[comvidyarthi]

I don’t understand the title of the article. The article essentially talks about what RepeatableRead isolation levels of Postgresql cannot do and how to remedy that by Serialize Isolation. But both need lock at some level, and tradeoff performance guarantees for weaker/stronger consistency guarantees.

[karmakaze]

It's not a great article, as some of the clarification comments and discussions are surfacing. It is an import topic. Often reading the database documentation directly around Transaction Isolation and Locking sections will be more precise, complete and be specific for the database/version you are using.