A Practical Introduction to Data Structures and Algorithm Analysis

326 Chap. 9 SearchingWhen a frequency distribution follows the 80/20 rule, the average searchlooks at about one tenth of the records in a table ordered by frequency.In most applications, we have no means of knowing in advance the frequenciesof access for the data records. To complicate matters further, certain records mightbe accessed frequently for a brief period of time, and then rarely thereafter. Thus,the probability of access for records might change over time (in most databasesystems, this is to be expected). Self-organizing lists seek to solve both of theseproblems.Self-organizing lists modify the order of records within the list based on theactual pattern of record access. Self-organizing lists use a heuristic for decidinghow to to reorder the list. These heuristics are similar to the rules for managingbuffer pools (see Section 8.3). In fact, a buffer pool is a form of self-organizinglist. Ordering the buffer pool by expected frequency of access is a good strategy,because typically we must search the contents of the buffers to determine if thedesired information is already in main memory. When ordered by frequency ofaccess, the buffer at the end of the list will be the one most appropriate for reusewhen a new page of information must be read. Below are three traditional heuristicsfor managing self-organizing lists:1. The most obvious way to keep a list ordered by frequency would be to storea count of accesses to each record and always maintain records in this order.This method will be referred to as count. Count is similar to the leastfrequently used buffer replacement strategy. Whenever a record is accessed,it might move toward the front of the list if its number of accesses becomesgreater than a record preceding it. Thus, count will store the records in theorder of frequency that has actually occurred so far. Besides requiring spacefor the access counts, count does not react well to changing frequency ofaccess over time. Once a record has been accessed a large number of timesunder the frequency count system, it will remain near the front of the listregardless of further access history.2. Bring a record to the front of the list when it is found, pushing all the otherrecords back one position. This is analogous to the least recently used bufferreplacement strategy and is called move-to-front. This heuristic is easy toimplement if the records are stored using a linked list. When records arestored in an array, bringing a record forward from near the end of the arraywill result in a large number of records changing position. Move-to-front’scost is bounded in the sense that it requires at most twice the number of accessesrequired by the optimal static ordering for n records when at least