Speaking of Postgres materialized views...The guys at sparkfun (makers of Arduino as well other fine products) migrated from MySQL to Postgres for various reasons, and decided to use the newish materialized views. Apparently it was all fun and games until their number of orders increased by an order of magnitude, bringing MV refreshes to their knees. (These MVs keep track of their available stock, it seems.)
MVs in Postgres are very basic -- no incremental refreshes like Oracle has, so they're crunching all the numbers from square one every single time you say "refresh."
I don't know anything about the sparkfun guys' particular requirements or infrastructure, but I probably would not try to keep Postgres MVs of any complexity constantly up to date in a transactional system. I'm imagining they're trying to update the MVs any time the dependent tables are modified, so too many orders kill them. What if they accepted some data staleness and set the MVs to refresh on a timer (every x minutes)? If the view of stock is out of date by a bit, that might be OK as long as there's a check in place at checkout time. Just kind of wondering out loud here.
This situation shows that testing to scale is necessary, by the way.
On a related note, their reasons for switching from MySQL to Postgres are pretty numerous.