Concurrency Control in Greenplum Database

Unlike traditional database systems which use locks for concurrency control, Greenplum Database (as does PostgreSQL) maintains data consistency by using a multiversion model (Multiversion Concurrency Control, MVCC). This means that while querying a database, each transaction sees a snapshot of data which protects the transaction from viewing inconsistent data that could be caused by (other) concurrent updates on the same data rows. This provides transaction isolation for each database session.
MVCC, by eschewing explicit locking methodologies of traditional database systems, minimizes lock contention in order to allow for reasonable performance in multiuser environments. The main advantage to using the MVCC model of concurrency control rather than locking is that in MVCC locks acquired for querying (reading) data do not conflict with locks acquired for writing data, and so reading never blocks writing and writing never blocks reading.
Greenplum Database provides various lock modes to control concurrent access to data in tables. Most Greenplum Database SQL commands automatically acquire locks of appropriate modes to ensure that referenced tables are not dropped or modified in incompatible ways while the command executes. For applications that cannot adapt easily to MVCC behavior, the LOCK command can be used to acquire explicit locks. However, proper use of MVCC will generally provide better performance than locks.