Is it possible to use DocumentLibrary with PostgreSQL (or MySql) as a database for storing files and file properties ?

No, the only options available are either to store the documents and properties the ZODB, or with the Robin Farine patch, you can store the documents on the local filesystem with the properties still in the ZODB (it uses the ExternalFile product). You can do searches on properties or content quite easily though, with the Catalog it builds of all of the documents... Happy Zoping, ;) Paul