操作内并行使用的slave process数量就是并行度dop，indextable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 ——
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
——————————————————————————–
——————————
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time
| TQ |IN-OUT| PQ Distrib |
——————————————————————————–
——————————
| 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):
—————————————————
PLAN_TABLE_OUTPUT
————————————————————————————————————————————————————————————————————————————————————————————————————
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）
