oracle – 为什么即使我禁用并行DML和并行DDL也会创建并行会话

在执行合并查询时,我禁用了并行DDL和DML
-in v $session创建了8个会话.

在执行合并查询时,我启用了并行DDL和DML
-in v $session创建了16个会话.

为什么会这样？对此有什么解释吗？

另外,我注意到如果启用了并行DDL和DML

>对于PARALLEL(a,2)：创建了总共4个会话
>对于PARALLEL(a,4)：创建了总共8个会话
>对于PARALLEL(a,8)：共创建了16个会话

ALTER SESSION DISABLE ALLALL QUERY;
ALTER SESSION DISABLE PARALLEL DML;
ALTER SESSION DISABLE PARALLEL DDL;

MERGE / * Parallel(a,8)* / BIGTABLE_1 a
使用BIGTABLE_2 b
ON(a.KEY = b.KEY)
当匹配更新时
SET a.Value1 = b.value1;

另外,在10g文档中我读到了这个

The default mode of a session is DISABLE PARALLEL DML. When parallel
DML is disabled,no DML will be executed in parallel even if the
PARALLEL hint is used.

https://docs.oracle.com/cd/B19306_01/server.102/b14223/usingpe.htm#CACCBEJC

提前致谢

alter session disable parallel dml;仅禁用语句的WRITE部分的并行性. READ部分可能仍然并行运行.由于这是MERGE操作,因此并行提示请求读写并行写入.此外,并行提示覆盖alter session disable parallel query;,即使它不覆盖alter session disable parallel dml;.

并行服务器的数量将是所请求的并行度的两倍,以支持producer and consumer operations,以便充分利用互操作并行性.对结果进行分组或排序的查询将使用两倍的线程.在某些情况下,即使没有显式的GROUP BY或ORDER BY,也可能会发生这种情况,因为某些操作可能隐式需要排序.

样本表

create table bigtable_1(key number,value1 number);
create table bigtable_2(key number,value1 number);

并行读写

注意操作#1的PX COORDINATOR.当该步骤高于MERGE时,意味着写入是并行完成的.

rollback;
alter session enable parallel dml;
alter session enable parallel query;
explain plan for merge /*+ parallel(a,8) */ into bigtable_1 a using bigtable_2 b
  on (a.key = b.key) when matched then update set a.value1 = b.value1;
select * from table(dbms_xplan.display(format => 'basic'));

Plan hash value: 827272579

------------------------------------------------------
| Id  | Operation                       | Name       |
------------------------------------------------------
|   0 | MERGE STATEMENT                 |            |
|   1 |  PX COORDINATOR                 |            |  <-- PARALLEL WRITE
|   2 |   PX SEND QC (RANDOM)           | :TQ10003   |
|   3 |    MERGE                        | BIGTABLE_1 |
|   4 |     PX RECEIVE                  |            |  <-- PARALLEL READ
|   5 |      PX SEND HYBRID (ROWID PKEY)| :TQ10002   |
|   6 |       VIEW                      |            |
|   7 |        HASH JOIN BUFFERED       |            |
|   8 |         BUFFER SORT             |            |
|   9 |          PX RECEIVE             |            |
|  10 |           PX SEND HASH          | :TQ10000   |
|  11 |            TABLE ACCESS FULL    | BIGTABLE_2 |
|  12 |         PX RECEIVE              |            |
|  13 |          PX SEND HASH           | :TQ10001   |
|  14 |           PX BLOCK ITERATOR     |            |
|  15 |            TABLE ACCESS FULL    | BIGTABLE_1 |
------------------------------------------------------

串行写入,并行读取

现在,MERGE操作首先是PX …操作.写操作是串行完成的,但读操作仍然是并行完成的.

rollback;
alter session disable parallel dml;
alter session disable parallel query;
explain plan for merge /*+ parallel(a,8) */ into bigtable_1 a using bigtable_2 b
  on (a.key = b.key) when matched then update set a.value1 = b.value1;
select * from table(dbms_xplan.display(format => 'basic'));

Plan hash value: 1648019208

------------------------------------------------
| Id  | Operation                 | Name       |
------------------------------------------------
|   0 | MERGE STATEMENT           |            |
|   1 |  MERGE                    | BIGTABLE_1 |  <-- SERIAL WRITE
|   2 |   PX COORDINATOR          |            |  <-- PARALLEL READ
|   3 |    PX SEND QC (RANDOM)    | :TQ10002   |
|   4 |     VIEW                  |            |
|   5 |      HASH JOIN BUFFERED   |            |
|   6 |       BUFFER SORT         |            |
|   7 |        PX RECEIVE         |            |
|   8 |         PX SEND HASH      | :TQ10000   |
|   9 |          TABLE ACCESS FULL| BIGTABLE_2 |
|  10 |       PX RECEIVE          |            |
|  11 |        PX SEND HASH       | :TQ10001   |
|  12 |         PX BLOCK ITERATOR |            |
|  13 |          TABLE ACCESS FULL| BIGTABLE_1 |
------------------------------------------------

（编辑：李大同）

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