Is there any method to perform something parallel in postgresql extension?

According to this link, to perform something parallel in postgresql extension, multi-thread isn't an option. So is there any other method? Such as some API and usage about multi-process? Maybe let me konw where can I find any example code? Or the expection itself of performing something parallel in Postgresql extension does not reasonable at all? Would you provide some explanation? Updated: Regarding the things I want to parallelize, it generally has nothing with Postgresql, but caculate hash of a bunch of data