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PG_UPGRADE: IN-PLACE UPGRADES FOR POSTGRESQL
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Upgrading a PostgreSQL database from one major release to another can be
an expensive process. For minor upgrades, you can simply install new
executables and forget about upgrading existing data. But for major
upgrades, you have to export all of your data using pg_dump, install the
new release, run initdb to create a new cluster, and then import your
old data. If you have a lot of data, that can take a considerable amount
of time. If you have too much data, you may have to buy more storage
since you need enough room to hold the original data plus the exported
data.  pg_upgrade can reduce the amount of time and disk space required
for many upgrades.

The URL http://momjian.us/main/writings/pgsql/pg_upgrade.pdf contains a
presentation about pg_upgrade internals that mirrors the text
description below.

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WHAT IT DOES
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pg_upgrade is a tool that performs an in-place upgrade of existing
data. Some upgrades change the on-disk representation of data;
pg_upgrade cannot help in those upgrades.  However, many upgrades do
not change the on-disk representation of a user-defined table.  In those
cases, pg_upgrade can move existing user-defined tables from the old
database cluster into the new cluster.

There are two factors that determine whether an in-place upgrade is
practical.

Every table in a cluster shares the same on-disk representation of the
table headers and trailers and the on-disk representation of tuple
headers. If this changes between the old version of PostgreSQL and the
new version, pg_upgrade cannot move existing tables to the new cluster;
you will have to pg_dump the old data and then import that data into the
new cluster.

Second, all data types should have the same binary representation
between the two major PostgreSQL versions.

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HOW IT WORKS
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To use pg_upgrade during an upgrade, start by installing a fresh
cluster using the newest version in a new directory. When you've
finished installation, the new cluster will contain the new executables
and the usual template0, template1, and postgres databases, but no
user-defined tables. At this point, you can shut down the old and new
postmasters and invoke pg_upgrade.

When pg_upgrade starts, it ensures that all required executables are
present and contain the expected version numbers. The verification
process also checks the old and new $PGDATA directories to ensure that
the expected files and subdirectories are in place.  If the verification
process succeeds, pg_upgrade starts the old postmaster and runs
pg_dumpall --schema-only to capture the metadata contained in the old
cluster. The script produced by pg_dumpall will be used in a later step
to recreate all user-defined objects in the new cluster.

Note that the script produced by pg_dumpall will only recreate
user-defined objects, not system-defined objects.  The new cluster will
contain the system-defined objects created by the latest version of
PostgreSQL.

Once pg_upgrade has extracted the metadata from the old cluster, it
performs a number of bookkeeping tasks required to 'sync up' the new
cluster with the existing data.

First, pg_upgrade copies the commit status information and 'next
transaction ID' from the old cluster to the new cluster. This step
ensures that the proper tuples are visible from the new cluster.
Remember, pg_upgrade does not export/import the content of user-defined
tables so the transaction IDs in the new cluster must match the
transaction IDs in the old data. pg_upgrade also copies the starting
address for write-ahead logs from the old cluster to the new cluster.

Now pg_upgrade begins reconstructing the metadata obtained from the old
cluster using the first part of the pg_dumpall output.

Next, pg_upgrade executes the remainder of the script produced earlier
by pg_dumpall --- this script effectively creates the complete
user-defined metadata from the old cluster to the new cluster.  It
preserves the relfilenode numbers so TOAST and other references
to relfilenodes in user data is preserved.  (See binary-upgrade usage
in pg_dump). We choose to preserve tablespace and database OIDs as well.

Finally, pg_upgrade links or copies each user-defined table and its
supporting indexes and toast tables from the old cluster to the new
cluster.

An important feature of the pg_upgrade design is that it leaves the
original cluster intact --- if a problem occurs during the upgrade, you
can still run the previous version, after renaming the tablespaces back
to the original names.