Indexing an array?

Hi,

I have a problem with arrays in Postgres. I want to create a really
large array, lets say 3 billion characters long.

As far I could read from the documentation - this should be possible.
But my question is, is there a kind of index on the array.

Lets say, I want to get element 2,675,345,328. Does Postgres have to
load the entire array into memory and then run through the 2.6 billion
characters to return the one I want or does Postgres have an index - as
where to find this element on disk?

Any advice is welcome and thanks in advance
Silke Trißl