Just so everyone's in agreement. Yes, FOR UPDATE blocks concurrent SELECT...FOR UPDATE (as well as SELECT...FOR SHARE/IN SHARE MODE). What the article said though was reads:

> Now, other transactions won’t be able to read this row until our transaction is committed or rolled back (for update can only be used inside a transaction).

Finally if transaction isolation is SERIALIZABLE, then a row lock can block normal SELECT reads. This is a consequence of SERIALIZABLE isolation preventing write skew. Transactions can be run in parallel, but there has to be some ordering of transactions that produces the same result as if each transaction was one run one at a time.