You can use parallel queries and parallel subqueries in
SELECT statements and execute in parallel the query portions of DDL statements and DML statements (
DELETE). You can also query external tables in parallel.
Parallelization has two components: the decision to parallelize and the degree of parallelism (DOP). These components are determined differently for queries, DDL operations, and DML operations. To determine the DOP, Oracle Database looks at the reference objects:
Parallel query looks at each table and index, in the portion of the query to be executed in parallel, to determine which is the reference table. The basic rule is to pick the table or index with the largest DOP.
For parallel DML (
DELETE), the reference object that determines the DOP is the table being modified by an insert, update, or delete operation. Parallel DML also adds some limits to the DOP to prevent deadlock. If the parallel DML statement includes a subquery, the subquery's DOP is equivalent to that for the DML operation.
For parallel DDL, the reference object that determines the DOP is the table, index, or partition being created, rebuilt, split, or moved. If the parallel DDL statement includes a subquery, the subquery's DOP is equivalent to the DDL operation.
This section contains the following topics:
For an explanation of how the processes perform parallel queries, refer to "Parallel Execution of SQL Statements". For examples of queries that reference a remote object, refer to "Distributed Transaction Restrictions". For information about the conditions for executing a query in parallel and the factors that determine the DOP, refer to "Rules for Parallelizing Queries".