PostgreSQL Backups

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Backups

Creating the 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, 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.

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 options similar to pg_dump for controlling the database server location and the user name. See psql's reference page for more information.

Not only must the target database already exist before starting to run the restore, but so must all the users who own objects in the dumped database or were granted permissions on the objects. 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.)

Once restored, it is wise to run ANALYZE on each database so the optimizer has useful statistics. An easy way to do this is to run vacuumdb -a -z to VACUUM ANALYZE all databases; this is equivalent to running VACUUM ANALYZE manually.

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.

Use compressed dumps

You can use your favorite compression program, for example gzip.

pg_dump dbname | gzip > filename.gz

Reload with

createdb dbname
gunzip -c filename.gz | psql dbname

or

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

Reload with

createdb dbname cat filename* | psql dbname


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