Oracle parallel DML

Oracle Database Tips by Donald Burleson

Oracle Parallel DML

Oracle parallel DML is very useful when you have an SMP server (lots of independent CPU processors, see your cpu_count) and multiple I/O channels on multiple physical disks. If you can bypass disk I/O bottlenecks by spreading data across multiple disks and controllers (ideally with a segment partition on each physical spindle), then parallel DML can greatly improve data loading speeds, especially using tools like SQL*Loader (sqlldr).

Also, note my tips for high-speed data loading with Oracle.

Oracle parallel DML does this by allocating multiple processes, each simultaneously performing DML (updates, inserts). You can invoke parallel DML (i.e. using the PARALLEL and APPEND hint) to have multiple inserts into the same table.

Oracle parallel DML configuration

To specify parallel directives, follow one of the following methods:

ALTER SESSION FORCE PARALLEL DML.
Use an update, merge, or delete parallel hint in the statement.
Use a parallel clause in the definition of the table being updated or deleted (the reference object).

Once it is decided to use parallel processing, the maximum degree of parallelism DOP that can be achieved in delete, merge, and update is equal to:

The number of partitions (or sub-partitions, in the case of composite sub-partitions) in the table.
The number of parallel processors
The number of disk channels

A parallel execution server can update or merge into, or delete from multiple partitions, but each partition can only be updated or deleted by one parallel execution server. Each parallel execution server creates a different parallel process transaction. As a result, parallel DML requires more than one rollback segment for performance.

However, a transaction can contain multiple parallel DML statements that modify different tables, but after a parallel DML statement modifies a table, no subsequent serial or parallel statement (DML or query) can access the same table again in that transaction.

Oracle append hint and parallel DML

You need to be careful when deciding to combine an APPEND with a PARALLEL hint. By using the APPEND hint, you ensure that Oracle always grabs "fresh" data blocks by raising the high-water-mark for the table. If you are doing parallel insert DML, the Append mode is the default and you don't need to specify an APPEND hint. Mark Bobak notes "Also, if you're going w/ APPEND, consider putting the table into NOLOGGING mode, which will allow Oracle to avoid almost all redo logging."

insert /*+ append */
into customer
values ('hello',';there');

For this INSERT optimization, make sure to define multiple freelists (or use automatic segment space management) and use the SQL "APPEND" option with the PARALLEL hint. Mark Bobak notes that if you submit parallel jobs to insert against the table (or same table partition) at the same time, using the APPEND hint may cause serialization, removing the benefit of parallel jobstreams.