Sophie: postgresql9.6-docs-9.6.22-1.mga7 noarch

postgresql9.6-docs-9.6.22-1.mga7.noarch.rpm

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<HTML
><HEAD
><TITLE
>Visibility Map</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 9.6.22 Documentation"
HREF="index.html"><LINK
REL="UP"
TITLE="Database Physical Storage"
HREF="storage.html"><LINK
REL="PREVIOUS"
TITLE="Free Space Map"
HREF="storage-fsm.html"><LINK
REL="NEXT"
TITLE="The Initialization Fork"
HREF="storage-init.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="2021-05-18T09:16:10"></HEAD
><BODY
CLASS="SECT1"
><DIV
CLASS="NAVHEADER"
><TABLE
SUMMARY="Header navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="4"
ALIGN="center"
VALIGN="bottom"
><A
HREF="index.html"
>PostgreSQL 9.6.22 Documentation</A
></TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
TITLE="Free Space Map"
HREF="storage-fsm.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
HREF="storage.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="60%"
ALIGN="center"
VALIGN="bottom"
>Chapter 65. Database Physical Storage</TD
><TD
WIDTH="20%"
ALIGN="right"
VALIGN="top"
><A
TITLE="The Initialization Fork"
HREF="storage-init.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="STORAGE-VM"
>65.4. Visibility Map</A
></H1
><P
>Each heap relation has a Visibility Map
(VM) to keep track of which pages contain only tuples that are known to be
visible to all active transactions; it also keeps track of which pages contain
only frozen tuples.  It's stored
alongside the main relation data in a separate relation fork, named after the
filenode number of the relation, plus a <TT
CLASS="LITERAL"
>_vm</TT
> suffix. For example,
if the filenode of a relation is 12345, the VM is stored in a file called
<TT
CLASS="FILENAME"
>12345_vm</TT
>, in the same directory as the main relation file.
Note that indexes do not have VMs.</P
><P
>The visibility map stores two bits per heap page.  The first bit, if set,
indicates that the page is all-visible, or in other words that the page does
not contain any tuples that need to be vacuumed.
This information can also be used
by <A
HREF="indexes-index-only-scans.html"
><I
CLASS="FIRSTTERM"
>index-only
scans</I
></A
> to answer queries using only the index tuple.
The second bit, if set, means that all tuples on the page have been frozen.
That means that even an anti-wraparound vacuum need not revisit the page.</P
><P
>The map is conservative in the sense that we make sure that whenever a bit is
set, we know the condition is true, but if a bit is not set, it might or
might not be true. Visibility map bits are only set by vacuum, but are
cleared by any data-modifying operations on a page.</P
><P
>The <A
HREF="pgvisibility.html"
>pg_visibility</A
> module can be used to examine the
information stored in the visibility map.</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="storage-fsm.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="storage-init.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>Free Space Map</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="storage.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>The Initialization Fork</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>