Parallel hint in Oracle

In Oracle, the PARALLEL hint is used to instruct the optimizer to consider parallel execution plans for SQL queries. Parallel execution allows Oracle to divide the work of a SQL statement among multiple parallel execution servers, which can potentially improve query performance for large data sets by leveraging multiple CPUs and disks.

Here's a basic example of how to use the PARALLEL hint in Oracle:

Suppose you have a table named employees and you want to retrieve data from it using a SELECT statement with parallel execution enabled. You can use the PARALLEL hint like this:

SELECT /*+ PARALLEL(employees, 4) */ * FROM employees;

In this example:

- employees is the name of the table.

- 4 is the degree of parallelism. This value specifies the number of parallel execution servers Oracle should use to process the query. You can adjust this value based on your system resources and the size of your data.

Keep in mind the following points when using the PARALLEL hint:

1. Parallel execution can significantly increase the resource usage on your database server, so it should be used judiciously.

2. The degree of parallelism specified should not exceed the available CPU cores and other system resources.

3. Not all queries benefit from parallel execution. It's typically most effective for large data sets and resource-intensive operations like full table scans and large aggregations.

4. It's essential to analyze the impact of parallel execution on your specific workload and tune accordingly.

Additionally, ensure that parallel execution is enabled at the system level and that parallel query settings are appropriately configured in your Oracle database instance. This includes parameters like PARALLEL_MAX_SERVERS, PARALLEL_MIN_SERVERS, and PARALLEL_THREADS_PER_CPU.