Postgres - Insert performance with indexes
No, a basic btree index on a small column is typically cheap to maintain. Of course it gets slightly more expensive when the table has accumulated some bloat from dead rows, but the difference should be small. And you have to consider additional storage on disk for the index.

One thing seems worth mentioning: Updates on columns involved in an index in any way cannot use H.O.T. updates ("heap-only tuples"), so this might add some cost for updates changing the indexed column. But still, this would just prevent one internal optimization, no big deal. And only relevant if you involve the column in updates.


Is a composite index also good for queries on the first field?
Redundant data in update statements