See Figure 11.5 on secondary key cname . To perform a lookup on Peterson, we must read all three records pointed to by entries in bucket 2. Only one entry points to a Peterson record, but three records need to be read. As file is not ordered physically by cname , this may take 3 block accesses. Secondary indices must be dense , with an index entry for every search-key value, and a pointer to every record in the file.
Secondary indices improve the performance of queries on non-primary keys.
Designer must decide whether to use secondary indices or not.