If you have a table that you basically always query by primary key, then Oracle offers a 'Hash Cluster table' which uses a hash instead of an index locate the data. It generally makes PK looks up very fast, but it has downsides.

1. You need to know roughly how many rows the table will have, and the row size at table creation time. 2. Oracle will allocate all that space at creation time, and so a full table scan will take a long time.

So long as you know the deal with them, hash cluster tables can prove very efficient for the correct problem.