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Upgrading PostgreSQL on openSUSE and SUSE Linux Enterprise Server
=================================================================

Current versions of PostgreSQL come with the pg_upgrade tool that
simplifies and speeds up the migration of a PostgreSQL installation to
a new version. Before version 9.1 dump and restore was needed which
was much slower.

pg_upgrade needs to have the server binaries of both versions
available. To allow this, we had to change the way PostgreSQL is
packaged as well as the naming of the packages, so that two or more
versions of PostgreSQL can be installed in parallel.  The package
names for PostgreSQL contain numbers indicating the major version.

In PostgreSQL terms for versions up to 9.6 the major version consisted
of the first two components of the three-component version number,
i.e. 8.3, 8.4, 9.0, or 9.1. So, the packages for Postgresql 9.1 are
named postgresql91, postgresql91-server, etc. Inside the packages the
files were moved from their standard locations to a versioned location
such as /usr/lib/postgresql83/bin or /usr/lib/postgresql91/bin to
avoid file conflicts if packages are installed in parallel.

Starting with version 10 the PostgreSQL project changed their
versioning scheme from from three components to two, which means one
component for the major version and one for the minor.  So, the
sequence of major version across the versioning scheme change will be:
9.4, 9.5, 9.6, 10, 11, 12. For versions that use the new versioning
scheme SUSE only puts the single component major version into the
package name, so the postgresql96 package (containg version 9.6
according to the old versioning scheme) will be followed by
postgresql10, then postgresql11, and so on.

The update-alternatives mechanism creates and maintains symbolic links
that cause one version (by default the highest installed version) to
re-appear in the standard locations. By default, database data are
stored under /var/lib/pgsql/data on SUSE Linux.

The following preconditions have to be fulfilled before data migration
can be started:

1. If not already done, the packages of the old PostgreSQL version
 must be upgraded to the new packaging scheme through a maintenance
 update.

2. The packages of the new PostgreSQL major version need to be
 installed. As pg_upgrade is contained in postgresql91-contrib, that
 one has to be installed as well, at least until the migration is
 done.

3. Unless pg_upgrade is used in link mode, the server must have
 enough free disk space to temporarily hold a copy of the database
 files. If the database instance was installed in the default
 location, the needed space in megabytes can be determined by running
 the follwing command as root: "du -hs /var/lib/pgsql/data". If space
 is tight, it might help to run the "VACUUM FULL" SQL command on each
 database in the instance to be migrated, but be aware that it might
 take very long.

The latest upstream documentation for pg_upgrade including step by
step instructions for performing a database migration can be found
online under https://www.postgresql.org/docs/current/pgupgrade.html ,
or locally under
file:///usr/share/doc/packages/postgresqlXX/html/pgupgrade.html , if
the postgresqlXX-docs package is installed. XX is a place holder for
the respective major version here.

NOTE: The online documentation starts with explaining how you can
install PostgreSQL from the upstream sources (which is not necessary
when you install the SUSE RPMs) and also uses other directory names
(/usr/local instead of the update-alternatives based path as described
above).

For background information about the inner workings of pg_upgrade and
a performance comparison with the old dump and restore method, see
http://momjian.us/main/writings/pgsql/pg_upgrade.pdf .

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