let' say we use string bytes long for the key (big enough for most last namesand bytes for the block number ( type int in javaeach entry in our index thus requires bytes this is only / the amount necessary for each record the entries in the index are arranged sequentially by last name the original records on the disk can be arranged in any convenient order this usually means that new records are simply appended to the end of the fileso the records are ordered by time of insertion this arrangement is shown in figure block # block # jones (other data benson (other datalloyd (other datakeller (other dataparker (other data abbot duncan (other (other datadata bytes index key blockjones , jordan , joslyn , joyce , jung figure bytes file index index file in memory the index is much smaller than the file containing actual records it may even be small enough to fit entirely in main memory in our example there are , records each one has -byte entry in the indexso the index will be , or , , bytes long ( megabytesin modern computers there' no problem fitting this in memory the index can be stored on the disk but read into memory whenever the database program is started up from then onoperations on the index can take place in memory at the end of each day (or perhaps more frequently)the index can be written back to disk for permanent storage searching the index-in-memory approach allows much faster operations on the phone book file than are possible with file in which the records themselves are arranged sequentially for examplea binary search requires index accesses at microseconds per accessthat' only about / , of second then there' (inevitablythe