13.1. Introduction

13.1. Introduction
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+ PostgreSQL provides a rich set of tools + for developers to manage concurrent access to data. Internally, + data consistency is maintained by using a multiversion + model (Multiversion Concurrency Control, MVCC). + This means that each SQL statement sees + a snapshot of data (a database version) + as it was some + time ago, regardless of the current state of the underlying data. + This prevents statements from viewing inconsistent data produced + by concurrent transactions performing updates on the same + data rows, providing transaction isolation + for each database session. MVCC, by eschewing + the locking methodologies of traditional database systems, + minimizes lock contention in order to allow for reasonable + performance in multiuser environments. +

+ The main advantage of 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. + PostgreSQL maintains this guarantee + even when providing the strictest level of transaction + isolation through the use of an innovative Serializable + Snapshot Isolation (SSI) level. +

+ Table- and row-level locking facilities are also available in + PostgreSQL for applications which don't + generally need full transaction isolation and prefer to explicitly + manage particular points of conflict. However, proper + use of MVCC will generally provide better + performance than locks. In addition, application-defined advisory + locks provide a mechanism for acquiring locks that are not tied + to a single transaction. +