Good/Bad query plans based on text criteria

Hello,

I am struggling to understand why for certain criteria that i supply for a
query alters the the query plan. In my "good" case i can see that an index
is used, in my bad case where i only change the text value of the criteria,
but not the criteria itslef (ie change/add the conditions) a hbitmap heap
scan of the table is performed.

Refer attached Good/Bad query plans.

The basic query is:

SELECT * FROM highrate_log_entry
WHERE
test_seq_number > 26668670
and udc = '2424'
AND (test_signal_number = 'D2030'
)
ORDER BY test_seq_number LIMIT 11

test_seq_number is the pk and is generated by a sequence.

The D2030 is the only thing that i vary between good/bad runs. The issue is
possibly related to the data spead is for the test-signal_number is not
uniform, but there does not appear to be that much difference in difference
between the first sequence number and the last sequence number (to achieve
the 11 results), when compared between the test_seq_number that yield good
or bad results.

I dont believe that the issue is to do with re-writing the query, but how
the planner chooses its path.

I am using Postgres 8.4 on windows with default postgres.conf. I have tried
changing(increasing) shared_buffers, work_mem and effective_cache_size
without success.

Any suggestions would be appreciated.

Thanks

Jason