What is the degree of parallelism:
Parallelism advantage is the ability to maximize the use of resources plurality cpu machines, multiple cpu simultaneously, so as to achieve the purpose of improving the efficiency of the database. In the system idle time, use parallel is a good choice, but the good thing is always relatively speaking, there is no absolute good or bad, improper use, it will also lead to a new generation of database problems.
1, the degree of parallelism View
select table_name, degree from user_tables; - parallelism tables are provided according to the user
2, modifying the degree of parallelism
alter table table_name parallel (degree 4) ; --- 4 is a modification of the table parallelism alter table table_name parallel; --- parallelism modify the default value
3, the degree of parallelism in the statement specifies
SELECT / * + PARALLEL (4) * / FROM T --- specified degree of parallelism. 4 the SELECT / * + * no_parallel / non-parallelism of the FROM T --- SELECT / * + parallel (auto) * / FROM T --- automatic parallelism
4, the use of parallel query
- Large table query, join, the index partition query.
- Create a large number of index.
- Create a large number of tables (including curing view).
- Batch insert, update, delete.
- Symmetric Multi-Processor cluster, parallel system,
- cpu underutilized,
- Enough memory for other operations, sort, hash, cache,
- Check line with dss and execution for data warehousing, OLTP system is also suitable for bulk operations, are not suitable for OLTP profile or select dml operations;
5, is not suitable for parallel execution scenarios:
- Very short queries or transactions