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.
PostgreSQL provides the utility program
pg_dump for this purpose. The basic usage of this
pg_dump dbname > outfile
As you see, pg_dump writes its results to the
standard output. We will see below how this can be useful.
pg_dump is a regular PostgreSQL
client application (albeit a particularly clever one). This means
that you can do this backup procedure from any remote host that has
access to the database. But remember that pg_dump
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.
To specify which database server pg_dump should
contact, use the command line options -h
host and -p port. The
default host is the local host or whatever your
PGHOST environment variable specifies. Similarly,
the default port is indicated by the PGPORT
environment variable or, failing that, by the compiled-in default.
(Conveniently, the server will normally have the same compiled-in
Like any other PostgreSQL client application,
pg_dump 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 -U option or set the
environment variable PGUSER. Remember that
pg_dump connections are subject to the normal
client authentication mechanisms (which are described in Chapter 19).
Dumps created by pg_dump are internally consistent,
that is, the dump represents a snapshot of the database as of the time
pg_dump begins running. pg_dump 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 ALTER TABLE.)
Important: If your database schema relies on OIDs (for instance as foreign
keys) you must instruct pg_dump to dump the OIDs
as well. To do this, use the -o command line
The text files created by pg_dump are intended to
be read in by the psql program. The
general command form to restore a dump is
psql dbname < infile
where infile is what
you used as outfile
for the pg_dump command. The database dbname will not be created by this
command, so you must create it yourself from template0
before executing psql (e.g., with
createdb -T template0 dbname). psql
supports options similar to pg_dump's for specifying
the database server to connect to and the user name to use. See
the psql reference page for more information.
Before restoring a 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, then 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.)
By default, the psql script will continue to
execute after an SQL error is encountered. You might wish to use the
following command at the top of the script to alter that
behaviour and have psql exit with an
exit status of 3 if an SQL error occurs:
Either way, you will have an only 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 -1 or --single-transaction
command-line options to psql. When using this
mode, be aware that even the smallest of errors 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.
The ability of pg_dump and psql to
write to or read from pipes makes it possible to dump a database
directly from one server to another, for example:
pg_dump -h host1 dbname | psql -h host2 dbname
Important: The dumps produced by pg_dump are relative to
template0. This means that any languages, procedures,
etc. added via template1 will also be dumped by
pg_dump. As a result, when restoring, if you are
using a customized template1, you must create the
empty database from template0, as in the example
After restoring a backup, it is wise to run ANALYZE on each
database so the query optimizer has useful statistics;
see Updating Planner Statistics
and The Autovacuum Daemon for more information.
For more advice on how to load large amounts of data
into PostgreSQL efficiently, refer to Section 14.4.
pg_dump 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 pg_dumpall program is provided.
pg_dumpall 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:
pg_dumpall > outfile
The resulting dump can be restored with psql:
psql -f infile postgres
(Actually, you can specify any existing database name to start from,
but if you are reloading into an empty cluster then postgres
should usually be used.) It is always necessary to have
database superuser access when restoring a pg_dumpall
dump, as that is required to restore the role and tablespace information.
If you use tablespaces, be careful that the tablespace paths in the
dump are appropriate for the new installation.
pg_dumpall works by emitting commands to re-create
roles, tablespaces, and empty databases, then invoking
pg_dump for each database. This means that while
each database will be internally consistent, the snapshots of
different databases might not be exactly in-sync.
Since PostgreSQL allows tables larger
than the maximum file size on your system, it can be problematic
to dump such a table to a file, since the resulting file will likely
be larger than the maximum size allowed by your system. Since
pg_dump can write to the standard output, you can
use standard Unix tools to work around this possible problem.
There are several ways to do it:
Use compressed dumps. You can use your favorite compression program, for example
pg_dump dbname | gzip > filename.gz
gunzip -c filename.gz | psql dbname
cat filename.gz | gunzip | psql dbname
Use split. The split command
allows you to split the output into pieces that are
acceptable in size to the underlying file system. For example, to
make chunks of 1 megabyte:
pg_dump dbname | split -b 1m - filename
cat filename* | psql dbname
Use pg_dump's custom dump format. If PostgreSQL was built on a system with the
zlib 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 gzip, but it
has the added advantage that tables can be restored selectively. The
following command dumps a database using the custom dump format:
pg_dump -Fc dbname > filename
A custom-format dump is not a script for psql, but
instead must be restored with pg_restore, for example:
pg_restore -d dbname filename
See the pg_dump and pg_restore reference pages for details.
For very large databases, you might need to combine split
with one of the other two approaches.