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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="BACKUP-DUMP"
>24.1. <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> Dump</A
></H1
><P
> The idea behind this dump method is to generate a text file with SQL
commands that, when fed back to the server, will recreate the
database in the same state as it was at the time of the dump.
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> provides the utility program
<A
HREF="app-pgdump.html"
>pg_dump</A
> for this purpose. The basic usage of this
command is:
</P><PRE
CLASS="SYNOPSIS"
>pg_dump <TT
CLASS="REPLACEABLE"
><I
>dbname</I
></TT
> > <TT
CLASS="REPLACEABLE"
><I
>outfile</I
></TT
></PRE
><P>
As you see, <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> writes its result to the
standard output. We will see below how this can be useful.
</P
><P
> <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> is a regular <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
client application (albeit a particularly clever one). This means
that you can perform this backup procedure from any remote host that has
access to the database. But remember that <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
>
does not operate with special permissions. In particular, it must
have read access to all tables that you want to back up, so in
practice you almost always have to run it as a database superuser.
</P
><P
> To specify which database server <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> should
contact, use the command line options <TT
CLASS="OPTION"
>-h
<TT
CLASS="REPLACEABLE"
><I
>host</I
></TT
></TT
> and <TT
CLASS="OPTION"
>-p <TT
CLASS="REPLACEABLE"
><I
>port</I
></TT
></TT
>. The
default host is the local host or whatever your
<TT
CLASS="ENVAR"
>PGHOST</TT
> environment variable specifies. Similarly,
the default port is indicated by the <TT
CLASS="ENVAR"
>PGPORT</TT
>
environment variable or, failing that, by the compiled-in default.
(Conveniently, the server will normally have the same compiled-in
default.)
</P
><P
> Like any other <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> client application,
<SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> will by default connect with the database
user name that is equal to the current operating system user name. To override
this, either specify the <TT
CLASS="OPTION"
>-U</TT
> option or set the
environment variable <TT
CLASS="ENVAR"
>PGUSER</TT
>. Remember that
<SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> connections are subject to the normal
client authentication mechanisms (which are described in <A
HREF="client-authentication.html"
>Chapter 19</A
>).
</P
><P
> An important advantage of <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> over the other backup
methods described later is that <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
>'s output can
generally be re-loaded into newer versions of <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>,
whereas file-level backups and continuous archiving are both extremely
server-version-specific. <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> is also the only method
that will work when transferring a database to a different machine
architecture, such as going from a 32-bit to a 64-bit server.
</P
><P
> Dumps created by <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> are internally consistent,
meaning, the dump represents a snapshot of the database at the time
<SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> began running. <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> does not
block other operations on the database while it is working.
(Exceptions are those operations that need to operate with an
exclusive lock, such as most forms of <TT
CLASS="COMMAND"
>ALTER TABLE</TT
>.)
</P
><DIV
CLASS="IMPORTANT"
><BLOCKQUOTE
CLASS="IMPORTANT"
><P
><B
>Important: </B
> If your database schema relies on OIDs (for instance, as foreign
keys) you must instruct <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> to dump the OIDs
as well. To do this, use the <TT
CLASS="OPTION"
>-o</TT
> command-line
option.
</P
></BLOCKQUOTE
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="BACKUP-DUMP-RESTORE"
>24.1.1. Restoring the Dump</A
></H2
><P
> The text files created by <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> are intended to
be read in by the <SPAN
CLASS="APPLICATION"
>psql</SPAN
> program. The
general command form to restore a dump is
</P><PRE
CLASS="SYNOPSIS"
>psql <TT
CLASS="REPLACEABLE"
><I
>dbname</I
></TT
> < <TT
CLASS="REPLACEABLE"
><I
>infile</I
></TT
></PRE
><P>
where <TT
CLASS="REPLACEABLE"
><I
>infile</I
></TT
> is the
file output by the <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> command. The database <TT
CLASS="REPLACEABLE"
><I
>dbname</I
></TT
> will not be created by this
command, so you must create it yourself from <TT
CLASS="LITERAL"
>template0</TT
>
before executing <SPAN
CLASS="APPLICATION"
>psql</SPAN
> (e.g., with
<TT
CLASS="LITERAL"
>createdb -T template0 <TT
CLASS="REPLACEABLE"
><I
>dbname</I
></TT
></TT
>). <SPAN
CLASS="APPLICATION"
>psql</SPAN
>
supports options similar to <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> for specifying
the database server to connect to and the user name to use. See
the <A
HREF="app-psql.html"
><SPAN
CLASS="APPLICATION"
>psql</SPAN
></A
> reference page for more information.
</P
><P
> Before restoring an SQL dump, all the users who own objects or were
granted permissions on objects in the dumped database must already
exist. If they do not, the restore will fail to recreate the
objects with the original ownership and/or permissions.
(Sometimes this is what you want, but usually it is not.)
</P
><P
> By default, the <SPAN
CLASS="APPLICATION"
>psql</SPAN
> script will continue to
execute after an SQL error is encountered. You might wish to run
<SPAN
CLASS="APPLICATION"
>psql</SPAN
> with
the <TT
CLASS="LITERAL"
>ON_ERROR_STOP</TT
> variable set to alter that
behavior and have <SPAN
CLASS="APPLICATION"
>psql</SPAN
> exit with an
exit status of 3 if an SQL error occurs:
</P><PRE
CLASS="PROGRAMLISTING"
>psql --set ON_ERROR_STOP=on dbname < infile</PRE
><P>
Either way, you will only have a partially restored database.
Alternatively, you can specify that the whole dump should be
restored as a single transaction, so the restore is either fully
completed or fully rolled back. This mode can be specified by
passing the <TT
CLASS="OPTION"
>-1</TT
> or <TT
CLASS="OPTION"
>--single-transaction</TT
>
command-line options to <SPAN
CLASS="APPLICATION"
>psql</SPAN
>. When using this
mode, be aware that even a minor error can rollback a
restore that has already run for many hours. However, that might
still be preferable to manually cleaning up a complex database
after a partially restored dump.
</P
><P
> The ability of <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> and <SPAN
CLASS="APPLICATION"
>psql</SPAN
> to
write to or read from pipes makes it possible to dump a database
directly from one server to another, for example:
</P><PRE
CLASS="PROGRAMLISTING"
>pg_dump -h <TT
CLASS="REPLACEABLE"
><I
>host1</I
></TT
> <TT
CLASS="REPLACEABLE"
><I
>dbname</I
></TT
> | psql -h <TT
CLASS="REPLACEABLE"
><I
>host2</I
></TT
> <TT
CLASS="REPLACEABLE"
><I
>dbname</I
></TT
></PRE
><P>
</P
><DIV
CLASS="IMPORTANT"
><BLOCKQUOTE
CLASS="IMPORTANT"
><P
><B
>Important: </B
> The dumps produced by <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> are relative to
<TT
CLASS="LITERAL"
>template0</TT
>. This means that any languages, procedures,
etc. added via <TT
CLASS="LITERAL"
>template1</TT
> will also be dumped by
<SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
>. As a result, when restoring, if you are
using a customized <TT
CLASS="LITERAL"
>template1</TT
>, you must create the
empty database from <TT
CLASS="LITERAL"
>template0</TT
>, as in the example
above.
</P
></BLOCKQUOTE
></DIV
><P
> After restoring a backup, it is wise to run <A
HREF="sql-analyze.html"
>ANALYZE</A
> on each
database so the query optimizer has useful statistics;
see <A
HREF="routine-vacuuming.html#VACUUM-FOR-STATISTICS"
>Section 23.1.3</A
>
and <A
HREF="routine-vacuuming.html#AUTOVACUUM"
>Section 23.1.6</A
> for more information.
For more advice on how to load large amounts of data
into <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> efficiently, refer to <A
HREF="populate.html"
>Section 14.4</A
>.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="BACKUP-DUMP-ALL"
>24.1.2. Using <SPAN
CLASS="APPLICATION"
>pg_dumpall</SPAN
></A
></H2
><P
> <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> dumps only a single database at a time,
and it does not dump information about roles or tablespaces
(because those are cluster-wide rather than per-database).
To support convenient dumping of the entire contents of a database
cluster, the <A
HREF="app-pg-dumpall.html"
><SPAN
CLASS="APPLICATION"
>pg_dumpall</SPAN
></A
> program is provided.
<SPAN
CLASS="APPLICATION"
>pg_dumpall</SPAN
> backs up each database in a given
cluster, and also preserves cluster-wide data such as role and
tablespace definitions. The basic usage of this command is:
</P><PRE
CLASS="SYNOPSIS"
>pg_dumpall > <TT
CLASS="REPLACEABLE"
><I
>outfile</I
></TT
></PRE
><P>
The resulting dump can be restored with <SPAN
CLASS="APPLICATION"
>psql</SPAN
>:
</P><PRE
CLASS="SYNOPSIS"
>psql -f <TT
CLASS="REPLACEABLE"
><I
>infile</I
></TT
> postgres</PRE
><P>
(Actually, you can specify any existing database name to start from,
but if you are loading into an empty cluster then <TT
CLASS="LITERAL"
>postgres</TT
>
should usually be used.) It is always necessary to have
database superuser access when restoring a <SPAN
CLASS="APPLICATION"
>pg_dumpall</SPAN
>
dump, as that is required to restore the role and tablespace information.
If you use tablespaces, make sure that the tablespace paths in the
dump are appropriate for the new installation.
</P
><P
> <SPAN
CLASS="APPLICATION"
>pg_dumpall</SPAN
> works by emitting commands to re-create
roles, tablespaces, and empty databases, then invoking
<SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> for each database. This means that while
each database will be internally consistent, the snapshots of
different databases might not be exactly in-sync.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="BACKUP-DUMP-LARGE"
>24.1.3. Handling Large Databases</A
></H2
><P
> Some operating systems have maximum file size limits that cause
problems when creating large <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> output files.
Fortunately, <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
> can write to the standard
output, so you can use standard Unix tools to work around this
potential problem. There are several possible methods:
</P
><DIV
CLASS="FORMALPARA"
><P
><B
>Use compressed dumps. </B
> You can use your favorite compression program, for example
<SPAN
CLASS="APPLICATION"
>gzip</SPAN
>:
</P><PRE
CLASS="PROGRAMLISTING"
>pg_dump <TT
CLASS="REPLACEABLE"
><I
>dbname</I
></TT
> | gzip > <TT
CLASS="REPLACEABLE"
><I
>filename</I
></TT
>.gz</PRE
><P>
Reload with:
</P><PRE
CLASS="PROGRAMLISTING"
>gunzip -c <TT
CLASS="REPLACEABLE"
><I
>filename</I
></TT
>.gz | psql <TT
CLASS="REPLACEABLE"
><I
>dbname</I
></TT
></PRE
><P>
or:
</P><PRE
CLASS="PROGRAMLISTING"
>cat <TT
CLASS="REPLACEABLE"
><I
>filename</I
></TT
>.gz | gunzip | psql <TT
CLASS="REPLACEABLE"
><I
>dbname</I
></TT
></PRE
><P>
</P
></DIV
><DIV
CLASS="FORMALPARA"
><P
><B
>Use <TT
CLASS="COMMAND"
>split</TT
>. </B
> The <TT
CLASS="COMMAND"
>split</TT
> command
allows you to split the output into smaller files that are
acceptable in size to the underlying file system. For example, to
make chunks of 1 megabyte:
</P><PRE
CLASS="PROGRAMLISTING"
>pg_dump <TT
CLASS="REPLACEABLE"
><I
>dbname</I
></TT
> | split -b 1m - <TT
CLASS="REPLACEABLE"
><I
>filename</I
></TT
></PRE
><P>
Reload with:
</P><PRE
CLASS="PROGRAMLISTING"
>cat <TT
CLASS="REPLACEABLE"
><I
>filename</I
></TT
>* | psql <TT
CLASS="REPLACEABLE"
><I
>dbname</I
></TT
></PRE
><P>
</P
></DIV
><DIV
CLASS="FORMALPARA"
><P
><B
>Use <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
>'s custom dump format. </B
> If <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> was built on a system with the
<SPAN
CLASS="APPLICATION"
>zlib</SPAN
> compression library installed, the custom dump
format will compress data as it writes it to the output file. This will
produce dump file sizes similar to using <TT
CLASS="COMMAND"
>gzip</TT
>, but it
has the added advantage that tables can be restored selectively. The
following command dumps a database using the custom dump format:
</P><PRE
CLASS="PROGRAMLISTING"
>pg_dump -Fc <TT
CLASS="REPLACEABLE"
><I
>dbname</I
></TT
> > <TT
CLASS="REPLACEABLE"
><I
>filename</I
></TT
></PRE
><P>
A custom-format dump is not a script for <SPAN
CLASS="APPLICATION"
>psql</SPAN
>, but
instead must be restored with <SPAN
CLASS="APPLICATION"
>pg_restore</SPAN
>, for example:
</P><PRE
CLASS="PROGRAMLISTING"
>pg_restore -d <TT
CLASS="REPLACEABLE"
><I
>dbname</I
></TT
> <TT
CLASS="REPLACEABLE"
><I
>filename</I
></TT
></PRE
><P>
See the <A
HREF="app-pgdump.html"
>pg_dump</A
> and <A
HREF="app-pgrestore.html"
>pg_restore</A
> reference pages for details.
</P
></DIV
><P
> For very large databases, you might need to combine <TT
CLASS="COMMAND"
>split</TT
>
with one of the other two approaches.
</P
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