Postgres won't let you perform any sufficiently long (read-only) queries on slaves. For example, I was unable to run pg_dumpall on a slave to have DB snapshots.
This is [well-explained](http://www.postgresql.org/message-id/201102272005.00234.jens...), but still inconvenient. This way, slaves are only useful for failover and possibly offloading very short read-only transactions, which is pretty limited.
Re:PostgreSQL as Schemaless Database [pdf]
by joevandyk on Sunday March 31, 2013 @12:37PM
(#5469151)
This used to be the case, but it's not anymore. You can run long-living queries, run pg_dump, etc on slaves.
Ah, that's really good news to hear. Seems it's time to upgrade. :)
Could you, please, provide a reference to a documentation or a changelog? Maybe I'm looking for a wrong keywords, but I can't find any mentions of such changes by myself.
Do you have a reference in which pg version this got fixed? We still pg_xlog_replay_pause() our slaves, it would be nice to drop that.
Also pgpool2 helps a lot with partitioning and parallel queries.
For a relational db, postgres does a pretty good job at distribution and fault tolerance.