Proven and powerful database
PostgreSQL is the most advanced open source object-relational database system. We choose it for its power, speed and maturity.
We store all our data in a SQL database. We had the choice between MySQL and PostgreSQL. We went with the latter, why? Important reason is that there is a great binary driver for PostgreSQL and only a text driver for MySQL. And we found PostgreSQL to be more complete and more robust.
As the performance of both databases is very similar it was an easy choice.
Why not CouchDB, Riak, or ... ?
In our opinion a SQL database is powerful and fast enough to handle the load of 99.9% of all websites.
And did we mention that triggers and cascading updates are very nice?
And that having uniqueness and foreign key constraints really helps keeping our data clean and tidy?
And that having SQL helped us write less code?
A NoSQL database will become interesting when we handle huge amounts of data. Till then the current crop of SQL databases will do the job just fine.
Maybe we will make a Riak version of Zotonic at some time, maybe, just maybe.