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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>68.1. Database File Layout</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="storage.html" title="Chapter 68. Database Physical Storage" /><link rel="next" href="storage-toast.html" title="68.2. TOAST" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">68.1. Database File Layout</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="storage.html" title="Chapter 68. Database Physical Storage">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="storage.html" title="Chapter 68. Database Physical Storage">Up</a></td><th width="60%" align="center">Chapter 68. Database Physical Storage</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11beta1 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="storage-toast.html" title="68.2. TOAST">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="STORAGE-FILE-LAYOUT"><div class="titlepage"><div><div><h2 class="title" style="clear: both">68.1. Database File Layout</h2></div></div></div><p>

This section describes the storage format at the level of files and

directories.

</p><p>

Traditionally, the configuration and data files used by a database

cluster are stored together within the cluster's data

directory, commonly referred to as <code class="varname">PGDATA</code> (after the name of the

environment variable that can be used to define it). A common location for

<code class="varname">PGDATA</code> is <code class="filename">/var/lib/pgsql/data</code>. Multiple clusters,

managed by different server instances, can exist on the same machine.

</p><p>

The <code class="varname">PGDATA</code> directory contains several subdirectories and control

files, as shown in <a class="xref" href="storage-file-layout.html#PGDATA-CONTENTS-TABLE" title="Table 68.1. Contents of PGDATA">Table 68.1</a>. In addition to

these required items, the cluster configuration files

<code class="filename">postgresql.conf</code>, <code class="filename">pg_hba.conf</code>, and

<code class="filename">pg_ident.conf</code> are traditionally stored in

<code class="varname">PGDATA</code>, although it is possible to place them elsewhere.

</p><div class="table" id="PGDATA-CONTENTS-TABLE"><p class="title"><strong>Table 68.1. Contents of <code class="varname">PGDATA</code></strong></p><div class="table-contents"><table class="table" summary="Contents of PGDATA" border="1"><colgroup><col /><col /></colgroup><thead><tr><th>

Item

</th><th>Description</th></tr></thead><tbody><tr><td><code class="filename">PG_VERSION</code></td><td>A file containing the major version number of <span class="productname">PostgreSQL</span></td></tr><tr><td><code class="filename">base</code></td><td>Subdirectory containing per-database subdirectories</td></tr><tr><td><code class="filename">current_logfiles</code></td><td>File recording the log file(s) currently written to by the logging

collector</td></tr><tr><td><code class="filename">global</code></td><td>Subdirectory containing cluster-wide tables, such as

<code class="structname">pg_database</code></td></tr><tr><td><code class="filename">pg_commit_ts</code></td><td>Subdirectory containing transaction commit timestamp data</td></tr><tr><td><code class="filename">pg_dynshmem</code></td><td>Subdirectory containing files used by the dynamic shared memory

subsystem</td></tr><tr><td><code class="filename">pg_logical</code></td><td>Subdirectory containing status data for logical decoding</td></tr><tr><td><code class="filename">pg_multixact</code></td><td>Subdirectory containing multitransaction status data

(used for shared row locks)</td></tr><tr><td><code class="filename">pg_notify</code></td><td>Subdirectory containing LISTEN/NOTIFY status data</td></tr><tr><td><code class="filename">pg_replslot</code></td><td>Subdirectory containing replication slot data</td></tr><tr><td><code class="filename">pg_serial</code></td><td>Subdirectory containing information about committed serializable transactions</td></tr><tr><td><code class="filename">pg_snapshots</code></td><td>Subdirectory containing exported snapshots</td></tr><tr><td><code class="filename">pg_stat</code></td><td>Subdirectory containing permanent files for the statistics

subsystem</td></tr><tr><td><code class="filename">pg_stat_tmp</code></td><td>Subdirectory containing temporary files for the statistics

subsystem</td></tr><tr><td><code class="filename">pg_subtrans</code></td><td>Subdirectory containing subtransaction status data</td></tr><tr><td><code class="filename">pg_tblspc</code></td><td>Subdirectory containing symbolic links to tablespaces</td></tr><tr><td><code class="filename">pg_twophase</code></td><td>Subdirectory containing state files for prepared transactions</td></tr><tr><td><code class="filename">pg_wal</code></td><td>Subdirectory containing WAL (Write Ahead Log) files</td></tr><tr><td><code class="filename">pg_xact</code></td><td>Subdirectory containing transaction commit status data</td></tr><tr><td><code class="filename">postgresql.auto.conf</code></td><td>A file used for storing configuration parameters that are set by

<code class="command">ALTER SYSTEM</code></td></tr><tr><td><code class="filename">postmaster.opts</code></td><td>A file recording the command-line options the server was

last started with</td></tr><tr><td><code class="filename">postmaster.pid</code></td><td>A lock file recording the current postmaster process ID (PID),

cluster data directory path,

postmaster start timestamp,

port number,

Unix-domain socket directory path (empty on Windows),

first valid listen_address (IP address or <code class="literal">*</code>, or empty if

not listening on TCP),

and shared memory segment ID

(this file is not present after server shutdown)</td></tr></tbody></table></div></div><br class="table-break" /><p>

For each database in the cluster there is a subdirectory within

<code class="varname">PGDATA</code><code class="filename">/base</code>, named after the database's OID in

<code class="structname">pg_database</code>. This subdirectory is the default location

for the database's files; in particular, its system catalogs are stored

there.

</p><p>

Each table and index is stored in a separate file. For ordinary relations,

these files are named after the table or index's <em class="firstterm">filenode</em> number,

which can be found in <code class="structname">pg_class</code>.<code class="structfield">relfilenode</code>. But

for temporary relations, the file name is of the form

<code class="literal">t<em class="replaceable"><code>BBB</code></em>_<em class="replaceable"><code>FFF</code></em></code>, where <em class="replaceable"><code>BBB</code></em>

is the backend ID of the backend which created the file, and <em class="replaceable"><code>FFF</code></em>

is the filenode number. In either case, in addition to the main file (a/k/a

main fork), each table and index has a <em class="firstterm">free space map</em> (see <a class="xref" href="storage-fsm.html" title="68.3. Free Space Map">Section 68.3</a>), which stores information about free space available in

the relation. The free space map is stored in a file named with the filenode

number plus the suffix <code class="literal">_fsm</code>. Tables also have a

<em class="firstterm">visibility map</em>, stored in a fork with the suffix <code class="literal">_vm</code>,

to track which pages are known to have no dead tuples. The visibility map is

described further in <a class="xref" href="storage-vm.html" title="68.4. Visibility Map">Section 68.4</a>. Unlogged tables and indexes

have a third fork, known as the initialization fork, which is stored in a fork

with the suffix <code class="literal">_init</code> (see <a class="xref" href="storage-init.html" title="68.5. The Initialization Fork">Section 68.5</a>).

</p><div class="caution"><h3 class="title">Caution</h3><p>

Note that while a table's filenode often matches its OID, this is

<span class="emphasis"><em>not</em></span> necessarily the case; some operations, like

<code class="command">TRUNCATE</code>, <code class="command">REINDEX</code>, <code class="command">CLUSTER</code> and some forms

of <code class="command">ALTER TABLE</code>, can change the filenode while preserving the OID.

Avoid assuming that filenode and table OID are the same.

Also, for certain system catalogs including <code class="structname">pg_class</code> itself,

<code class="structname">pg_class</code>.<code class="structfield">relfilenode</code> contains zero. The

actual filenode number of these catalogs is stored in a lower-level data

structure, and can be obtained using the <code class="function">pg_relation_filenode()</code>

function.

</p></div><p>

When a table or index exceeds 1 GB, it is divided into gigabyte-sized

<em class="firstterm">segments</em>. The first segment's file name is the same as the

filenode; subsequent segments are named filenode.1, filenode.2, etc.

This arrangement avoids problems on platforms that have file size limitations.

(Actually, 1 GB is just the default segment size. The segment size can be

adjusted using the configuration option <code class="option">--with-segsize</code>

when building <span class="productname">PostgreSQL</span>.)

In principle, free space map and visibility map forks could require multiple

segments as well, though this is unlikely to happen in practice.

</p><p>

A table that has columns with potentially large entries will have an

associated <em class="firstterm">TOAST</em> table, which is used for out-of-line storage of

field values that are too large to keep in the table rows proper.

<code class="structname">pg_class</code>.<code class="structfield">reltoastrelid</code> links from a table to

its <acronym class="acronym">TOAST</acronym> table, if any.

See <a class="xref" href="storage-toast.html" title="68.2. TOAST">Section 68.2</a> for more information.

</p><p>

The contents of tables and indexes are discussed further in

<a class="xref" href="storage-page-layout.html" title="68.6. Database Page Layout">Section 68.6</a>.

</p><p>

Tablespaces make the scenario more complicated. Each user-defined tablespace

has a symbolic link inside the <code class="varname">PGDATA</code><code class="filename">/pg_tblspc</code>

directory, which points to the physical tablespace directory (i.e., the

location specified in the tablespace's <code class="command">CREATE TABLESPACE</code> command).

This symbolic link is named after

the tablespace's OID. Inside the physical tablespace directory there is

a subdirectory with a name that depends on the <span class="productname">PostgreSQL</span>

server version, such as <code class="literal">PG_9.0_201008051</code>. (The reason for using

this subdirectory is so that successive versions of the database can use

100

the same <code class="command">CREATE TABLESPACE</code> location value without conflicts.)

101

Within the version-specific subdirectory, there is

102

a subdirectory for each database that has elements in the tablespace, named

103

after the database's OID. Tables and indexes are stored within that

104

directory, using the filenode naming scheme.

105

The <code class="literal">pg_default</code> tablespace is not accessed through

106

<code class="filename">pg_tblspc</code>, but corresponds to

107

<code class="varname">PGDATA</code><code class="filename">/base</code>. Similarly, the <code class="literal">pg_global</code>

108

tablespace is not accessed through <code class="filename">pg_tblspc</code>, but corresponds to

109

<code class="varname">PGDATA</code><code class="filename">/global</code>.

110

</p><p>

111

The <code class="function">pg_relation_filepath()</code> function shows the entire path

112

(relative to <code class="varname">PGDATA</code>) of any relation. It is often useful

113

as a substitute for remembering many of the above rules. But keep in

114

mind that this function just gives the name of the first segment of the

115

main fork of the relation — you may need to append a segment number

116

and/or <code class="literal">_fsm</code>, <code class="literal">_vm</code>, or <code class="literal">_init</code> to find all

117

the files associated with the relation.

118

</p><p>

119

Temporary files (for operations such as sorting more data than can fit in

120

memory) are created within <code class="varname">PGDATA</code><code class="filename">/base/pgsql_tmp</code>,

121

or within a <code class="filename">pgsql_tmp</code> subdirectory of a tablespace directory

122

if a tablespace other than <code class="literal">pg_default</code> is specified for them.

123

The name of a temporary file has the form

124

<code class="filename">pgsql_tmp<em class="replaceable"><code>PPP</code></em>.<em class="replaceable"><code>NNN</code></em></code>,

125

where <em class="replaceable"><code>PPP</code></em> is the PID of the owning backend and

126

<em class="replaceable"><code>NNN</code></em> distinguishes different temporary files of that backend.

127

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