Sophie: postgresql8.4-docs-8.4.12-0.1mdv2010.2 i586

postgresql8.4-docs-8.4.12-0.1mdv2010.2.i586.rpm

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<HTML
><HEAD
><TITLE
>Introduction</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK
REV="MADE"
HREF="mailto:pgsql-docs@postgresql.org"><LINK
REL="HOME"
TITLE="PostgreSQL 8.4.12 Documentation"
HREF="index.html"><LINK
REL="UP"
TITLE="Concurrency Control"
HREF="mvcc.html"><LINK
REL="PREVIOUS"
TITLE="Concurrency Control"
HREF="mvcc.html"><LINK
REL="NEXT"
TITLE="Transaction Isolation"
HREF="transaction-iso.html"><LINK
REL="STYLESHEET"
TYPE="text/css"
HREF="stylesheet.css"><META
HTTP-EQUIV="Content-Type"
CONTENT="text/html; charset=ISO-8859-1"><META
NAME="creation"
CONTENT="2012-05-31T23:30:11"></HEAD
><BODY
CLASS="SECT1"
><DIV
CLASS="NAVHEADER"
><TABLE
SUMMARY="Header navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="5"
ALIGN="center"
VALIGN="bottom"
>PostgreSQL 8.4.12 Documentation</TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
HREF="mvcc.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
HREF="mvcc.html"
>Fast Backward</A
></TD
><TD
WIDTH="60%"
ALIGN="center"
VALIGN="bottom"
>Chapter 13. Concurrency Control</TD
><TD
WIDTH="10%"
ALIGN="right"
VALIGN="top"
><A
HREF="mvcc.html"
>Fast Forward</A
></TD
><TD
WIDTH="10%"
ALIGN="right"
VALIGN="top"
><A
HREF="transaction-iso.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="MVCC-INTRO"
>13.1. Introduction</A
></H1
><A
NAME="AEN20918"
></A
><P
>    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 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, <ACRONYM
CLASS="ACRONYM"
>MVCC</ACRONYM
>). 
    This means that while querying a database each transaction sees
    a snapshot of data (a <I
CLASS="FIRSTTERM"
>database version</I
>)
    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 <I
CLASS="FIRSTTERM"
>transaction isolation</I
>
    for each database session.  <ACRONYM
CLASS="ACRONYM"
>MVCC</ACRONYM
>, by eschewing
    explicit locking methodologies of traditional database systems,
    minimizes lock contention in order to allow for reasonable 
    performance in multiuser environments.
   </P
><P
>    The main advantage of using the <ACRONYM
CLASS="ACRONYM"
>MVCC</ACRONYM
> model of
    concurrency control rather than locking is that in
    <ACRONYM
CLASS="ACRONYM"
>MVCC</ACRONYM
> 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.
   </P
><P
>    Table- and row-level locking facilities are also available in
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> for applications that cannot
    adapt easily to <ACRONYM
CLASS="ACRONYM"
>MVCC</ACRONYM
> behavior.  However, proper
    use of <ACRONYM
CLASS="ACRONYM"
>MVCC</ACRONYM
> 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.
   </P
></DIV
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="mvcc.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="index.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="transaction-iso.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>Concurrency Control</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="mvcc.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>Transaction Isolation</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>