."OBJECT_NAME"="HAO1"."OBJECT_NAME")
4 - access("HAO3"."OBJECT_ID"=999)
6 - access("HAO1"."OBJECT_ID"="HAO2"."OBJECT_ID")


接着,我把haoview放到outer join的右侧,这是haoview就属于unmergeable view了,

优化器默认无法将谓词merge进这个haoview中,于是就看到了haoview单独先执行:

select hao3.object_name
from hao3,haoview
where hao3.object_name=haoview.object_name(+)
and hao3.object_id=999;
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 86 | 153 (5)| 00:00:02 |
|* 1 | HASH JOIN OUTER | | 1 | 86 | 153 (5)| 00:00:02 |
| 2 | TABLE ACCESS BY INDEX ROWID| HAO3 | 1 | 20 | 2 (0)| 00:00:01 |
|* 3 | INDEX RANGE SCAN | HAO3IDX | 1 | | 1 (0)| 00:00:01 |
| 4 | VIEW | HAOVIEW | 36309 | 2340K| 150 (4)| 00:00:02 |
|* 5 | HASH JOIN | | 36309 | 850K| 150 (4)| 00:00:02 |
| 6 | INDEX FAST FULL SCAN | HAO2IDX | 36309 | 141K| 22 (5)| 00:00:01 |
| 7 | TABLE ACCESS FULL | HAO1 | 36311 | 709K| 125 (2)| 00:00:02 |
----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("HAO3"."OBJECT_NAME"="HAOVIEW"."OBJECT_NAME"(+))
3 - access("HAO3"."OBJECT_ID"=999)
5 - access("HAO1"."OBJECT_ID"="HAO2"."OBJECT_ID")
接着,我们来使用这里的hint push_pred强制优化器将谓词merge进view中,可见到“VIEW PUSHED PREDICATE”:
select /*+push_pred(haoview)*/ hao3.object_name
from hao3,haoview
where hao3.object_name=haoview.object_name(+)
and hao3.object_id=999;

----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 40 | 128 (2)| 00:00:02 |
| 1 | NESTED LOOPS OUTER | | 1 | 40 | 128 (2)| 00:00:02 |
| 2 | TABLE ACCESS BY INDEX ROWID| HAO3 | 1 | 36 | 2 (0)| 00:00:01 |
|* 3 | INDEX RANGE SCAN | HAO3IDX | 1 | | 1 (0)| 00:00:01 |
| 4 | VIEW PUSHED PREDICATE | HAOVIEW | 1 | 4 | 126 (2)| 00:00:02 |
| 5 | NESTED LOOPS | | 1 | 24 | 126 (2)| 00:00:02 |
|* 6 | TABLE ACCESS FULL | HAO1 | 1 | 20 | 125 (2)| 00:00:02 |
|* 7 | INDEX RANGE SCAN | HAO2IDX | 1 | 4 | 1 (0)| 00:00:01 |
----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("HAO3"."OBJECT_ID"=999)
6 - filter("HAO1"."OBJECT_NAME"="HAO3"."OBJECT_NAME")
7 - access("HAO1"."OBJECT_ID"="HAO2"."OBJECT_ID")
是,会有同学问,那么merge hint能否有同样的效果呢?答案是,对于这种unmergeable view来说,merge hint无效。
select /*+merge(haoview)*/ hao3.object_name
from hao3,haoview
where hao3.object_name=haoview.object_name(+)
and hao3.object_id=999;

----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 86 | 153 (5)| 00:00:02 |
|* 1 | HASH JOIN OUTER | | 1 | 86 | 153 (5)| 00:00:02 |
| 2 | TABLE ACCESS BY INDEX ROWID| HAO3 | 1 | 20 | 2 (0)| 00:00:01 |
|* 3 | INDEX RANGE SCAN | HAO3IDX | 1 | | 1 (0)| 00:00:01 |
| 4 | VIEW | HAOVIEW | 36309 | 2340K| 150 (4)| 00:00:02 |
|* 5 | HASH JOIN | | 36309 | 850K| 150 (4)| 00:00:02 |
| 6 | INDEX FAST FULL SCAN | HAO2IDX | 36309 | 141K| 22 (5)| 00:00:01 |
| 7 | TABLE ACCESS FULL | HAO1 | 36311 | 709K| 125 (2)| 00:00:02 |
----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("HAO3"."OBJECT_NAME"="HAOVIEW"."OBJECT_NAME"(+))
3 - access("HAO3"."OBJECT_ID"=999)
5 - access("HAO1"."OBJECT_ID"="HAO2"."OBJECT_ID")

可见,对于此种身处outger join右侧的view来说,merge hint已经无能为力了。

 

---后续忽略

1、创建视图

SQL> create view v_p as select * from emp where deptno=10;
View created.
SQL> select * from v_p   ;--视图走全表
Execution Plan
----------------------------------------------------------
Plan hash value: 3956160932

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     3 |   117 |     3   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| EMP  |     3 |   117 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("DEPTNO"=10)
Statistics
----------------------------------------------------------
          8  recursive calls
          0  db block gets
         10  consistent gets
          0  physical reads
          0  redo size
       1109  bytes sent via SQL*Net to client
        492  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          3  rows processed
SQL> select empno,dname from v_p,dept
  2    where v_p.deptno=dept.deptno; --此语句有谓词推入

Execution Plan
----------------------------------------------------------
Plan hash value: 568005898

--------------------------------------------------------------------------------
--------

| Id  | Operation                    | Name    | Rows  | Bytes | Cost (%CPU)| Ti
me     |

--------------------------------------------------------------------------------
--------

|   0 | SELECT STATEMENT             |         |     3 |    54 |     4   (0)| 00
:00:01 |

|   1 |  NESTED LOOPS  (嵌套循环、效率一般不高)              |         |     3 |    54 |     4   (0)| 00
:00:01 |

|   2 |   TABLE ACCESS BY INDEX ROWID| DEPT    |     1 |    11 |     1   (0)| 00
:00:01 |

|*  3 |    INDEX UNIQUE SCAN         | PK_DEPT |     1 |       |     0   (0)| 00
:00:01 |

|*  4 |   TABLE ACCESS FULL          | EMP     |     3 |    21 |     3   (0)| 00
:00:01 |

--------------------------------------------------------------------------------
--------
Predicate Information (identified by operation id):
---------------------------------------------------
   3 - access("DEPT"."DEPTNO"=10) --筛选语句从emp.deptno=10推送至dept表的deptno。
   4 - filter("DEPTNO"=10)
Statistics
----------------------------------------------------------
          8  recursive calls
          0  db block gets
         12  consistent gets
          0  physical reads
          0  redo size
        662  bytes sent via SQL*Net to client
        492  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          3  rows processed

使用hint,强制sql走hash连接(是否使用谓词推入要看具体的需求、慎用!!)
SQL> select /*+use_hash(v_p,dept)*/empno,dname from v_p,dept
  2    where v_p.deptno=dept.deptno;
Execution Plan
----------------------------------------------------------
Plan hash value: 2263032238
--------------------------------------------------------------------------------
--------

| Id  | Operation                    | Name    | Rows  | Bytes | Cost (%CPU)| Ti
me     |
--------------------------------------------------------------------------------
--------
|   0 | SELECT STATEMENT             |         |     3 |    54 |     5  (20)| 00
:00:01 |

|*  1 |  HASH JOIN                   |         |     3 |    54 |     5  (20)| 00
:00:01 |

|   2 |   TABLE ACCESS BY INDEX ROWID| DEPT    |     1 |    11 |     1   (0)| 00
:00:01 |

|*  3 |    INDEX UNIQUE SCAN         | PK_DEPT |     1 |       |     0   (0)| 00
:00:01 |

|*  4 |   TABLE ACCESS FULL          | EMP     |     3 |    21 |     3   (0)| 00
:00:01 |
--------------------------------------------------------------------------------
--------
Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("DEPTNO"="DEPT"."DEPTNO")
   3 - access("DEPT"."DEPTNO"=10)
   4 - filter("DEPTNO"=10)
Statistics
----------------------------------------------------------
          8  recursive calls
          0  db block gets
         12  consistent gets
          0  physical reads
          0  redo size
        662  bytes sent via SQL*Net to client
        492  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          3  rows processed

Oracle数据库中,**谓词越界(predicate out of range)** 是一个与查询优化和统计信息相关的性能问题。它通常发生在查询条件中的值超出了表列的统计范围,导致优化器无法准确估算查询结果集的大小,从而可能选择不理想的执行计划。 ### 问题释 当SQL查询中使用了某个列上的过滤条件(例如 `WHERE column = value`),而该值超出了该列在统计信息中的最大或最小值时,就会触发“谓词越界”现象。这种情况下,优化器可能会低估或高估返回的行数[^1]。 例如,假设某列 `col1` 的统计信息显示其最大值为 100,但查询使用了 `WHERE col1 = 200`,此时 Oracle 就会标记该谓词为“out of range”。 此行为会影响以下方面: - **执行计划的选择**:优化器基于错误的行数预估可能导致索引扫描与全表扫描之间的误判。 - **性能下降**:不准确的统计信息会导致次优执行计划,影响查询响应时间。 ### 决方案 #### 1. 更新统计信息 最直接的方法是重新收集相关对象的统计信息,确保它们反映最新的数据分布情况。 ```sql EXEC DBMS_STATS.GATHER_TABLE_STATS('SCHEMA_NAME', 'TABLE_NAME'); ``` 如果表有分区,应考虑对特定分区进行统计更新: ```sql EXEC DBMS_STATS.GATHER_TABLE_STATS('SCHEMA_NAME', 'TABLE_NAME', PARTNAME=>'PARTITION_NAME'); ``` #### 2. 使用直方图 对于数据分布不均匀的列,创建频率直方图可以帮助优化器更准确地评估谓词的选择性。 ```sql EXEC DBMS_STATS.GATHER_TABLE_STATS('SCHEMA_NAME', 'TABLE_NAME', METHOD_OPT=>'FOR COLUMNS SIZE AUTO COLUMN_NAME'); ``` #### 3. 固定统计信息(适用于静态数据) 如果某些列的数据很少变化,可以考虑手动锁定统计信息以防止自动收集过程覆盖这些值。 ```sql EXEC DBMS_STATS.LOCK_TABLE_STATS('SCHEMA_NAME', 'TABLE_NAME'); ``` #### 4. 使用动态采样(Dynamic Sampling) 在查询执行前,Oracle 可以通过动态采样临时获取缺失的统计信息,有助于优化器做出更好的决策。 ```sql SELECT /*+ DYNAMIC_SAMPLING(4) */ * FROM table_name WHERE column = value; ``` #### 5. 查询重写 如果某些查询频繁使用超出范围的值,考虑将这些值映射到已知的有效范围内,或者引入额外的条件来引导优化器。 --- ### 性能监控建议 可以通过以下方式识别是否发生了谓词越界问题: - 检查执行计划中的 `cardinality` 是否明显偏离际返回行数。 - 分析 AWR 报告中 SQL 执行效率偏低的语句。 - 启用 10053 事件跟踪优化器决策过程,查看谓词如何被处理。 ---
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