| 释义 |
update in site amendment file.It is not suitable for updating file with variable length records or when new records must be added, unless considerable unused space is left to accommodate them.It has the disadvantage of requiring a copy to be made before the update begins, in order to retain the previous version.It is used mainly for updating low hitrate disc files and, in transaction processing systems, it is often done as the changes occur.；一种更新主文件的方法，即利用修正文件中的新版本记录来覆盖主文件中的记录.除非有相当大未用空间可装下新记录，这种方法一般不适合具有变长记录的更新文件及必须要加一些新记录的场合.该方法的缺点是，为了保留原文件，在更新前必须做一个拷贝.这种方法主要用于更新低命中率的磁盘文件以及交易处理系统中发生变化时的更新.参阅update by copy |