Large number of lookups

I have a large table (>2 Mrows) against which my application runs some 1M
queries per day.  The queries are almost all of the nature of

select PRIMARY_KEY_FLD from MY_TABLE where SECONDARY_KEY_FLD = 'something';

I've optimized the table to the limits of what I can think of, now I'm
looking to optimize the application.  I'm really only interested in the
yes/no question of whether the row exists (the returned PRIMARY_KEY_FLD
value is pretty much just for debug documentation).  What I'm wondering
is whether my application would get better (faster) results if I ran a
number of queries at once.

For example, I could build SQL that looks like:

select PRIMARY_KEY_FLD, SECONDARY_KEY_FLD
from MY_TABLE WHERE SECONDARY_KEY_FLD in(first_val, second_val, ...);

so that the list contains some number of values (either a limited "slice"
of the values I care about, or the whole pile--usually 100-200), and
iterate this until I've checked them all.

Another technique would be to construct a "UNION" table:

select PRIMARY_KEY_FLD, SECONDARY_KEY_FLD
from MY_TABLE
join ( select first_val as "SECONDARY_KEY_FLD" union select second_val union ...) as TEMP_TABLE using
(SECONDARY_KEY_FLD)

likewise, either taking some number of my desired values in "chunks" or
all at once.

What's likely to work better, and why?  (I'm going off to write a
benchmark script, but I'd like to hear some theoretical answers, too.)

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