Not really. I had Oracle refuse to use the index written exactly for the query in question (select id from contracts where auth_status = 'U', with index on auth

Not really. I had Oracle refuse to use the index written exactly for the query in question (select id from contracts where auth_status = 'U', with index on auth_status) because the planner decided that the index was useless (the statistics were gathered at night when the table contained no unauthorized records). But in Oracle you can use hints to force the planner to use the index, while in Postgres you can't.