You must consider the following topics when parallelizing DDL statements:
DDL operations can be executed in parallel if a PARALLEL clause (declaration) is specified in the syntax. For CREATE INDEX and ALTER INDEX REBUILD or ALTER INDEX REBUILD PARTITION, the parallel declaration is stored in the data dictionary.
You can use the ALTER SESSION FORCE PARALLEL DDL statement to override the parallel clauses of subsequent DDL statements in a session.
The DOP is determined by the specification in the PARALLEL clause, unless it is overridden by an ALTER SESSION FORCE PARALLEL DDL statement. A rebuild of a partitioned index is never executed in parallel.
Parallel clauses in CREATE TABLE and ALTER TABLE statements specify table parallelism. If a parallel clause exists in a table definition, it determines the parallelism of DDL statements and queries. If the DDL statement contains explicit parallel hints for a table, however, those hints override the effect of parallel clauses for that table. You can use the ALTER SESSION FORCE PARALLEL DDL statement to override parallel clauses.