> if you are a startup you are not even considering db2 or oracle.
Why not? The free versions exceed postgresql. If you succeed and face heavy load, then you just throw some money at the problem rather than blow hundreds of man-hours fidling with the hacks people use to scale the free DBMSes.
You can just throw money at Postgres problems and make them go away. There are a number of very capable firms that do nothing but Postgresql support. And, they don't hold you captive w/ licensing. And if your needs are really that unique they can help you with building extensions to postgresql that support the capabilities you need.
Why not? The free versions exceed postgresql. If you succeed and face heavy load, then you just throw some money at the problem rather than blow hundreds of man-hours fidling with the hacks people use to scale the free DBMSes.