数据库外文参考文献及翻译
SQL ALL-IN-ONE DESK REFERENCE FOR DUMMIES
Data Files and Databases
I. Irreducible complexity
Any software system that performs a useful function is going to be complex. The more valuable the function, the more complex its implementation will be. Regardless of how the data is stored, the complexity remains. The only question is where that complexity resides. Any non-trivial computer application has two major components: the program the data. Although an application’s level of complexity depends on the task to be performed, developers have some control over the location of that complexity. The complexity may reside primarily in the program part of the overall system, or it may reside in the data part.
Operations on the data can be fast. Because the program interacts directly with the data, with no DBMS in the middle, well-designed applications can run as fast as the hardware permits. What could be better? A data organization that minimizes storage requirements and at the same time maximizes speed of operation seems like the best of all possible worlds. But wait a minute . Flat file systems came into use in the 1940s. We have known about them for a long time, and yet today they have been almost entirely replaced by database systems. What’s up with that? Perhaps it is the not-so-beneficial consequences。
The simple program/complex data organization
The major selling point of database systems is that the metadata resides on the data end of the system rather than in the program. The program need to know what data they want to operate on, and not where that data is located, they are unaffected when the physical details of where data is stored changes.
✦ Portability across platforms, even when they are highly dissimilar, is easy as long as the DBMS used by the first platform is also available on the second. Generally, the programs need not be changed at all.
✦ Placing a database management system in between the application program and the data slows down operations on that data significantly.
✦ Databases take up more space on disk storage than the same amount of data would take up in a flat file system. This is due to the fact that metadata is stored along with the data. The metadata contains information
about how the data is stored, so that the application programs don’t have to include it.
Applications
What are the chances that a person could actually find a needle in a haystack? Not very good. Finding the proverbial needle is so hard because the haystack is a random pile of hay with individual pieces of hay going in every direction, and the needle is located at some random place among all that hay. A flat file system is not really very much like a haystack, but it does lack structure, and in order to find a particular record in such a file, you must use tools that lie outside of the file itself. This is like applying a powerful magnet
to the haystack to find the needle.
The first database system
The first true database system was developed by IBM in the 1960s in support of NASA’s Apollo moon landing program. The number of components in the Saturn V launch vehicle, the Apollo
Command and Service Module, and the Lunar Lander far exceeded anything that had been
build up to that time. Every component had to be tested more exhaustively than anything had
ever been tested before because they were all going to have to withstand the rigors of an environment that was more hostile and more unforgiving than any environment that humans had
ever attempted to work in. Flat file systems were out of the question. IBM’s solution, which
it later transformed into a commercial database product named IMS, kept track of each individual
component, as well its complete history. When the ill-fated Apollo 13’s main oxygen tank ruptured on the way to the Moon, engineers worked frantically to come up with a plan to save the lives of the three astronauts onboard. The engineers succeeded and transmitted a plan to the astronauts that worked. After the crew had returned safely to Earth, querying IMS about the oxygen tank that failed showed that somewhere between its manufacture and its installation in Apollo 13, it had been dropped on the floor. It was retested for its ability to withstand the pressure it would have to contain during the mission, and then replaced in stock after passing the test. In this case, the test did not detect the hidden damage to the tank, but at least the history stored in IMS showed that passing a pressure test is not enough to assure that a dropped tank is undamaged. No dropped tanks were ever used on subsequent Apollo missions.1669