Parallel hint in PostgreSQL

In PostgreSQL, there isn't a direct parallel hint like in some other database systems. PostgreSQL's query optimizer makes decisions about parallelism automatically based on factors like available system resources, query complexity, and the configuration settings.

However, PostgreSQL provides a configuration parameter called max_parallel_workers_per_gather that controls the maximum number of workers that can be used in parallel query execution. You can adjust this parameter to influence the degree of parallelism for your queries indirectly.

Here's how you can set max_parallel_workers_per_gather in PostgreSQL:

SET max_parallel_workers_per_gather = 4;

In this example, you're setting the maximum number of parallel workers per gather node to 4. This means that PostgreSQL can use up to 4 parallel workers to execute a parallel query.

However, it's important to note that simply setting this parameter won't force PostgreSQL to execute all queries in parallel. PostgreSQL's query planner will still decide whether parallelism is appropriate for a given query based on various factors.

To actually see parallelism in action, you'll need a query that meets certain criteria, typically involving large data sets and operations that can be parallelized effectively, such as full table scans or certain types of joins and aggregations.

Here's an example of a query that might benefit from parallel execution in PostgreSQL:

SELECT /*+ PARALLEL(4) */ * FROM large_table;

In this example, large_table is a large table in your database, and you're hinting to the optimizer that parallelism with a degree of 4 would be beneficial for this query. However, as mentioned earlier, PostgreSQL's query planner will ultimately decide whether to use parallelism based on its own cost estimation and criteria.

While PostgreSQL doesn't provide a direct hint for forcing parallelism like some other databases, you can still indirectly influence parallel execution through configuration settings and query design.