low cardinality column

Hi,

I have a select like this:

SELECT MAX(transactionid) FROM cbntransaction WHERE transactiontypeid=0;

in the query:
transactionid is the primary key of cbntransaction table,
But transactiontypeid is a low cardinality column, there're over 100,000
records has the same trnsactiontypeid.
I was trying to create an index on (transactiontypeid, transactionid), but
no luck on that, postgresql will still scan the table.
I'm wondering if there's solution for this query:
Maybe something like if I can partition the table using transactiontypeid,
and do a local index on transactionid on each partition, but I couldnt'
find any doc on postgresql to do that.

Thanks in advance,
rong :-)