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Don Burleson Blog 







Parallel_index hint tips

Oracle Database Tips by Donald BurlesonDecember 3,  2015


Question:  What does the parallel_index hint do?

Answer:  The parallel_index hint is effective only on partitioned indexes and only when the range scans are being performed, AND ONLY when partitioned indexes are defined for the table.

The single most common mistake with parallel_index hint is using it against a non-partitioned index, where it will be ignored.

The parallel_index hint specifies the desired number of concurrent servers that can be used to parallelize index range scans for partitioned indexes.

When using the parallel_index hint, specifying DEFAULT or no DEGREE value tells the query coordinator check the initialization parameter default degree of parallelism (parallel_automatic_tuning).

/*+ PARALLEL_INDEX([@queryblock] <tablespec> <index_name> <degreee | DEFAULT>) */

Here is an example of invoking the parallel_index hint:

SELECT /*+ PARALLEL_INDEX(mytab, myindex, 3) */ stuff FROM mytab;


Using the default DEGREE with parallel_index

If you are using an SMP or MPP database server with many CPUs, you can issue a parallel request and leave it up to each Oracle instance to use its default degree of parallelism. For example, consider the following parallel_index query:

SELECT /*+ PARALLEL_INDEX(emp, type_idx, 31) */
   emp_type = 'SALARIED';

Oracle also provides the parallel_automatic_tuning init.ora parameter to assist in setting the best degree of parallelism. When setting parallel_automatic_tuning, you only need to specify parallelism for a table, and Oracle will dynamically change the parallel_adaptive_multi_user parameter to override the execution plan in favor of maintaining an acceptable overall load on the database.

WARNING:  The parallel_automatic_tuning parameter is not always appropriate for OLTP and online systems, where setting parallel_automatic_tuning may change the SQL optimizers perception of the costs of full scan operations, causing indexes not to be used.



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