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
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