[R] Indexes on dataframe columns?

[Ranjan Bagchi]

Hi --

I'm working with some data frames with fairly high nrows (call it 8 
columns, by 20,000 rows).  Are there any indexes on these columns?

When I do a df[df$foo == 42,] [which I think is idiomatic], am I doing a linear 
search or something better?  If the column contents is ordered, I'd like 
to at least be doing a naive binary search.

Thanks!

Ranjan