A Practical Introduction to Data Structures and Algorithm Analysis

10IndexingMany large-scale computing applications are centered around data sets that are toolarge to fit into main memory. The classic example is a large database of recordswith multiple search keys, requiring the ability to insert, delete, and search forrecords. Hashing provides outstanding performance for such situations, but onlyin the limited case in which all searches are of the form “find the record with keyvalue K.” Unfortunately, many applications require more general search capabilities.One example is a range query search for all records whose key lies within somerange. Other queries might involve visiting all records in order of their key value,or finding the record with the greatest key value. Hash tables are not organized tosupport any of these queries efficiently.This chapter introduces file structures used to organize a large collection ofrecords stored on disk. Such file structures support efficient insertion, deletion, andsearch operations, for exact-match queries, range queries, and largest/smallest keyvalue searches.Before discussing such file structures, we must become familiar with some basicfile-processing terminology. An entry-sequenced file stores records in the orderthat they were added to the file. Entry-sequenced files are the disk-based equivalentto an unsorted list and so do not support efficient search. The natural solution is tosort the records by order of the search key. However, a typical database, such as acollection of employee or customer records maintained by a business, might containmultiple search keys. To answer a question about a particular customer mightrequire a search on the name of the customer. Businesses often wish to sort andoutput the records by zip code order for a bulk mailing. Government paperworkmight require the ability to search by Social Security number. Thus, there mightnot be a single “correct” order in which to store the records.Indexing is the process of associating a key with the location of a correspondingdata record. Section 8.5 discussed the concept of a key sort, in which an index357