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Chapter 8. Backup and Restore

As everything that contains valuable data, Postgres databases should be backed up regularly. While the procedure is essentially simple, it is important to have a basic understanding of the underlying techniques and assumptions.

There are two fundamentally different approaches to backing up Postgres data:

  • SQL dump

  • File system level backup

8.1. SQL Dump

The idea behind this 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. Postgres provides the utility program pg_dump for this purpose. The basic usage of this command is:

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 Postgres 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, you must have read access to all tables that you want to back up, so in practice you almost always have to be 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 default.)

As any other Postgres client application, pg_dump will by default connect with the database user name that is equal to the current Unix user name. To override this, either specify the -u option to force a prompt for the user name, or set the environment variable PGUSER. Remember that pg_dump connections are subject to the normal client authentication mechanisms (which are described in Chapter 4).

Dumps created by pg_dump are internally consistent, that is, updates to the database while pg_dump is running will not be in the dump. 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 VACUUM.)

Important: When your database schema relies on OIDs (for instances as foreign keys) you must instruct pg_dump to dump the OIDs as well. To do this, use the -o command line option.

8.1.1. Restoring the dump

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, you must create it yourself from template0 before executing psql (e.g., with createdb -T template0 dbname). psql supports similar options to pg_dump for controlling the database server location and the user names. See its reference page for more information.

If the objects in the original database were owned by different users, then the dump will instruct psql to connect as each affected user in turn and then create the relevant objects. This way the original ownership is preserved. This also means, however, that all these user must already exist, and furthermore that you must be allowed to connect as each of them. It might therefore be necessary to temporarily relax the client authentication settings.

The ability of pg_dump and psql to write or read from pipes also make it possible to dump a database directory 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 to 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 above.

8.1.2. Using pg_dumpall

The above mechanism is cumbersome and inappropriate when backing up an entire database cluster. For this reason the pg_dumpall program is provided. pg_dumpall backs up each database in a given cluster and also makes sure that the state of global data such as users and groups is preserved. The call sequence for pg_dumpall is simply

pg_dumpall > outfile
The resulting dumps can be restored with psql as described above. But in this case it is definitely necessary that you have database superuser access, as that is required to restore the user and group information.

pg_dumpall has one little flaw: It is not prepared for interactively authenticating to each database it dumps. If you are using password authentication then you need to set it the environment variable PGPASSWORD to communicate the password the the underlying calls to pg_dump. More severely, if you have different passwords set up for each database, then pg_dumpall will fail. You can either choose a different authentication mechanism for the purposes of backup or adjust the pg_dumpall shell script to your needs.

8.1.3. Large Databases

Acknowledgement: Originally written by Hannu Krosing () on 1999-06-19

Since Postgres allows tables larger than the maximum file size on your system, it can be problematic to dump the table to a file, since the resulting file will likely be larger than the maximum size allowed by your system. As pg_dump writes to the standard output, you can just use standard *nix tools to work around this possible problem.

Use compressed dumps. Use your favorite compression program, for example gzip.

pg_dump dbname | gzip > filename.gz
Reload with
createdb dbname
gunzip -c filename.gz | psql dbname
cat filename.gz | gunzip | psql dbname

Use split. This 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
Reload with
createdb dbname
cat filename.* | psql dbname

Use the custom dump format (V7.1). 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. For large databases, this will produce similar dump sizes to using gzip, but has the added advantage that the tables can be restored selectively. The following command dumps a database using the custom dump format:

pg_dump -Fc dbname > filename
See the pg_dump and pg_restore reference pages for details.

8.1.4. Caveats

pg_dump (and by implication pg_dumpall) has a few limitations which stem from the difficulty to reconstruct certain information from the system catalogs.

Specifically, the order in which pg_dump writes the objects is not very sophisticated. This can lead to problems for example when functions are used as column default values. The only answer is to manually reorder the dump. If you created circular dependencies in your schema then you will have more work to do.

For reasons of backward compatibility, pg_dump does not dump large objects by default. To dump large objects you must use either the custom or the TAR output format, and use the -B option in pg_dump. See the reference pages for details. The directory contrib/pg_dumplo of the Postgres source tree also contains a program that can dump large objects.

Please familiarize yourself with the pg_dump reference page.