Reverse scans?

Would it be doable, and would it be useful, to try to alternate the
directions of table and index scans every time each table/index was
fully scanned?

I was thinking that it could help cache performance at various levels
in cases where data at the end of a large table, say, that remained in
memory after a scan, would otherwise be flushed out by a new scan of the
same table. If the next scan of the same table was to go in the other
direction, any remains of the last time around that were still in the
filesystem cache, buffer pool, hard disk cache etc. would stand a greater
chance of being reused.

Does that make sense at all? I tried searching the archives for some of
the mailing lists but couldn't find anything related.

Jeroen