As everything that contains valuable data, PostgreSQL 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 PostgreSQL data:
File system level backup
The idea behind the SQL-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 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, 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 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, 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 FULL.)
Important: When 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. "Large objects" are not dumped by default, either. See pg_dump's command reference page if you use large objects.
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 name. 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 users 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.
Once restored, it is wise to run ANALYZE on each database so the optimizer has useful statistics. You can also run vacuumdb -a -z to ANALYZE all databases.
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 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.
Tip: Restore performance can be improved by increasing the configuration parameter sort_mem (see Section 126.96.36.199).
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 preserves cluster-wide data such as users and groups. The call sequence for pg_dumpall is simply
pg_dumpall > outfile
The resulting dump can be restored with psql:
psql template1 < infile
(Actually, you can specify any existing database name to start from, but if you are reloading in an empty cluster then template1 is the only available choice.) It is always necessary to have database superuser access when restoring a pg_dumpall dump, as that is required to restore the user and group information.
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. As pg_dump can write to the standard output, you can just use standard Unix tools to work around this possible problem.
Use compressed dumps. You can use your favorite compression program, for example gzip.
pg_dump dbname | gzip > filename.gz
createdb dbname 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
createdb dbname cat filename* | psql dbname
Use the 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. 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.
pg_dump (and by implication pg_dumpall) has a few limitations which stem from the difficulty of reconstructing 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 PostgreSQL source tree also contains a program that can dump large objects.
Please familiarize yourself with the pg_dump reference page.
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