If you had 20 databases spread across 20 servers, could you replicate them all into a single PostgreSQL instance? And can this be done on a server costing less than $100,000?
The databases total 30 TB on disk. There are ~30,000 tables. The maximum number of rows in a single table is 1.2 billion. The number of rows changed per second is modest (100s or less).
Some of the servers run PostgreSQL, some MySQL. The versions are different, but they're relatively current (PostgreSQL >= 9.1, MySQL >= 5.6).