SELECT from a set of values really slow?

I've been benchmarking some very simple databases.  By simple, I mean a table
like this:

    CREATE TABLE bench (
        id SERIAL,
        data TEXT )
    CREATE INDEX bench_data_index ON bench (data)

which is filled with 100k records, where the data values for each record are
distinct (the data for record N is "text_item_N").

I generate a random data value known to be in the table and lookup its id,
with "SELECT id FROM bench WHERE data = 'whatever'".

I run this for three seconds.  PostgreSQL does OK: 4300 selects in 2999
msec.  MySQL does a little better, but not much: 5500 selects in 2999 msec.

In my application, this kind of lookup turns out to be the bottleneck on
performance (which is why I'm benchmarking).  On MySQL, I can speed it up
quite a bit by looking up more than one thing at a time:

    SELECT id FROM bench WHERE data IN ('X', 'Y')

for example, gets about 4050 selects in 3 seconds, so that's 8100 records
looked up, compared to 5300 when they were done one at a time.  It continues
to improve selecting more at a time.  To my surprise, when I tried this
trick with PostgreSQL, it did not speed things up.  In fact, it *massively*
slowed down--it only is getting 13 selects in 3 seconds, searching for two
at a time.

What's going on here?

PostgreSQL 7.4.2 on SuSE 9.1 Linux.  (The version that comes with SuSE).

--
--Tim Smith