Oracle Database cannot return results to a user process in parallel. If a query returns a large number of rows, execution of the query might indeed be faster. However, the user process can receive the rows only serially. To optimize parallel execution performance for queries that retrieve large result sets, use
SELECT or direct-path
INSERT to store the result set in the database. At a later time, users can view the result set serially.
SELECT in parallel does not influence the
CREATE statement. If the
CREATE statement is executed in parallel, however, the optimizer tries to make the
SELECT run in parallel also.
When combined with the
NOLOGGING option, the parallel version of
SELECT provides a very efficient intermediate table facility, for example:
CREATE TABLE summary PARALLEL NOLOGGING AS SELECT dim_1, dim_2 ..., SUM (meas_1) FROM facts GROUP BY dim_1, dim_2;
These tables can also be incrementally loaded with parallel
INSERT. You can take advantage of intermediate tables using the following techniques:
Common subqueries can be computed once and referenced many times. This can allow some queries against star schemas (in particular, queries without selective
WHERE-clause predicates) to be better parallelized. Star queries with selective
WHERE-clause predicates using the star-transformation technique can be effectively parallelized automatically without any modification to the SQL.
Decompose complex queries into simpler steps to provide application-level checkpoint or restart. For example, a complex multitable join on a one terabyte database could run for dozens of hours. A failure during this query would mean starting over from the beginning. Using
SELECT, you can rewrite the query as a sequence of simpler queries that run for a few hours each. If a system failure occurs, the query can be restarted from the last completed step.
Implement manual parallel delete operations efficiently by creating a new table that omits the unwanted rows from the original table, and then dropping the original table. Alternatively, you can use the convenient parallel delete feature, which directly deletes rows from the original table.
Create summary tables for efficient multidimensional drill-down analysis. For example, a summary table might store the sum of revenue grouped by month, brand, region, and salesman.
Reorganize tables, eliminating chained rows, compressing free space, and so on, by copying the old table to a new table. This is much faster than export/import and easier than reloading.
Be sure to use the
DBMS_STATS package to gather optimizer statistics on newly created tables. To avoid I/O bottlenecks, specify a tablespace that is striped across at least as many physical disks as CPUs. To avoid fragmentation in allocating space, the number of files in a tablespace should be a multiple of the number of CPUs.
Oracle Database Data Warehousing Guide for information about parallel execution in data warehouses