Impact of table scan on shared buffers

Suppose I have a large table with no indexes, and I scan the entire
thing. What is the impact on the shared buffers? I'm interested in
three scenarios:

- Scan done in SQL using SELECT, (via JDBC if it matters).
- Scan done using SQL COPY.
- Scan done using psql COPY.

I suspect that the SELECT scan will tend to displace everything else
in shared buffers, and I hope that the two forms of COPY do not. Is
that correct?

I'm using 7.4 now, but I'm also interested in the answer for 8.x.

Morris