This chapter describes the behavior of the PostgreSQL database system when two or more sessions try to access the same data at the same time. The goals in that situation are to allow efficient access for all sessions while maintaining strict data integrity. Every developer of database applications should be familiar with the topics covered in this chapter.

Postgres-XL inherited concurrency control from PostgreSQL and extended it globally to all of the Coordinators and Datanodes involved. Regardless of which Coordinator is connected to, all of the transactions in the Postgres-XL database cluster behaves in a consistent way as if they are running in single database.