| Server IP : 103.119.228.120 / Your IP : 18.223.125.236 Web Server : Apache System : Linux v8.techscape8.com 3.10.0-1160.119.1.el7.tuxcare.els2.x86_64 #1 SMP Mon Jul 15 12:09:18 UTC 2024 x86_64 User : nobody ( 99) PHP Version : 5.6.40 Disable Function : shell_exec,symlink,system,exec,proc_get_status,proc_nice,proc_terminate,define_syslog_variables,syslog,openlog,closelog,escapeshellcmd,passthru,ocinum cols,ini_alter,leak,listen,chgrp,apache_note,apache_setenv,debugger_on,debugger_off,ftp_exec,dl,dll,myshellexec,proc_open,socket_bind,proc_close,escapeshellarg,parse_ini_filepopen,fpassthru,exec,passthru,escapeshellarg,escapeshellcmd,proc_close,proc_open,ini_alter,popen,show_source,proc_nice,proc_terminate,proc_get_status,proc_close,pfsockopen,leak,apache_child_terminate,posix_kill,posix_mkfifo,posix_setpgid,posix_setsid,posix_setuid,dl,symlink,shell_exec,system,dl,passthru,escapeshellarg,escapeshellcmd,myshellexec,c99_buff_prepare,c99_sess_put,fpassthru,getdisfunc,fx29exec,fx29exec2,is_windows,disp_freespace,fx29sh_getupdate,fx29_buff_prepare,fx29_sess_put,fx29shexit,fx29fsearch,fx29ftpbrutecheck,fx29sh_tools,fx29sh_about,milw0rm,imagez,sh_name,myshellexec,checkproxyhost,dosyayicek,c99_buff_prepare,c99_sess_put,c99getsource,c99sh_getupdate,c99fsearch,c99shexit,view_perms,posix_getpwuid,posix_getgrgid,posix_kill,parse_perms,parsesort,view_perms_color,set_encoder_input,ls_setcheckboxall,ls_reverse_all,rsg_read,rsg_glob,selfURL,dispsecinfo,unix2DosTime,addFile,system,get_users,view_size,DirFiles,DirFilesWide,DirPrintHTMLHeaders,GetFilesTotal,GetTitles,GetTimeTotal,GetMatchesCount,GetFileMatchesCount,GetResultFiles,fs_copy_dir,fs_copy_obj,fs_move_dir,fs_move_obj,fs_rmdir,SearchText,getmicrotime MySQL : ON | cURL : ON | WGET : ON | Perl : ON | Python : ON | Sudo : ON | Pkexec : ON Directory : /usr/share/doc/postgresql-9.2.24/html/ |
Upload File : |
<!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 9.2.24 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="2017-11-06T22:43:11"></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"
><A
HREF="index.html"
>PostgreSQL 9.2.24 Documentation</A
></TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
TITLE="Concurrency Control"
HREF="mvcc.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
HREF="mvcc.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="60%"
ALIGN="center"
VALIGN="bottom"
>Chapter 13. Concurrency Control</TD
><TD
WIDTH="20%"
ALIGN="right"
VALIGN="top"
><A
TITLE="Transaction Isolation"
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
><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
the 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.
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> maintains this guarantee
even when providing the strictest level of transaction
isolation through the use of an innovative <I
CLASS="FIRSTTERM"
>Serializable
Snapshot Isolation</I
> (<ACRONYM
CLASS="ACRONYM"
>SSI</ACRONYM
>) level.
</P
><P
> Table- and row-level locking facilities are also available in
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> for applications which don't
generally need full transaction isolation and prefer to explicitly
manage particular points of conflict. 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
>