PostgreSQL: How to set number of parallel workers in a session

SET max_parallel_workers_per_gather=num;

Default is num=2, which will give you 3 processes in total (1 master + 2 workers). If set to 3, you will have 5 processes in total. This parameter can be set on a per session basis.

Remember the global cap on parallelism that is represented by the parameter max_parallel_workers. Default is 8. max_parallel_workers_per_gather cannot exceed max_parallel_workers, which can only be changed in the configuration file and requires a full restart of the postgres server.

The documentation can be found here

See also "When can parallel query be used?" in the documentation

This post is based on the PostgreSQL 11 server.