The idea behind this dump method is to generate a 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_dumpdbname>dumpfile
As you see, pg_dump writes its result to the standard output. We will see below how this can be useful. While the above command creates a text file, pg_dump can create files in other formats that allow for parallelism and more fine-grained control of object restoration.
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 order
to back up the entire database you almost always have to run it as a
database superuser. (If you do not have sufficient privileges to back up
the entire database, you can still back up portions of the database to which
you do have access using options such as
-n
or schema-t .)
table
To specify which database server pg_dump should
contact, use the command line options -h
and host-p . The
default host is the local host or whatever your
portPGHOST 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 20).
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.)
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
psqldbname<dumpfile
where dumpfile 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 ). 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.
Non-text file dumps are restored using the pg_restore utility.
dbname
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=ondbname<dumpfile
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 -hhost1dbname| psql -hhost2dbname
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 24.1.3 and Section 24.1.6 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 > dumpfile
The resulting dump can be restored with psql:
psql -f dumpfile 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 are not synchronized.
Cluster-wide data can be dumped alone using the
pg_dumpall --globals-only option.
This is necessary to fully backup the cluster if running the
pg_dump command on individual databases.
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_dumpdbname| gzip >filename.gz
Reload with:
gunzip -cfilename.gz | psqldbname
or:
catfilename.gz | gunzip | psqldbname
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_dumpdbname| split -b 1m -filename
Reload with:
catfilename* | psqldbname
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 -Fcdbname>filename
A custom-format dump is not a script for psql, but instead must be restored with pg_restore, for example:
pg_restore -ddbnamefilename
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.
Use pg_dump's parallel dump feature.
To speed up the dump of a large database, you can use
pg_dump's parallel mode. This will dump
multiple tables at the same time. You can control the degree of
parallelism with the -j parameter. Parallel dumps
are only supported for the "directory" archive format.
pg_dump -jnum-F d -fout.dirdbname
You can use pg_restore -j to restore a dump in parallel.
This will work for any archive of either the "custom" or the "directory"
archive mode, whether or not it has been created with pg_dump -j.