extremly bad select performance on huge table

Hi newsgroup,

I have a very huge table (70 mio rows ) with a key (text length about 30
characters each key). A select on this indexed column "myprimkey" (index
on column mycolumn) took more than 30 mins.

Here is the explain (analyze,buffers) select mycolumn from myhugetable

"Index Only Scan using myprimkey on myhugetable (cost=0.00..8224444.82
rows=71768080 width=33) (actual time=16.722..2456300.778 rows=71825999
loops=1)"

" Heap Fetches: 356861"

"Total runtime: 2503009.611 ms"

Even repeating the query does not show a performance improvement. I
assume that the index itself is too large for my db cache. What can I do
to gain performance? Which parameters can I adapt? Having a huge Linux
machine with 72 GB RAM.

Note: This select is just for testing. My final statement will be a join
on this table via the "mycolumn" column.

Thanks for your help
Björn