alt Hacker NewsTwo-phase commit provides an eventual consistency guarantee only....
Other clients (readers) have to be able to deal with inconsistencies in the meantime.
Also, 2PC in postgres is incompatible with temporary tables, which rules out use with longrunning batch analysis jobs which might use temporary tables for intermediate work and then save results. Eg. "We want to send this marketing campaign to the top 10% of users" doesn't work with the naive approach.
These are limitations in the current PostgreSQL implementation. It's quite possible to have consistent commits and snapshots across sharded databases. Hopefully some day in PostgreSQL too.