[R] merging pre-sorted data frames

[Mike Miller]

I have many pairs of data frames each with about 15 million records each 
and about 10 million records in common.  They are sorted by two of their 
fields and will be merged by those same fields.

The fact that the data are sorted could be used to greatly speed up a 
merge, but I have the impression that merge() cannot "know" in advance 
that the fields are already sorted.

I'm sure that I can use merge(), but I suspect that it is doing a lot of 
unnecessary work and that it will take much more time than the job really 
should require.  Is that correct?  Can anything be done about it?

The inspiration for my question comes partly from the way GNU comm works.

If you have any ideas about this, I'd love to hear them.

Thanks in advance.

Mike

-- 
Michael B. Miller, Ph.D.
University of Minnesota
http://scholar.google.com/citations?user=EV_phq4AAAAJ