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Parallel DDL Tips

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.

DDL operations can be paralleled if a parallel clause or declaration is specified in the syntax.  The parallel DDL statements for non-partitioned tables and indexes are:

CREATE TABLE ... AS select

The parallel DDL statements for partitioned tables and indexes are:

CREATE TABLE ... AS select

The ALTER SESSION FORCE PARALLEL DDL statement can be used to override the parallel clauses of subsequent DDL statements in a session. The degree of parallelism is determined by the specification in the parallel clause, unless an ALTER SESSION FORCE PARALLEL DDL statement overrides it. A rebuild of a partitioned index is never paralleled.

When indexes and tables are created in parallel, each parallel execution server allocates a new extent and fills the extent with the table or index data. Hence, if an index is created with a degree of parallelism of four, the index will have at least four extents initially. The parallel allocation of extents is the same for indexes are built in parallel or partitions are being moved, split, or rebuilt in parallel.

While serial operations only require that the schema object has at least one extent, parallel creations initially require that tables or indexes have at least as many extents as there are parallel execution servers creating the schema object. After creation, the extents may be coalesced, but there must be a plan for the additional space that may be required for the initial multiple extents caused by parallel DDL operations.

Rules for Paralleling Create Table as Select (CTAS)

CTAS is a very widely used and useful operation while dealing with the reorganization of large tables. The CTAS statement contains two parts: a CREATE part (DDL) and a SELECT part (query). Oracle can parallellize both parts of the statement. The CREATE part of the CTAS must follow the same rules as apply to other DDL operations.  The following conditions must be satisfied if the query part of a CTAS statement is to be parallellized:

  • The query includes a parallel hint specification, the CREATE part of the statement has a parallel clause, or objects referred to in the query have a parallel declaration.

  • At least one of the tables specified in the query requires a full table scan or an index range scan spanning multiple partitions.

The degree of parallelism for the query part of CTAS is then determined as follows:

  • The query part of the CTAS uses the settings specified in the parallel clause of the CREATE part.

  • When the parallel clause is not specified, the default degree of parallelism will be the number of CPUs.

  • When the CREATE is done as a serial operation, the degree of parallelism is determined by the query settings.

The CREATE operation of CTAS can be paralleled only by a parallel clause or an ALTER SESSION FORCE PARALLEL DDL statement.

The degree of parallelism for the CREATE operation, and for the SELECT operation if it is paralleled, is specified by the parallel clause of the CREATE statement, unless it is overridden by an ALTER SESSION FORCE PARALLEL DDL statement.

SQL*LOADER: Parallel Load

Placing large data loads into database tables often involves considerable time. However, with the help of concurrent or parallel loading, the whole loading process can be accomplished faster.


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


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