In some RDBMSs, primary and secondary indexes are very different. MsSql clustered tables store the tuples themselves in the primary index B Tree. Traversing t

In some RDBMSs, primary and secondary indexes are very different. MsSql clustered tables store the tuples themselves in the primary index B Tree. Traversing the data in the order of the primary index incurs no extra disk seeks. Also, you can add extra columns to secondary indexes to support queries without searching the primary index at all. The emulation of secondary indexes is inefficient. In a RDBMS without built in index maintenance you would create table (LeaderboardId, ScoreId), not (*LeaderboardId, ScoreIds). There's no need for comma separated fields.