Parallel execution is a technique used to optimize database query performance by dividing a query into smaller tasks that can be executed simultaneously on multiple processors or cores. This approach can help to reduce the total execution time of a query by allowing multiple operations to be performed at the same time.

Here are some examples of parallel execution techniques that can be used to optimize database queries:

1. ## Parallel query execution

This technique involves breaking down a large query into smaller, parallelizable tasks that can be executed simultaneously on multiple processors or cores. For example, a query that scans a large table could be broken down into multiple smaller queries, each of which is executed on a separate core.

1. ## Parallel table scans

This technique involves scanning multiple tables simultaneously to perform a join operation. For example, a query that joins two large tables could be executed by scanning each table in parallel and then combining the results.

1. ## Parallel aggregation

This technique involves dividing an aggregation query (such as COUNT, SUM, or AVG) into smaller tasks that can be executed in parallel. For example, a query that aggregates data across multiple tables could be executed by dividing the aggregation task into smaller, parallelizable sub-tasks.

1. ## Parallel indexing

This technique involves building or updating indexes in parallel to improve query performance. For example, a database system could use multiple processors or cores to build indexes on a large table.

Practical Steps

1. ### Enable parallel query processing

In most RDBMS, parallel query processing is disabled by default. To enable parallel query processing, the database administrator can use a system-specific command or modify the RDBMS settings. For example, in Oracle Database, the degree of parallelism can be set using the PARALLEL parameter in the CREATE TABLE or ALTER TABLE statements.

1. ### Break down the query into parallelizable tasks

To utilize parallel query processing, the query needs to be broken down into parallelizable tasks that can be executed simultaneously. For example, a large query that joins multiple tables can be broken down into smaller queries that join two or three tables at a time.

1. ### Use parallel hint

The SQL parallel hint can be used to instruct the RDBMS to execute the query in parallel. The parallel hint specifies the degree of parallelism for the query, which is the number of parallel threads or processes that will be used to execute the query. For example, the following SQL query uses the parallel hint to instruct the Oracle Database to execute the query with a degree of parallelism of four:

SELECT /*+ PARALLEL(4) */ *
FROM orders
WHERE order_date BETWEEN TO_DATE('01-JAN-2022', 'DD-MON-YYYY') AND TO_DATE('31-DEC-2022', 'DD-MON-YYYY');

1. ### Monitor query performance

After executing the parallel query, it is important to monitor the performance to ensure that it is executing efficiently. The database administrator can use tools such as the query execution plan, SQL trace, or system performance metrics to monitor the query performance.