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Document the backup manifest file format.
Patch by me, at the request of Andres Freund. Reviewed by Justin Pryzby, Erik Rijkers, Álvaro Herrera, and Andrew Dunstan. Discussion: http://postgr.es/m/20200327203225.hcm6ag4grwsiruea@alap3.anarazel.de
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‎doc/src/sgml/backup-manifest.sgml

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<!-- doc/src/sgml/backupmanifest.sgml -->
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<chapter id="backup-manifest-format">
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<title>Backup Manifest Format</title>
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<para>
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The backup manifest generated by <xref linkend="app-pgbasebackup" /> is
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primarily intended to permit the backup to be verified using
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<xref linkend="app-pgverifybackup" />. However, it is
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also possible for other tools to read the backup manifest file and use
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the information contained therein for their own purposes. To that end,
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this chapter describes the format of the backup manifest file.
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</para>
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<para>
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A backup manifest is a JSON document encoded as UTF-8. (Although in
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general JSON documents are required to be Unicode, PostgreSQL permits
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the <type>json</type> and <type>jsonb</type> data types to be used with any
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supported server encoding. There is no similar exception for backup
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manifests.) The JSON document is always an object; the keys that are present
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in this object are described in the next section.
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</para>
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<sect1 id="backup-manifest-toplevel">
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<title>Backup Manifest Toplevel Object</title>
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<para>
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The backup manifest JSON document contains the following keys.
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</para>
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<variablelist>
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<varlistentry>
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<term><literal>PostgreSQL-Backup-Manifest-Version</literal></term>
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<listitem>
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<para>
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The associated value is always the integer 1.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>Files</literal></term>
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<listitem>
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<para>
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The associated value is always a list of objects, each describing one
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file that is present in the backup. No entries are present in this
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list for the WAL files that are needed in order to use the backup,
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or for the backup manifest itself. The structure of each object in the
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list is described in <xref linkend="backup-manifest-files" />.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>WAL-Ranges</literal></term>
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<listitem>
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<para>
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The associated value is always a list of objects, each describing a
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range of WAL records that must be readable from a particular timeline
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in order to make use of the backup. The structure of these objects is
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further described in <xref linkend="backup-manifest-wal-ranges" />.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>Manifest-Checksum</literal></term>
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<listitem>
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<para>
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This key is always present on the last line of the backup manifest file.
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The associated value is a SHA256 checksum of all the preceding lines.
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We use a fixed checksum method here to make it possible for clients
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to do incremental parsing of the manifest. While a SHA256 checksum
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is significantly more expensive than a CRC32C checksum, the manifest
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should normally be small enough that the extra computation won't matter
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very much.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect1>
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<sect1 id="backup-manifest-files">
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<title>Backup Manifest File Object</title>
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<para>
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The object which describes a single file contains either a
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<literal>Path</literal> key or an <literal>Encoded-Path</literal> key.
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Normally, the <literal>Path</literal> key will be present. The
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associated string value is the path of the file relative to the root
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of the backup directory. Files located in a user-defined tablespace
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will have paths whose first two components are pg_tblspc and the OID
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of the tablespace. If the path is not a string that is legal in UTF-8,
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or if the user requests that encoded paths be used for all files, then
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the <literal>Encoded-Path</literal> key will be present instead. This
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stores the same data, but it is encoded as a string of hexadecimal
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digits. Each pair of hexadecimal digits in the string represents a
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single octet.
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</para>
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<para>
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The following two keys are always present:
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</para>
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<variablelist>
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<varlistentry>
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<term><literal>Size</literal></term>
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<listitem>
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<para>
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The expected size of this file, as an integer.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>Last-Modified</literal></term>
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<listitem>
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<para>
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The last modification time of the file as reported by the server at
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the time of the backup. Unlike the other fields stored in the backup,
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this field is not used by <xref linkend="app-pgverifybackup" />.
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It is included only for informational purposes.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>
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If the backup was taken with file checksums enabled, the following
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keys will be present:
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</para>
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<variablelist>
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<varlistentry>
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<term><literal>Checksum-Algorithm</literal></term>
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<listitem>
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<para>
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The checksum algorithm used to compute a checksum for this file.
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Currently, this will be the same for every file in the backup
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manifest, but this may change in future releases. At present, the
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supported checksum algorithms are <literal>CRC32C</literal>,
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<literal>SHA224</literal>,
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<literal>SHA256</literal>,
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<literal>SHA384</literal>, and
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<literal>SHA512</literal>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>Checksum</literal></term>
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<listitem>
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<para>
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The checksum computed for this file, stored as a series of
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hexadecimal characters, two for each byte of the checksum.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect1>
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<sect1 id="backup-manifest-wal-ranges">
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<title>Backup Manifest WAL Range Object</title>
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<para>
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The object which describes a WAL range always has three keys:
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</para>
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<variablelist>
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<varlistentry>
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<term><literal>Timeline</literal></term>
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<listitem>
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<para>
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The timeline for this range of WAL records, as an integer.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>Start-LSN</literal></term>
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<listitem>
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<para>
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The LSN at which replay must begin on the indicated timeline in order to
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make use of this backup. The LSN is stored in the format normally used
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by <productname>PostgreSQL</productname>; that is, it is a string
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consisting of two strings of hexadecimal characters, each with a length
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of between 1 and 8, separated by a slash.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>End-LSN</literal></term>
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<listitem>
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<para>
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The earliest LSN at which replay on the indicated timeline may end when
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making use of this backup. This is stored in the same format as
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<literal>Start-LSN</literal>.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>
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Ordinarily, there will be only a single WAL range. However, if a backup is
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taken from a standby which switches timelines during the backup due to an
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upstream promotion, it is possible for multiple ranges to be present, each
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with a different timeline. There will never be multiple WAL ranges present
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for the same timeline.
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</para>
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</sect1>
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</chapter>

‎doc/src/sgml/filelist.sgml

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<!ENTITY storage SYSTEM "storage.sgml">
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<!ENTITY tablesample-method SYSTEM "tablesample-method.sgml">
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<!ENTITY generic-wal SYSTEM "generic-wal.sgml">
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<!ENTITY backup-manifest SYSTEM "backup-manifest.sgml">
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<!-- contrib information -->
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<!ENTITY contrib SYSTEM "contrib.sgml">

‎doc/src/sgml/postgres.sgml

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&storage;
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&bki;
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&planstats;
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&backup-manifest;
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</part>
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