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.
Unlike traditional database systems which use locks for concurrency control,
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 (a database version)
as it was some
time ago, regardless of the current state of the underlying data.
This protects the transaction from viewing inconsistent data that
could be caused by (other) concurrent transaction updates on the same
data rows, providing transaction isolation
for each database session.
The main difference between multiversion and lock models is that
in MVCC locks acquired for querying (reading) data don't conflict
with locks acquired for writing data, and so reading never blocks
writing and writing never blocks reading.
Table- and row-level locking facilities are also available in
PostgreSQL for applications that cannot
adapt easily to MVCC behavior. However, proper use of MVCC will
generally provide better performance than locks.
