For web and mobile applications (here, let's talk only about them, not touching analytical workloads, background processing, and so on), engineers usually aim to have as few SQL queries per each HTTP(S) request as possible, and keep the execution time of those queries very low.
How much low?
This article discusses what subtransactions are, how to create them, and how widely they are used nowadays. Next, we try to understand why they have a terrible reputation among PostgreSQL experts who operate heavily loaded systems. We overview four problems that may arise in a Postgres database in which subtransactions are used, and load grows. In the end, we discuss the short-term and long-term options for PostgreSQL users willing to solve the ongoing or prevent future problems related to subtransactions.