A method of updating a master file by overwriting changed records with their new versions from an 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 hitrate disc files and, in transaction processing systems, it is often done as the changes occur.; 一種更新主文件的方法，即利用修正文件中的新版本記錄來覆蓋主文件中的記錄.除非有相當大未用空間可裝下新記錄，這 種方法一般不適合具有變長記錄的更新文件及必須要加一些新記錄的場合.該方法的缺點 是，為了保留原文件，在更新前必須做一個拷貝.這種方法主要用于更新低命中率的磁盤文 件以及交易處理系統(tǒng)中發(fā)生變化時的更新.參閱update by copy