For benchmarking and testing, I am looking for an open SQL database that can be installed in various database servers (e.g. Postgres, MySQL, etc).
One good option for RDMS (relational databases, aka SQL) is the Chinook Database, which is based on an iTunes library.
Chinook is a sample database available for SQL Server, Oracle, MySQL, etc. It can be created by running a single SQL script. Chinook database is an alternative to the Northwind database, being ideal for demos and testing ORM tools targeting single and multiple database servers.
Supported Database Servers
SQL Server Compact
If you want to use the data from your own iTunes library, then replace the file ChinookDatabase\DataSources_Xml\Source\iTunes Music Library.xml with your version.
Downside seems to be that most scripts are for windows, although the
.sql database statements can be loaded directly.