It's been over a year since I last blogged about parallelism, so I think I'm past due for an update, especially because some exciting things are happening.
First, Amit Kapila has published a draft patch for parallel sequential scan. Many things remain to be improved about this patch, which is neither as robust as it needs to be nor as performant as we'd like it to be nor as well-modularized as it really should be. But it exists, and it passes simple tests, and that is a big step forward. Even better, on most of Amit's tests, it shows a very substantial speed-up over a non-parallel sequential scan.