postgresql shared buffers

hai...

I have installed postgresql from sourcecode.I would like to know
how pages are replaced in the bufferpool when we join two relations.I
tried to trace it by editing files
pgsql/src/backend/storage/buffer/(bufmgr.c,freelist.c).But i feel still I
am missing some information after observing extracted information abt
buffer replacement.My input datasize is 10times more than main memory/RAM
size.When I joined two relations,postgresql accessed both relations
sequentially one by one and that too only once.Then how is it joining two
relations by accessing only once? Is it storing that accessed relations
some where other than main memory/bufferpool(Becos they cant fit into main
memory).

So can anybdy tell me is there is ne thing I am
missing? Is there any concept like postgresql cache similar to kernel
cache otherthan sharedbuffers.If so how can we figure it out.

Is there any way by which postgresql is accessing database
relations through,other than rotines in bufmgr.c nd freelist.c(I
mean any other routines like ReadBuffer,StrategyGet etc.)

thanks in anticipation.

--
N Praveen Kumar
Btech-IV CSE
IIIT,Hyd
AP,India

Imagination is more important than knowledge...
--Albert Einstein