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17.6. Upgrading a PostgreSQL Cluster

This section discusses how to upgrade your database data from one PostgreSQL release to a newer one.

PostgreSQL major versions are represented by the first two digit groups of the version number, e.g., 8.4. PostgreSQL minor versions are represented by the third group of version digits, e.g., 8.4.2 is the second minor release of 8.4. Minor releases never change the internal storage format and are always compatible with earlier and later minor releases of the same major version number, e.g., 8.4.2 is compatible with 8.4, 8.4.1 and 8.4.6. To update between compatible versions, you simply replace the executables while the server is down and restart the server. The data directory remains unchanged — minor upgrades are that simple.

For major releases of PostgreSQL, the internal data storage format is subject to change, thus complicating upgrades. The traditional method for moving data to a new major version is to dump and reload the database. Other methods are available, as discussed below.

New major versions also typically introduce some user-visible incompatibilities, so application programming changes might be required. All user-visible changes are listed in the release notes (Appendix E); pay particular attention to the section labeled "Migration". If you are upgrading across several major versions, be sure to read the release notes for each intervening version.

Cautious users will want to test their client applications on the new version before switching over fully; therefore, it's often a good idea to set up concurrent installations of old and new versions. When testing a PostgreSQL major upgrade, consider the following categories of possible changes:

Administration: The capabilities available for administrators to monitor and control the server often change and improve in each major release.
SQL: Typically this includes new SQL command capabilities and not changes in behavior, unless specifically mentioned in the release notes.
Library API: Typically libraries like libpq only add new functionality, again unless mentioned in the release notes.
System Catalogs: System catalog changes usually only affect database management tools.
Server C-language API: This involves changes in the backend function API, which is written in the C programming language. Such changes affect code that references backend functions deep inside the server.

17.6.1. Upgrading Data via pg_dump

To dump data from one major version of PostgreSQL and reload it in another, you must use pg_dump; file system level backup methods will not work. (There are checks in place that prevent you from using a data directory with an incompatible version of PostgreSQL, so no great harm can be done by trying to start the wrong server version on a data directory.)

It is recommended that you use the pg_dump and pg_dumpall programs from the newer version of PostgreSQL, to take advantage of enhancements that might have been made in these programs. Current releases of the dump programs can read data from any server version back to 7.0.

These instructions assume that your existing installation is under the /usr/local/pgsql directory, and that the data area is in /usr/local/pgsql/data. Substitute your paths appropriately.

If making a backup, make sure that your database is not being updated. This does not affect the integrity of the backup, but the changed data would of course not be included. If necessary, edit the permissions in the file /usr/local/pgsql/data/pg_hba.conf (or equivalent) to disallow access from everyone except you. See Chapter 19 for additional information on access control.
To back up your database installation, type:
```
pg_dumpall > outputfile
```
If you need to preserve OIDs (such as when using them as foreign keys), then use the -o option when running pg_dumpall.
To make the backup, you can use the pg_dumpall command from the version you are currently running. For best results, however, try to use the pg_dumpall command from PostgreSQL 9.1.24lts2, since this version contains bug fixes and improvements over older versions. While this advice might seem idiosyncratic since you haven't installed the new version yet, it is advisable to follow it if you plan to install the new version in parallel with the old version. In that case you can complete the installation normally and transfer the data later. This will also decrease the downtime.
Shut down the old server:
```
pg_ctl stop
```
On systems that have PostgreSQL started at boot time, there is probably a start-up file that will accomplish the same thing. For example, on a Red Hat Linux system one might find that this works:
```
/etc/rc.d/init.d/postgresql stop
```
See Chapter 17 for details about starting and stopping the server.
If restoring from backup, rename or delete the old installation directory. It is a good idea to rename the directory, rather than delete it, in case you have trouble and need to revert to it. Keep in mind the directory might consume significant disk space. To rename the directory, use a command like this:
```
mv /usr/local/pgsql /usr/local/pgsql.old
```
(Be sure to move the directory as a single unit so relative paths remain unchanged.)
Install the new version of PostgreSQL as outlined in Section 15.4.
Create a new database cluster if needed. Remember that you must execute these commands while logged in to the special database user account (which you already have if you are upgrading).
```
/usr/local/pgsql/bin/initdb -D /usr/local/pgsql/data
```
Restore your previous pg_hba.conf and any postgresql.conf modifications.
Start the database server, again using the special database user account:
```
/usr/local/pgsql/bin/postgres -D /usr/local/pgsql/data
```
Finally, restore your data from backup with:
```
/usr/local/pgsql/bin/psql -d postgres -f outputfile
```
using the new psql.

The least downtime can be achieved by installing the new server in a different directory and running both the old and the new servers in parallel, on different ports. Then you can use something like:

pg_dumpall -p 5432 | psql -d postgres -p 5433

to transfer your data.

17.6.2. Non-Dump Upgrade Methods

The pg_upgrade module allows an installation to be migrated in-place from one major PostgreSQL version to the next. Upgrades can be performed in minutes.

It is also possible to use certain replication methods, such as Slony, to create a standby server with the updated version of PostgreSQL. This is possible because Slony supports replication between different major versions of PostgreSQL. The standby can be on the same computer or a different computer. Once it has synced up with the master server (running the older version of PostgreSQL), you can switch masters and make the standby the master and shut down the older database instance. Such a switch-over results in only several seconds of downtime for an upgrade.

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