Question

我有一个简单的查询，它一直在运行。有一个日期条件，一旦我删除，查询返回结果。它是一个格式为'31 -MAR-15'的日期字段。我不明白为什么这种情况会使查询变得如此缓慢。提前谢谢。

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

在TABLE1上应用这4个谓词后，看起来优化器显着低估了返回的行数。

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

（稍微偏离主题：使用ANSI文字date '2015-03-31'代替隐式转换后的字符串'31-MAR-15'会更安全。而且语句有一些错误，比如错过前两个错误之间的条件谓词并错过最后一个谓词前面的A.。）

首先，确保所有表格都有准确的统计数据，看看是否会改变解释计划：

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

“智能列”ID使得很难估计应用条件后返回的行数。如果更改数据模型为时已晚，您至少可以为Oracle提供一些扩展统计信息来帮助它处理谓词：

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'是一个受欢迎的值，但如果没有扩展统计数据，Oracle就不会知道这一点。额外的直方图可以显着增加基数。然后优化器不会选择两个不相关的表之间的笛卡尔连接。

Answer 2

我在WHERE子句中简化了 A.ID 字段

的条件

A.ID LIKE 'US_20150303%'

与

具有相同的效果

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

并且，如果列 A.ID 被编入索引，则应用 SUBSTR（a.ID，..）函数的事实会使索引无效。

另一方面， a.dump_dt 似乎是DATE类型列，因此在此列上应用过滤器的首选方法可能是

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

而不是

a.dump_dt = '31-MAR-15'

后者主要取决于运行查询的Oracle客户端的NLS_DATE_FORMAT，并且在某些情况下可能会忽略使用 a.dump_dt 上的索引而对性能产生负面影响。

所以重写的查询如下所示：

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);

Answer 3

尝试使用oracle提示来稳定选择计划，或者您可以使用该技巧：

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

Oracle - 查询运行速度很慢

3 个答案: