五种提高SQL性能的方法----译文+原文.docVIP

译文 Five Ways to Rev up Your SQL Performance Sometimes all it takes is a little tweak here or there to make your application run much faster. Ah, but the key is figuring out how to tweak it! Sooner or later youll face a situation where a SQL query in your application isnt responding the way you intended. Either it doesnt return the data you want or it takes entirely too long to be reasonable. If it slows down a report or your enterprise application, users wont be pleased if they have to wait inordinate amounts of time. And just like your parents didnt want to hear why you were coming in past curfew, users dont want to hear why your query is taking so long. (Sorry, Mom, I used too many LEFT JOINs.) Users want applications to respond quickly and their reports to return analytical data in a flash. I myself get impatient when I surf the Web and a page takes more than ten seconds to load (OK, more like five seconds). To resolve these issues, it is important to get to the root of the problem. So where do you start? The root cause is usually in the database design and the queries that access it. In this months column Ill demonstrate four techniques that can be used to either improve your SQL Server?-based applications performance or improve its scalability. Ill examine the use of LEFT JOINs, CROSS JOINs, and retrieving an IDENTITY value. Keep in mind that there is no magic solution. Tuning your database and its queries takes time, analysis, and a lot of testing. While the techniques here are proven, some may work better than others in your application. Returning an IDENTITY From an INSERT I figured I would start with something I get a lot of questions about: how to retrieve an IDENTITY value after performing a SQL INSERT. Often, the problem is not how to write the query to retrieve the value, but rather where and when to do it. In SQL Server, the statement to retrieve the IDENTITY value created by the most recent SQL statement run on the active database connection is as fol