Muhammad Azeez - Backing up and restoring databases in Postgres

To get a dump of a database you can use pg_dump or pg_dumpall for dumping an entire cluster. It supports 4 formats:

Format	Description	Restore via
`plain`	Output a plain-text SQL script file (the default).	`psql`
`custom`	Output a custom-format archive suitable for input into pg_restore. Together with the directory output format, this is the most flexible output format in that it allows manual selection and reordering of archived items during restore. This format is also compressed by default.	`pg_restore`
`directory`	Output a directory-format archive suitable for input into pg_restore. This will create a directory with one file for each table and blob being dumped, plus a so-called Table of Contents file describing the dumped objects in a machine-readable format that pg_restore can read. A directory format archive can be manipulated with standard Unix tools; for example, files in an uncompressed archive can be compressed with the gzip tool. This format is compressed by default.	`pg_restore`
`tar`	Output a `tar`-format archive suitable for input into pg_restore. The tar format is compatible with the directory format: extracting a tar-format archive produces a valid directory-format archive. However, the tar format does not support compression. Also, when using tar format the relative order of table data items cannot be changed during restore.	`pg_restore`

How to backup a database

To create a dump of sample-db in custom format and save it to sample-db.dump:

pg_dump -U postgres --encoding utf8 -F c -f sample-db.dump sample-db

To create a dump of sample-db in plain format and save it to sample-db.sql:

pg_dump -U postgres --encoding utf8 -F p -f stoplist.sql stoplist

How to restore a database dump

First create an empty database to restore the dump to.

# We use template0 because it's is empty and it doesn't conflict with the schemas and tables in the dump.
createdb -U postgres restored-db --template=template0

Restore custom, directory, and tar format dumps using pg_restore:

pg_restore -U postgres -d restored-db < ./sample-db.dump

Restore plain format dumps using psql:

psql -U postgres -d restored-db < ./sample-db.sql

Note: In Powershell the < operator doesn't work. So you'll have to use cmd on Windows.

Errors you might come across:

Corrupted dumps

pg_restore: [archiver] found unexpected block id (x) when reading data -- expected y

pg_restore: error unrecognized data block type

This might mean the dump is corrupted. One possible reason is the database contained Unicode data and the dump was not encoded in utf8. Use --encoding utf8 when running pg_dump to fix that.

Restoring plain format dumps using pg_restore:

pg_restore: [archiver] did not find magic string in file header

pg_restore: [archiver] input file does not appear to be a valid archive

This happens if you run pg_restore on a plain format dump. Use psql to restore it instead.

If you have any other tips/tricks, please write the down in the comments!