Chapter 36. Backup and Restore

Table of Contents

As everything that contains valuable data, EnterpriseDB 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 three fundamentally different approaches to backing up EnterpriseDB data:

SQL dump
File system level backup
On-line backup

Each has its own strengths and weaknesses.

36.1. SQL Dump

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. EnterpriseDB provides the utility program pg_dump for this purpose. Following is an example of this command:

pg_dump -f "C:\newfile.backup" -F c -X use-set-session-authorization "edb" -h localhost -p 5444 -U "edbuser"

Here -F c after the file name indicates that the file has been dumped in a compressed format. -F t here would have saved the file in tar (uncompressed) format

pg_dump is a regular EnterpriseDB 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 EnterpriseDB 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 33).

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.

36.1.1. Restoring the dump

The text files created by pg_dump are intended to be read in by EnterpriseDB PSQL. Following is an example of this command:

pg_restore -F c -d "edb" "C:\newfile.backup" -X use-set-session-authorization -h localhost -p 5444 -U "edbuser"

Here -F c after pg_restore indicates that the dump file being restored is of compressed format. -F t here would have indicated that the file being restored is in tar (uncompressed) format.

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 EnterpriseDB PSQL (e.g., with createdb -T template0 dbname). EnterpriseDB 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 EnterpriseDB 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 VACUUM ANALYZE all databases; this is equivalent to running VACUUM ANALYZE manually.

The ability of pg_dump and edb-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 | edb-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.

For advice on how to load large amounts of data into EnterpriseDB efficiently, refer to Section 13.5.

36.1.2. Using pg_dumpall

The above mechanism is cumbersome and inappropriate when backing up an entire database cluster. pg_dumpall backs up each database in a given cluster, and also preserves cluster-wide data such as users and groups. The basic usage of this command is:

pg_dumpall > outfile

The resulting dump can be restored with EnterpriseDB PSQL:

edb-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.

36.1.3. Large Databases

Since EnterpriseDB 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. Since 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

Reload with

createdb dbname
gunzip -c filename.gz | edb-psql dbname

cat filename.gz | gunzip | edb-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* | edb-psql dbname