|
In computing, the Halloween Problem refers to a phenomenon in databases in which an update operation causes a change in the physical location of a row, potentially allowing the row to be visited more than once during the operation. This could even cause an infinite loop in some cases where updates continually place the updated record ahead of the scan performing the update operation. The potential for this database error was first discovered by Don Chamberlin, Pat Selinger, and Morton Astrahan in 1976, on Halloween day while working on a query that was supposed to give a ten percent raise to every employee who earned less than $25,000. This query would run successfully, with no errors, but when finished all the employees in the database earned at least $25,000, because it kept giving them a raise until they reached that level. The expectation was that the query would iterate over each of the employee records with a salary less than $25,000 precisely once. In fact, because even updated records were visible to the query execution engine and so continued to match the query's criteria, salary records were matching multiple times and each time being given a 10% raise until they were all greater than $25,000. The name is not descriptive of the nature of the problem but rather was given due to the day it was discovered. As recounted by Don Chamberlin:
== References == * (The 1995 SQL Reunion (Protokoll) ) * (The "Halloween Problem" for XML APIs ), Mike Champion's weblog. * ''(archive)'' (A well-intentioned query and the Halloween Problem ), Los Alamos National Laboratory, Anecdotes, IEEE Annals of the History of Computing * Quotation from Donald D. Chamberlin, Charles Babbage Institute, OH 329 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Halloween Problem」の詳細全文を読む スポンサード リンク
|