We're doing something very similar to store RDF data. Our reason for doing it this way isn't performance but rather RDF's pretty intricate schema requirements (and opportunities). The drawback of using this scheme is that it roughly doubles the amount of data stored.
Contrary to Friendfeed, we have to use joins a lot because analysing data is the purpose of our application. We tried to do it with mysql, but mysql turns out to be completely unsuitable for the task due to its lack of merge or hash joins.
I'm quite surprised that someone like Friendfeed would change their entire data model for performance reasons instead of considering a stronger RDBMS (of which there are many). Their problem with index maintainance isn't exactly new. It's a solved problem that needs no wheel reinventing and doesn't merit the increased complexity of asynchronous index updates in my view.
Contrary to Friendfeed, we have to use joins a lot because analysing data is the purpose of our application. We tried to do it with mysql, but mysql turns out to be completely unsuitable for the task due to its lack of merge or hash joins.
I'm quite surprised that someone like Friendfeed would change their entire data model for performance reasons instead of considering a stronger RDBMS (of which there are many). Their problem with index maintainance isn't exactly new. It's a solved problem that needs no wheel reinventing and doesn't merit the increased complexity of asynchronous index updates in my view.