(I'm having trouble with e-mail, so if you get this twice, sorry)
I was looking at some Oracle configuration files today, and it occurred to me
how Postgres can be made to pre-fork, similarly to Oracle.
Oracle has "listener" processes that listen on a port for Oracle clients. The
listeners are configured for a database. Postgres could work the same way. It
could start up on port 5432 and work as it always has, and, in addition, it
could read a configuration script which directs it to "pre-fork" listeners on
other ports, one port per database. This would work because they already know
the database that they should be ready to use. The back-end does not need to be
involved.
Once you connect to the pre-forked back end, it will already be ready to
perform a query because it has already loaded the database. The file which
configures the "pre-forked" database could also contain run-time changeable
tuning options for each "pre-forked" instance, presumably, because you would
tune it for each database on which it would operate.