Primary B +-tree, a variant of B +-tree structure with row data in leaf blocks, is an ideal storage organization for queries involving exact match and/or range search on primary keys. Commercially, primary B +-tree like structures have been supported in DBMSs like Compaq Non-Stop SQL, Sybase Adaptive Server, and Microsoft SQL Server. Oracle’s index-organized table is like a primary B +-tree; however, it differs from its commercial counterparts in the following respects: 1) The storage organization does not require the entire row to be stored in the primary key index. Infrequently accessed columns can be selectively pushed into an overflow storage area to speed up access to columns that are frequently accessed. 2) Secondary indexes on index-organized tables support logical primary key-based row identifiers, and still provide performance comparable to secondary indexes with physical row identifiers by storing and making use of guess-DBA (Database Block Address). 3) Support for primary key compression leads to reduced storage requirements. This paper Permission to copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage, the VLDB copyright notice and the title of the publication and its date appear, and notice is given that copying is by