Unify distant Postgres databases

Hi,

We desire to implement a multi-site server that unifies data from
distant Postgres databases in a nightly batch. The distant databases
have all the same architecture (schema). The size of data exchanged
between distant servers and the multi-site manager has to be reduced to
the strictly usefully data.

We naturally have been interested by the WAL archiving (PITR). But after
a deep analysis of this skill, we still encounter problems. The main
problem is unifying the data from many databases in a common database.

Has anyone experienced a solution to this problem... maybe not with WAL?

Thanks

Hugues