oracle 并行之并行度篇

oracle 并行(并行度)

操作内并行使用的slave process数量就是并行度dop，index&table都有dop 作为默认操作并行度default 1表示不使用并行处理

SQL> create table t1 (a int) parallel 6;

Table created.

SQL> select degree from user_tables where table_name=’T1′;

DEGREE
———-
6

SQL> alter table t1 parallel 3;

Table altered.

SQL> select degree from user_tables where table_name=’T1′;

DEGREE
———-
3
*禁用alter table(index) parallel 1 (noprallel)
#create 时候使用parallel不仅会在创建table&index时使，后续的操作ddl,dml也会使用（如果只想建表时使用，建好后修改）

SQL> create table t2 (a int) parallel;（未指定并行度）

Table created.

SQL> select degree from user_tables where table_name=’T2′;

DEGREE
———-
DEFAULT ~~~这样使用default并行度=(cpu_count*parallel_threads_per_cpu)

SQL> show parameter cpu_count

NAME TYPE VALUE
———————————— ———– ——————————
cpu_count integer 2
SQL> show parameter parallel_thread

NAME TYPE VALUE
———————————— ———– ——————————
parallel_threads_per_cpu integer 2
SQL>

SQL> insert into t2 values (1);

1 row created.

SQL> commit;

Commit complete.

SQL> set autotrace trace exp
SQL> select * from t2;

Execution Plan
———————————————————-
Plan hash value: 1216610266

——————————————————————————–
——————————

——————————————————————————–
——————————

| 0 | SELECT STATEMENT | | 1 | 13 | 2 (0)| 00:00:01
| | | |

| 1 | PX COORDINATOR | | | | |
| | | |

| 2 | PX SEND QC (RANDOM)| :TQ10000 | 1 | 13 | 2 (0)| 00:00:01
| Q1,00 | P->S | QC (RAND) |

| 3 | PX BLOCK ITERATOR | | 1 | 13 | 2 (0)| 00:00:01
| Q1,00 | PCWC | |

| 4 | TABLE ACCESS FULL| T2 | 1 | 13 | 2 (0)| 00:00:01
| Q1,00 | PCWP | |

——————————————————————————–
——————————

Note
—–
– dynamic sampling used for this statement

SQL>
SQL> set autotrace off
SQL> select process from v$pq_tqstat;

no rows selected

可以看到用set autotrace 后查v$pq_tqstat norows ，这是2个原因造成的
1.set autotrace的原理
开启autotrace时候一个process对应 2个 session
通常情况下是一个 session对应一个 server processs，但SErVER PORCESSS 可以对应多个session

SQL> conn xh/a831115
已连接。
SQL> select distinct sid from v$mystat;

SID
———-
144

SQL> select username, sid, serial#, server, paddr, status from v$session where s
id=144;

USERNAME SID SERIAL# SERVER PADDR STATUS
—————————— ———- ———- ——— ——– ——–

XH 144 27 DEDICATED 20E4CC3C INACTIVE

SQL> select program ,addr from v$process where addr=(select paddr from v$session
where sid=144);

PROGRAM ADDR
—————————————————————- ——–
ORACLE.EXE (SHAD) 20E4CC3C

SQL> select sid from v$session where paddr=’20E4CC3C’;

SID
———-
144

SQL> set autotrace on
SQL> select sid from v$session where paddr=’20E4CC3C’;

SID
———-
144
154

2.v$pq_tqstat只提供当前session最后一次并行执行sql语句的信息
综合2点可以看出set autotrace 最后一个session 是执行计划的，所以v$pq_tqstat为no rows

所以直接执行
SQL> select * from t2;

A
———-
1

SQL> SELECT dfo_number, tq_id, server_type, process, num_rows, bytes
2 FROM v$pq_tqstat
3 ORDER BY dfo_number, tq_id, server_type DESC, process;

DFO_NUMBER TQ_ID SERVER_TYP PROCESS NUM_ROWS BYTES
———- ———- ———- ———- ———- ———-
1 0 Producer P000 1 24
1 0 Producer P001 0 20
1 0 Producer P002 0 20
1 0 Producer P003 0 20
1 0 Consumer QC 1 84

可以看到启动了4个slave process，这4个process=cpu_count*parallel_threads_per_cpu（2*2）

相关的hint:
parallel(10g,11g),no_parallel(10g,11g),parallel_index(10g,11g,9i),no_parallel_index(10g,11g)
noparallel(9i),noparallel_index(9i)
##需要知道并行hint只是告诉优化器可以使用并行，但不是强制使用并行一切还是从cost出发（重点）
declare
begin
for i in 1..1000 loop
insert into t2 values(i);
end loop;
commit;
end;

SQL> explain plan set statement_id=’t2_pp’ for select /*+parallel(t2 2)*/* from t2 where a>900;

Explained.

SQL> create index t2_id on t2 (a);

Index created.

SQL> explain plan set statement_id=’t2_id’ for select /*+parallel(t2 2)*/* from t2 where a>900;

Explained.

SQL> set linesize 1000
SQL> select * from table (dbms_xplan.display(null,’t2_pp’));

PLAN_TABLE_OUTPUT
————————————————————————————————————————————————————————————————————————————————————————————————————
Plan hash value: 1216610266

————————————————————————————————————–
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | TQ |IN-OUT| PQ Distrib |
————————————————————————————————————–
| 0 | SELECT STATEMENT | | 100 | 1300 | 2 (0)| 00:00:01 | | | |
| 1 | PX COORDINATOR | | | | | | | | |
| 2 | PX SEND QC (RANDOM)| :TQ10000 | 100 | 1300 | 2 (0)| 00:00:01 | Q1,00 | P->S | QC (RAND) |
| 3 | PX BLOCK ITERATOR | | 100 | 1300 | 2 (0)| 00:00:01 | Q1,00 | PCWC | |
|* 4 | TABLE ACCESS FULL| T2 | 100 | 1300 | 2 (0)| 00:00:01 | Q1,00 | PCWP | |
————————————————————————————————————–

PLAN_TABLE_OUTPUT
————————————————————————————————————————————————————————————————————————————————————————————————————

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

4 – filter(“A”>900)

Note
—–
– dynamic sampling used for this statement

20 rows selected.

SQL> select * from table (dbms_xplan.display(null,’t2_id’));

PLAN_TABLE_OUTPUT
————————————————————————————————————————————————————————————————————————————————————————————————————
Plan hash value: 523330294

————————————————————————–
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
————————————————————————–
| 0 | SELECT STATEMENT | | 100 | 1300 | 2 (0)| 00:00:01 |
|* 1 | INDEX RANGE SCAN| T2_ID | 100 | 1300 | 2 (0)| 00:00:01 |
————————————————————————–

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

1 – access(“A”>900)

Note
—–
– dynamic sampling used for this statement

17 rows selected.

可以看到即使在使用并行与走index时cost一样时候还是选择了走index（cost表面一样，若使用trace alter session set events ’10053 trace name context forever’;看的更详细，并行cpu cost要高些，所以选择了走index，所以hint parallel,parallel_index只是告诉query optimizer
可以考虑并行不是强制使用）

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oracle 并行之并行度篇

oracle 并行(并行度)

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