Partial indexes ... any good theoretical discussion?

I've only just now noticed that CREATE INDEX accepts a 'WHERE' clause.
This is used to create something called a "partial index".  Hmm, ever
being one who sees the world as made of nails when first given a hammer ...

One of our tables, with a couple hundred thousand rows) has a
date-column index. We expire things out of the table every day (the vast
majority, but not exclusively, stuff that's a couple days old). We're
frequently running queries against the table, looking for "everything
since this time yesterday"; we hardly ever look back more than 24 hours.

If I created the index as something like:

    CREATE INDEX ix_foo ON foo(the_date)
      WHERE the_date >= now() - interval '24 hours';

what might I expect as the impact?  Do index values older than 24 hours
drop out? Or must I "refresh" the index from time to time (in our
application, probably a couple dozen times a day)?

And, absent pat answers to this, is there anything out there in PG-land
that documents partial indexes, and when to use them?

--
Jeff Boes                                      vox 269.226.9550 ext 24
Database Engineer                                     fax 269.349.9076
Nexcerpt, Inc.                                 http://www.nexcerpt.com
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