many instances or many databases or many users?

[ismo(dot)tuononen(at)solenovo(dot)fi]

Hi,

I have used postgresql some years now, but only small databases and only 
one database per instance and one user per database.

Now we have a server reserved only for postgresql, and I'm wondering if it 
is better to set up:
- only one instance and many databases or
- many instances and only one database/instance or
- one instance, one database and many users

server will have 8G memory and 2 processors.

Earlier we have had problems with difficult queries, some query can take 
100% cpu and all other processes have slowed down.

I have used oracle many years and in oracle it's better have one 
instance and many users, but oracle can handle many difficult queries in 
same time. no process (=query) can slow other queries as much as in postgesql.

there is no need think safety, maintenance, ... only pure performance!

is one instance capable to use that 8G of memory? and share it with 
different databases/users as needed?
or will one big and difficult query take all memory and slow down whole 
server?

if there is 2 instances one query can't take all memory, but downside is 
that if instance1 is inactive and instance2 is active, there will be much 
unused memory (reverved for instance1) and that can produce disk io when 
instance2 reads lots of data and sorts it.

how you have set up your postgresql server?

Ismo