For a long time now we have been planning to extend the amount of information we store in the database for the next version, 0.10, which will run on KDE4. As the database is the central storage and digikam is built around it, such a move involves deep structural changes. We have done preparations, discussed on the mailing list what we want and what we need, made up documentation, and then sat down to code.
This is a companion discussion topic for the original entry at https://www.digikam.org/news/2007-10-17_renovating_our_foundations_new_database_schema/