break table into portions for writing to separate files

Hello,

I've been looking for a way to write a table into multiple files, and am
wondering if there are some clever suggestions.  Say we have a table
that is too large (several Gb) to write to a file that can be used for
further analyses in other languages.  The table consists of a timestamp
field and several numeric fields, with records every 10th of a second.
It could be meaningfully broken down into subsets of say 20 minutes
worth of records.  One option is to write a shell script that loops
through the timestamp, selects the corresponding subset of the table,
and writes it as a unique file.  However, this would be extremely slow
because each select takes several hours, and there can be hundreds of
subsets.  Is there a better way?

Cheers,

--
Seb