Parallel technology of Oracle

stay Oracle When dealing with big tasks , Generally, a serial process is used to execute , When the system resources are sufficient , Can handle
This big task is divided into several small tasks , Use several processes at the same time / Threads to process these small tasks in parallel , These concurrent processes
It is called parallel execution server (parallel executeion server), Coordinated by a concurrent coordinator Process to manage .
In general , Enabling concurrent processing must be done with sufficient system resources , otherwise , Multiple concurrent processes will take up a lot
System resources for , Causing other sessions to wait , Make the overall performance of the system decline .

Parallel technology is mainly applied in the following three aspects ： Parallel queries , parallel DML, parallel DDL.

1. Parallel queries
Parallel queries can be used for query statements and sub statements , Not available on remotely referenced objects （ such as DBLINK）.
The condition of parallel query is ：
1)SQL There are Hint Tips , such as Parallel perhaps Parallel_index;
select /*+parallel(test1 4) */ * from test1;
2) stay SQL The retrieved object in the statement is retrieved after setting the parallel property ：
ALTER TABLE test1 PARALLEL;
3) In the association query of multiple tables , At least one of the tables is full table scan or cross partition index range SCAN.

2. parallel DML
insert /*+ parallel(test1 4) */ into test1 select /*+ parallel(leo6 
2) */ * from test1.sal_range;;

3. parallel DDL（ parallel DDL operation , Such as creating tables or indexes ）

Parallel operation of creating tables ：
CREATE TABLE test2 parallel 4 AS SELECT * from test1;
ALTER TABLE test2 move partition part1 parallel 4;
Alter table test2 coalesce partition parallel 4;
Related operations can be done through trace File view its execution process .

Parallel execution method of index creation ：
create index ind1 on test1(col1) parallel 4;
Alter index ind1 rebuild parallel 4;
Alter index index_name rebuild partition partition_name parallel 4;

Some considerations when using parallel technology ：
1、RAC Database , It is recommended that parallel processes be local only instance start-up , In order to reduce cluster wait.
alter system set instance_groups='racdb1, racdb' scope=spfile
sid='racdb1';
alter system set parallel_instance_group='racdb1' scope=spfile
sid='racdb1';
2、 General setting parameters parallel_max_servers At most cpu Subtract one from the number , Not too large , Prevent different
Frequent concurrent processes lead to CPU Excessive resource consumption . stay OLTP The application of , It is not suitable to open parallel , It is easy to cause application hang
rise .
3、Oracle Do not open by default PDML, So I want to execute in parallel DML Statement must be enabled manually .
alter session enable dml parallel;
4、 if cpu Very few , There are not many benefits to using parallelism .
View parallel tasks , The commonly used views are v$px_session.