[agentgt]

Actually it is sort of the opposite.... They needed better OLTP to deal with synchronizing billing ... and they wanted schema flexibility. Document/Column stores are traditionally not ideal for this because A+P (CAP theorm) are preferred over "C"onsistency (very broadly speaking).

They could have done it with a transactional message queue and a decent RDBMS (there are far bigger companies that use RDBMS for far more transactions than Uber does) but they clearly did not have the in house expertise for that (and they also wanted rapid schema changes).

Part of the problem is Postgres was a little behind on scaling in previous years but that has changed. IMO they could have stuck with Postgres by making an addon to that instead but they found extending MySQL easier.