Backup and Restore

Remarks

Backing up the filesystem instead of using pg_dumpall and pg_dump

It's very important that if you use this, you call the pg_start_backup() function before and pg_stop_backup() function after. Doing filesystem backups is not safe otherwise; even a ZFS or FreeBSD snapshot of the filesystem backed up without those function calls will place the database in recovery mode and may lose transactions.

I would avoid doing filesystem backups instead of regular Postgres backups, both for this reason, and because Postgres backup files (especially in the custom format) are extremely versatile in supporting alternate restores. Since they're single files, they're also less hassle to manage.

Backing up one database

pg_dump -Fc -f DATABASE.pgsql DATABASE

The -Fc selects the "custom backup format" which gives you more power than raw SQL; see pg_restore for more details. If you want a vanilla SQL file, you can do this instead:

pg_dump -f DATABASE.sql DATABASE

or even

pg_dump DATABASE > DATABASE.sql

Backing up the whole cluster

$ pg_dumpall -f backup.sql

This works behind the scenes by making multiple connections to the server once for each database and executing pg_dump on it.

Sometimes, you might be tempted to set this up as a cron job, so you want to see the date the backup was taken as part of the filename:

$ postgres-backup-$(date +%Y-%m-%d).sql

However, please note that this could produce large files on a daily basis. Postgresql has a much better mechanism for regular backups - WAL archives

The output from pg_dumpall is sufficient to restore to an identically-configured Postgres instance, but the configuration files in $PGDATA (pg_hba.conf and postgresql.conf) are not part of the backup, so you'll have to back them up separately.

postgres=# SELECT pg_start_backup('my-backup');
postgres=# SELECT pg_stop_backup();

To take a filesystem backup, you must use these functions to help ensure that Postgres is in a consistent state while the backup is prepared.

Restoring backups

psql < backup.sql

A safer alternative uses -1 to wrap the restore in a transaction. The -f specifies the filename rather than using shell redirection.

psql -1f backup.sql

Custom format files must be restored using pg_restore with the -d option to specify the database:

pg_restore -d DATABASE DATABASE.pgsql

The custom format can also be converted back to SQL:

pg_restore backup.pgsql > backup.sql

Usage of the custom format is recommended because you can choose which things to restore and optionally enable parallel processing.

You may need to do a pg_dump followed by a pg_restore if you upgrade from one postgresql release to a newer one.

Using Copy to export

To Copy table to standard o/p

COPY <tablename> TO STDOUT (DELIMITER '|');

To export table user to Standard ouput:

COPY user TO STDOUT (DELIMITER '|');

To Copy table to file

COPY user FROM '/home/user/user_data' WITH DELIMITER '|';

To Copy the output of SQL statement to file

COPY (sql statement) TO '<filename with path>';

COPY (SELECT * FROM user WHERE user_name LIKE 'A%') TO '/home/user/user_data';

To Copy into a compressed file

COPY user TO PROGRAM 'gzip > /home/user/user_data.gz';

Here program gzip is executed to compress user table data.

Using Copy to import

To Copy Data from a CSV file to a table

COPY <tablename> FROM '<filename with path>';

To insert into table user from a file named user_data.csv placed inside /home/user/:

COPY user FROM '/home/user/user_data.csv';

To Copy data from pipe separated file to table

COPY user FROM '/home/user/user_data' WITH DELIMITER '|';

Note: In absence of the option with delimiter, the default delimiter is comma ,

To ignore header line while importing file

Use the Header option:

COPY user FROM '/home/user/user_data' WITH DELIMITER '|' HEADER;

Note: If data is quoted, by default data quoting characters are double quote. If the data is quoted using any other character use the QUOTE option; however, this option is allowed only when using CSV format.

Using psql to export data

Data can be exported using copy command or by taking use of command line options of psql command.

To Export csv data from table user to csv file:

psql -p \<port> -U \<username> -d \<database> -A -F<delimiter> -c\<sql to execute> \> \<output filename with path>

psql -p 5432 -U postgres -d test_database -A -F, -c "select * from user" > /home/user/user_data.csv

Here combination of -A and -F does the trick.

-F is to specify delimiter

-A or --no-align

Switches to unaligned output mode. (The default output mode is otherwise aligned.)