tailieunhanh - Managing time in relational databases- P6

Tham khảo tài liệu 'managing time in relational databases- p6', công nghệ thông tin, cơ sở dữ liệu phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Chapter 4 THE ORIGINS OF ASSERTED VERSIONING IT BEST PRACTICES 83 representing that object during some period of its existence. The one non-temporal row and the set of version rows cover exactly the same period of time. But basic versioning is the least frequently used kind of versioning in real-world databases. The reason is that it preserves a history of changes to an object for only as long as the object exists in the database. When a delete transaction for the object is applied all the information about that object is removed. One type of versioning that is frequently seen in real-world databases is logical delete versioning. It is similar to basic versioning but it uses logical deletes instead of physical deletes. As a result the history of an object remains in the table even after a delete transaction is applied. Logical Delete Versioning In this variation on versioning a logical delete flag is included in the version table. It has two values one marking the row as not being a delete and the other marking the row as being a delete. We will use the values Y and N . After the same insert and the same update transactions our non-temporal and logical delete version tables look as shown in Figure . We are now at one clock tick before December 2010 . at November 2010. Although we have chosen to use a one-month clock in our examples primarily because a full timestamp or 84 Chapter 4 THE ORIGINS OF ASSERTED VERSIONING IT BEST PRACTICES of the page a 1-month clock is not completely unrealistic. It corresponds to a database that is updated only in batch mode and only at one-month intervals. Nonetheless the reader should be aware that all these examples and all these discussions would remain valid if any other granularity such as a full timestamp were used instead. Let us assume that it is now December 2010 and time to apply the logical delete transaction. The result is shown in Figure . However the non-temporal table is not shown in Figure or in any of the