我们面临的问题是一个连接在几个表之间的查询。 即使表格中有数百条记录，计划也要进行合并加入，只考虑表格中的一条记录，请查看以下计划。 当使用合并排序计划时，查询将失败并出现临时空间问题。
oracle仅在作业加载到新创建的分区时选择合并计划。但其余的旧分区正在选择Hash Join，我们可以在几秒钟内获得结果。
获取信息。所有连接的表都具有相同的音量。
你能解释一下为什么会这样吗？Merg join( query hung)
-----------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
-----------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 712 | 36 (3)| 00:00:01 | | |
|* 1 | HASH JOIN | | 1 | 712 | 36 (3)| 00:00:01 | | |
| 2 | MERGE JOIN CARTESIAN | | 1 | 679 | 28 (0)| 00:00:01 | | |
| 3 | MERGE JOIN CARTESIAN | | 1 | 615 | 21 (0)| 00:00:01 | | |
| 4 | MERGE JOIN CARTESIAN | | 1 | 388 | 14 (0)| 00:00:01 | | |
| 5 | PARTITION RANGE SINGLE | | 1 | 105 | 7 (0)| 00:00:01 | 4 | 4 |
|* 6 | TABLE ACCESS FULL | PCA_DCM_CLNT_BSPKE_REFS_M_LND | 1 | 105 | 7 (0)| 00:00:01 | 4 | 4 |
| 7 | BUFFER SORT | | 1 | 283 | 7 (0)| 00:00:01 | | |
| 8 | PARTITION RANGE SINGLE| | 1 | 283 | 7 (0)| 00:00:01 | 4 | 4 |
|* 9 | TABLE ACCESS FULL | PCA_DCM_INDBTDNS_BLK_M_LND | 1 | 283 | 7 (0)| 00:00:01 | 4 | 4 |
| 10 | BUFFER SORT | | 1 | 227 | 14 (0)| 00:00:01 | | |
| 11 | PARTITION RANGE SINGLE | | 1 | 227 | 7 (0)| 00:00:01 | 4 | 4 |
|* 12 | TABLE ACCESS FULL | PCA_DCM_DELPHI_BLK_M_LND | 1 | 227 | 7 (0)| 00:00:01 | 4 | 4 |
| 13 | BUFFER SORT | | 1 | 64 | 21 (0)| 00:00:01 | | |
| 14 | PARTITION RANGE SINGLE | | 1 | 64 | 7 (0)| 00:00:01 | 4 | 4 |
|* 15 | TABLE ACCESS FULL | PCA_DCM_APACS_BLK_M_LND | 1 | 64 | 7 (0)| 00:00:01 | 4 | 4 |
| 16 | PARTITION RANGE SINGLE | | 1 | 33 | 7 (0)| 00:00:01 | 4 | 4 |
|* 17 | TABLE ACCESS FULL | PCA_DCM_SCORE_BLK_M_LND | 1 | 33 | 7 (0)| 00:00:01 | 4 | 4 |
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哈希联接（我们得到结果的时间很短）
----------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
----------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 491 | 341K| 74 (3)| 00:00:01 | | |
|* 1 | HASH JOIN | | 491 | 341K| 74 (3)| 00:00:01 | | |
| 2 | PARTITION RANGE SINGLE | | 493 | 109K| 14 (0)| 00:00:01 | 3 | 3 |
|* 3 | TABLE ACCESS FULL | PCA_DCM_DELPHI_BLK_M_LND | 493 | 109K| 14 (0)| 00:00:01 | 3 | 3 |
|* 4 | HASH JOIN | | 491 | 232K| 60 (4)| 00:00:01 | | |
|* 5 | HASH JOIN | | 492 | 99384 | 45 (3)| 00:00:01 | | |
|* 6 | HASH JOIN | | 492 | 47724 | 31 (4)| 00:00:01 | | |
| 7 | PARTITION RANGE SINGLE| | 493 | 16269 | 14 (0)| 00:00:01 | 3 | 3 |
|* 8 | TABLE ACCESS FULL | PCA_DCM_SCORE_BLK_M_LND | 493 | 16269 | 14 (0)| 00:00:01 | 3 | 3 |
| 9 | PARTITION RANGE SINGLE| | 493 | 31552 | 16 (0)| 00:00:01 | 3 | 3 |
|* 10 | TABLE ACCESS FULL | PCA_DCM_APACS_BLK_M_LND | 493 | 31552 | 16 (0)| 00:00:01 | 3 | 3 |
| 11 | PARTITION RANGE SINGLE | | 493 | 51765 | 14 (0)| 00:00:01 | 3 | 3 |
|* 12 | TABLE ACCESS FULL | PCA_DCM_CLNT_BSPKE_REFS_M_LND | 493 | 51765 | 14 (0)| 00:00:01 | 3 | 3 |
| 13 | PARTITION RANGE SINGLE | | 493 | 136K| 14 (0)| 00:00:01 | 3 | 3 |
|* 14 | TABLE ACCESS FULL | PCA_DCM_INDBTDNS_BLK_M_LND | 493 | 136K| 14 (0)| 00:00:01 | 3 | 3 |
----------------------------------------------------------------------------------------------------------------------------
请查询
SELECT
substr(BLK.ACC_NUM,1,14) AS ACCOUNT_NUMBER,
CASE WHEN SUBSTR(BLK.ACC_NUM,20,1) = '1' THEN 'F1'
WHEN SUBSTR(BLK.ACC_NUM,20,1) = ' ' THEN 'F1'
WHEN SUBSTR(BLK.ACC_NUM,20,1) = '0' THEN 'F1'
WHEN SUBSTR(BLK.ACC_NUM,20,1) = '2' THEN 'F2'
END FLTR,
DELPHI.ND_SPA_CII_SPA
FROM
BUR_LND.PCA_DCM_SCORE_BLK_M_LND BLK
INNER JOIN BUR_LND.PCA_DCM_CLNT_BSPKE_REFS_M_LND REFFS
ON BLK.ACC_NUM= REFFS.ACC_NUM
INNER JOIN BUR_LND.PCA_DCM_INDBTDNS_BLK_M_LND IND
ON BLK.ACC_NUM= IND.ACC_NUM
INNER JOIN BUR_LND.PCA_DCM_DELPHI_BLK_M_LND DELPHI
ON BLK.ACC_NUM= DELPHI.ACC_NUM
INNER JOIN BUR_LND.PCA_DCM_APACS_BLK_M_LND APACS
ON BLK.ACC_NUM= APACS.ACC_NUM
WHERE
BLK.EFF_DT=TO_DATE('2013-10-30','YYYY-MM-DD')
AND REFFS.EFF_DT=TO_DATE('2013-10-30','YYYY-MM-DD')
AND IND.EFF_DT=TO_DATE('2013-10-30','YYYY-MM-DD')
AND DELPHI.EFF_DT=TO_DATE('2013-10-30','YYYY-MM-DD')
AND APACS.EFF_DT=TO_DATE('2013-10-30','YYYY-MM-DD')
提前感谢您的帮助。
由于 arkesh
答案 0 :(得分：1)
计划很糟糕，因为新分区缺少统计信息。应在分区更改后更新统计信息，最好使用增量统计信息。如果那不可能，那么像/*+ dynamic_sampling(4) */这样的提示可以提供帮助。
统计数据可能准确，不准确或缺失。由于动态采样，缺少统计数据通常不是一个大问题。使用默认动态采样级别2，如果语句包含没有统计信息的表，Oracle将自动收集统计信息。
不幸的是，对于这种情况，Oracle仅考虑每个表缺少统计信息，而不是每个分区。 （这将是一个很好的功能请求，但这对你现在没有帮助。）使用SQL语句中的文字，Oracle似乎确切地知道要查看哪个分区。由于没有该分区的统计信息，它假设有没有行，导致糟糕的计划。
示例
创建一个包含1000行但没有收集统计信息的样本分区表。
create table partition_test
(
a number,
b number
)
partition by range (a)
(
partition p1 values less than (2)
);
insert into partition_test select 1, level from dual connect by level <= 1000;
如果没有统计信息，Oracle将自动使用动态采样并获得良好的行数。你无法在这个简单的计划中看到它，但通常这会带来更好的访问方法和更好的连接操作。
explain plan for select * from partition_test where a = 1 and b <= 1000;
select * from table(dbms_xplan.display);
Plan hash value: 4097357352
---------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
---------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1000 | 26000 | 16 (0)| 00:00:01 | | |
| 1 | PARTITION RANGE SINGLE| | 1000 | 26000 | 16 (0)| 00:00:01 | 1 | 1 |
|* 2 | TABLE ACCESS FULL | PARTITION_TEST | 1000 | 26000 | 16 (0)| 00:00:01 | 1 | 1 |
---------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("A"=1 AND "B"<=1000)
Note
-----
- dynamic statistics used: dynamic sampling (level=2)
收集统计信息，然后使用数据创建新分区。虽然该分区有1000行且没有统计信息，但Oracle并不知道这一点，只是假设它是空的。
begin
dbms_stats.gather_table_stats(user, 'partition_test');
end;
/
alter table partition_test add partition p2 values less than (3);
insert into partition_test select 2, level from dual connect by level <= 1000;
explain plan for select * from partition_test where a = 2 and b <= 1000;
select * from table(dbms_xplan.display);
Plan hash value: 4097357352
---------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
---------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 7 | 9 (0)| 00:00:01 | | |
| 1 | PARTITION RANGE SINGLE| | 1 | 7 | 9 (0)| 00:00:01 | 2 | 2 |
|* 2 | TABLE ACCESS FULL | PARTITION_TEST | 1 | 7 | 9 (0)| 00:00:01 | 2 | 2 |
---------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("A"=2 AND "B"<=1000)
明确请求动态采样将修复基数估算，这可能会解决您的执行计划问题。
explain plan for select /*+ dynamic_sampling(4) */ * from partition_test where a = 2 and b <= 1000;
select * from table(dbms_xplan.display);
Plan hash value: 4097357352
---------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
---------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1000 | 7000 | 9 (0)| 00:00:01 | | |
| 1 | PARTITION RANGE SINGLE| | 1000 | 7000 | 9 (0)| 00:00:01 | 2 | 2 |
|* 2 | TABLE ACCESS FULL | PARTITION_TEST | 1000 | 7000 | 9 (0)| 00:00:01 | 2 | 2 |
---------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("A"=2 AND "B"<=1000)
Note
-----
- dynamic statistics used: dynamic sampling (level=4)