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 command is:
pg_dump dbname > outfile
As you see, pg_dump writes its result 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 perform 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 default.)
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).
An important advantage of pg_dump over the other backup methods described later is that pg_dump's output can generally be re-loaded into newer versions of PostgreSQL, whereas file-level backups and continuous archiving are both extremely server-version-specific. pg_dump 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.
Dumps created by pg_dump are internally consistent, meaning, the dump represents a snapshot of the database at the time pg_dump began 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 option.
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 the file output by 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 for specifying the database server to connect to and the user name to use. See the psql reference page for more information.
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.)
By default, the psql script will continue to execute after an SQL error is encountered. You might wish to run psql with the ON_ERROR_STOP variable set to alter that behavior and have psql exit with an exit status of 3 if an SQL error occurs:
psql --set ON_ERROR_STOP=on dbname < infile
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 -1 or --single-transaction command-line options to psql. 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.
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 above.
After restoring a backup, it is wise to run ANALYZE on each database so the query optimizer has useful statistics; see Section 23.1.3 and Section 23.1.6 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 loading 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, make sure 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.
Some operating systems have maximum file size limits that cause problems when creating large pg_dump output files. Fortunately, pg_dump can write to the standard output, so you can use standard Unix tools to work around this potential problem. There are several possible methods:
Use compressed dumps. You can use your favorite compression program, for example gzip:
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 smaller files 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
For very large databases, you might need to combine split with one of the other two approaches.