pgpool-II is a inherited project of pgpool (to classify from
pgpool-II, it is sometimes called as pgpool-I). For those of
you not familiar with pgpool-I, it is a multi-functional
middle ware for PostgreSQL that features connection pooling,
replication and load balancing functions. pgpool-I allows a
user to connect at most two PostgreSQL servers for higher
availability or for higher search performance compared to a
single PostgreSQL server.
pgpool-II, on the other hand, allows multiple PostgreSQL
servers (DB nodes) to be connected, which enables queries
to be executed simultaneously on all servers. In other words,
it enables "parallel query" processing. Also, pgpool-II can
be started as pgpool-I by changing configuration parameters.
pgpool-II that is executed in pgpool-I mode enables multiple
DB nodes to be connected, which was not possible in pgpool-I.