Single-user database queries are concerned with getting the job done. Multiuser queries must be designed to gracefully handle multiple users accessing the same data.
Multiuser interaction can be very confusing, because the database is constantly changing. In a multiuser environment, improperly constructed queries can randomly fail when users perform simultaneous operations. Queries cannot assume that rows from previous transactions still exist.
By learning about POSTGRESQL'S multiuser behavior, you are now prepared to create robust queries. POSTGRESQL has the features necessary to construct reliable multiuser queries.