Just to be clear, this isn't for actually running postgres to serve your database, right? This is just more or less a tool to upgrade your data files to a new postgres version?
It does both. As part of the startup process it first checks if the data files are for an old version, and upgrades them first (if needed).
Then it starts PostgreSQL 15.3 as per normal.
It's intended as being a drop-in replacement for existing (alpine based) PostgreSQL containers.
That being said, it's still pretty new so don't use it on production data until you've tested it first (etc).
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Btw, it uses the "--link" option when it runs pg_upgrade so should be reasonably suitable even for databases of a fairly large size. That option means it processes the database files "in place" to avoid needing to make a 2nd copy.
At the moment it only process a single data directory, so if you have multiple then it's not yet suitable.
