Distributed / Linked with Open Source DB

Does anybody out there have experience implementing either a distributed
homogenous database or linked databases with MYSQL or POSTGRES.

The reason I ask is I will have 4 distant regions connected over high speed
data lines, that will have seperate databases, if the databases had the same
table structure, ie table names and field types and widths the same, would
it easily be possible to link these together so that they can be queried as
one database (union), but the one region can only perform DML on their own
databases data. Has anybody out there done this?

I assume this would require a lot of traffic between the servers.
Is this efficient, and reliable?

I am looking at Oracle, they offer all of the capabilities, but are pretty
complex, and way more expensive than the open source solutions.

Lee D.

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