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Databases - Practical PostgreSQL
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When to Backup and Restore Data

An important consideration to the use of the pg_dump , pg_dumpall , and pg_restore commands is when to use them, and when not to. Fortunately, in respect to each of these procedures, PostgreSQL is quite accommodating.

When to backup

With regards to backing up data with either pg_dump or pg_dumpall , there are few considerations necessary for when they may be performed. PostgreSQL has supported hot backup procedures since Version 6.5—these allow you to request data without blocking the normal activity of other concurrent users. It is called a hot backup because it is performed while the system is running, uninterrupted.

Therefore, the only potential considerations for backup PostgreSQL center around performance. An exceptionally large database may take a while to dump all of its contents. The use of large objects may also be a factor if you intend to back up large object data with pg_dump 's -b flag (thus, adding to the amount of data needing to be output).

If you have a large database that takes a substantial amount of time to complete a dump, it is recommended that you schedule the pg_dump execution for a time when the database is not heavily used. Even though a dump does not block users from requesting and completing transactions, it can still slow down the general performance of such a system during heavy, or even medium usage.

When to restore

With respect to restoration there are several more considerations to be taken into account than when merely backing up data. Specifically, these apply to how "deep" a restoration must go; restoring just the data is a very different operation from totally recreating the database from scratch, and the restrictions involved scale with the depth of the operation.

The least restrictive kind of restoration is one which restores data only . This can be executed while users are actively connected to the database. It may even be executed while connected users are in the middle of transactions. This is possible through PostgreSQL's multiversion control. Such a restore can be performed on the fly, without having to restart the database system. Once modifications are synchronized with the database, the changes are immediately available to connected users.

A restoration involving dropping and recreating database schema (e.g., tables, views, etc.) may also be performed while the system is running. This method is not as seamless as a data-only restoration, because database objects will briefly be removed from the system, which may cause temporary problems to applications relying on certain objects to exist. The exact nature of such a restriction is dependent on the nature of the application accessing the database.

The most restrictive kind of restoration is one which involves dropping the database itself. If such a restoration is scheduled, it must be done at a time when no other user is connected to that database. The DROP DATABASE command will fail if any user is actively connected at the time it is executed.

It may in fact be necessary to shut down and restart PostgreSQL with TCP/IP connections disabled if a highly-used database is intended to be dropped and recreated from scratch; this will prevent any external machine from connecting to the database server until the work is completed.

Databases - Practical PostgreSQL
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