Oracle - 查询运行速度很慢

Question

我有一个简单的查询正在运行。有一个日期条件，我一旦删除，查询就返回结果。它的格式为'31 -MAR-15'的日期字段。我不明白为什么这种情况会使查询速度变慢。提前致谢。

SELECT 
    substr(a.id, 1, 2) AS country, 
    count(DISTINCT a.id) AS id_count, 
    sum(a.amount)  AS amount 
FROM table1 a 
    JOIN table2 b ON a.id = b.id 
    JOIN table3 c ON b.party_id = c.party_id 
WHERE a.prod_type = 'INS' 
    AND c.acct_type = 'LON' 
    AND substr(a.id, 1, 2) = 'US' 
    AND a.dump_dt = '31-MAR-15' 
    AND substr(id, 4, 8) = '20150303' 
GROUP BY substr(a.id, 1, 2); 

解释计划：

PLAN_TABLE_OUTPUT 
Plan hash value: 255044277 

------------------------------------------------------------------------------------------------------------ 
| Id | Operation       | Name     | Rows | Bytes | Cost (%CPU)| Time  | 
------------------------------------------------------------------------------------------------------------ 
| 0 | SELECT STATEMENT     |      |  1 | 121 | 125K (1)| 00:25:08 | 
| 1 | HASH GROUP BY     |      |  1 | 121 | 125K (1)| 00:25:08 | 
| 2 | VIEW       | VW_DAG_0    |  1 | 121 | 125K (1)| 00:25:08 | 
| 3 | HASH GROUP BY     |      |  1 | 98 | 125K (1)| 00:25:08 | 
| 4 |  NESTED LOOPS     |      |  |  |   |   | 
| 5 |  NESTED LOOPS     |      |  1 | 98 | 125K (1)| 00:25:08 | 
| 6 |  MERGE JOIN CARTESIAN  |      | 12613 | 800K| 21133 (2)| 00:04:14 | 
|* 7 |  TABLE ACCESS BY INDEX ROWID| TABLE1     |  1 | 45 | 46 (0)| 00:00:01 | 
|* 8 |   INDEX RANGE SCAN   | DATA_DATE__STG_BACKUP2 | 1040 |  |  6 (0)| 00:00:01 | 
| 9 |  BUFFER SORT    |      | 182K| 3564K| 21087 (2)| 00:04:14 | 
|* 10 |   TABLE ACCESS FULL   | TABLE3     | 182K| 3564K| 21087 (2)| 00:04:14 | 
|* 11 |  INDEX RANGE SCAN   | BSB_PARTYID_IDX  | 22 |  |  3 (0)| 00:00:01 | 
|* 12 |  TABLE ACCESS BY INDEX ROWID | TABLE2     |  1 | 33 | 10 (0)| 00:00:01 | 
------------------------------------------------------------------------------------------------------------ 

Predicate Information (identified by operation id): 
--------------------------------------------------- 

    7-filter(SUBSTR(A.ID, 4, 8) = '20150303' AND SUBSTR(A.ID, 1, 2) = 'US' 
       AND A.PROD_TYPE = 'INS') 
    8 - access(A.DUMP_DT = '31-MAR-15') 
    10 - filter(C.ACCT_TYPE = 'LON') 
    11 – access(B.PARTY_ID = C.PARTY_ID) 
    12 - filter(A.ID = B.ID) 

Answer 1

貌似优化器是显著低估的行数上TABLE1应用这些4个谓词后返回。

A.PROD_TYPE = 'INS' 
SUBSTR(A.ID, 1, 2) = 'US' 
A.DUMP_DT = '31-MAR-15' 
SUBSTR(ID, 4, 8) = '20150303' 

（略题外话：它的安全使用ANSI文字date '2015-03-31'而不是隐式转换字符串'31-MAR-15'和声明有一些错误，好像缺少了前2个谓语之间的状态和失踪的。 A.在过去的谓语前）

首先，确保有对所有表准确的统计数据，看看是否改变了解释计划：

begin 
    dbms_stats.gather_table_stats(user, 'TABLE1'); 
    dbms_stats.gather_table_stats(user, 'TABLE2'); 
    dbms_stats.gather_table_stats(user, 'TABLE3'); 
end; 
/

的“SMA rt列“，ID，使得很难估计应用条件后返回的行数。如果是来不及更改数据模型，你至少可以为Oracle提供了一些扩展统计信息，以帮助其应对谓词：

select dbms_stats.create_extended_stats(user, 'TABLE1', '(SUBSTR(ID, 1, 2))') from dual; 
select dbms_stats.create_extended_stats(user, 'TABLE1', '(SUBSTR(ID, 4, 8))') from dual; 

我猜测SUBSTR(A.ID, 1, 2) = 'US'是一种流行的价值，但没有扩展统计信息Oracle不会知道这一点。额外的直方图可能会显着增加基数。然后优化器不会选择两个不相关表之间的笛卡尔连接。

Answer 2

尝试使用Oracle提示来选择稳定计划或您可以使用招：

.... 
And A.DUMP_DT+0 = to_date('31-MAR-15','dd-mon- rr') 
... 

Answer 3

我在WHERE子句超过A.ID场

A.ID LIKE 'US_20150303%' 

简化条件与

substr(a.id, 1, 2) = 'US' AND substr(id, 4, 8) = '20150303' 

相同的效果，并且在情况下列A.ID被索引，应用SUBSTR（a.ID，..）函数的事实使索引无用。

在另一方面，a.dump_dt似乎是一个DATE类型列，所以在此列应用过滤器的优选方式可能是

a.dump_dt = TO_DATE('31-MAR-15', 'DD-MON-RR') 

，而不是

a.dump_dt = '31-MAR-15' 

后者主要依赖于运行查询的Oracle客户端的NLS_DATE_FORMAT，并且在某些情况下，可能会通过忽略对索引的使用而对性能产生负面影响，该索引的使用范围为a.dump_dt。

因此改写查询看起来是这样的：

SELECT 
    SUBSTR(A.ID, 1, 2) AS country, 
    COUNT(DISTINCT A.ID) AS id_count, 
    SUM(A.amount)  AS amount 
FROM table1 A 
    JOIN table2 b ON A.ID = b.ID 
    JOIN table3 c ON b.party_id = c.party_id 
WHERE A.prod_type = 'INS' 
    AND c.acct_type = 'LON' 
    AND A.ID LIKE 'US_20150303%' 
    AND A.dump_dt = TO_DATE('31-MAR-15', 'DD-MON-RR') 
GROUP BY SUBSTR(A.ID, 1, 2);