PostgreSQL supports synchronous multi-master, MongoDB supports write concern, but this causes a performance penalty).
Anyways I doubt that "PostgreSQL supports synchronous multi-master"
Postgres core taken as such does not support multi-master. The fork of PostgreSQL called Postgres-XC somewhat does, for OLTP applications.
I know about PostgresXC, but I thought it is distributed database (similar to shards of mongoDB). [Though if I am correct there could be tables which are shared across different nodes, but that is not the best way of utilizing features of PostgresXC] I think it is not apt to call it "synchronous" (since there is no replication happening).
XC supports table sharding with distribution of data using a column key, or even data replication. Table definitions and schemas are the same across all the nodes, it is just that the data is present in a portion of the nodes (Datanodes more precisely) of the cluster. Operations are done *synchronously* through the cluster using what is called a GTM (global transaction manager) that is unique and feeds all the other nodes with globally-consistent transaction IDs and snapshots. GTM SPOF is solved with the presence of a Standby.
