barcodelib.barcode.rdlc reports.dll Locks and Cursors in VS .NET

Printing QR in VS .NET Locks and Cursors

CHAPTER
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Locks and Cursors
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In this chapter, you will learn about: Locks Deadlocks Transaction isolation levels Hints Cursors
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A good database is always seeking a balance between: 1 Allowing as many users as possible to use the database at the same time (high concurrency) and, 2 Preventing problems from multiple users accessing the same data when one or more users are trying to modify the data (through the judicious use of locks) For the many SQL Server databases, locks occur automatically within the database, and database developers often don t need to know how to troubleshoot locks However, blocking locks cause problems, and when they do occur you need to know how to find and kill them For high-end databases we sometimes need to create our own locks We can establish our own locks by setting individual transaction isolation levels, or by using query or table hints Deadlocks occur primarily due to the way objects are accessed within an application Implementing good programming practices within an application can often eliminate almost all possible deadlock conditions However, when they do occur you need to know how to find them Cursors are used to retrieve a result set and process it one row at a time While developers still often use cursors because they are easy to use and easy to conceptualize, they can often be rewritten as queries or batches that perform much more efficiently Not always, but very often, the use of cursors are simple easy-to-understand wrong solutions for complex problems Still, they re on the 70-442 exam, so you need to know them
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MCITP SQL Server 2005 Database Developer All-in-One Exam Guide
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Locks and Deadlocks
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From an optimization perspective, locks and deadlocks become very important to understand, identify, and manage Many times I ve been asked about a slow database that often seems to just lock up After investigation, I ve often found that locks and deadlocks were the culprit Because of this, whenever I hear about a slow database that is periodically unresponsive, I almost always suspect locks or deadlocks For the MCITP tests, you need to understand what locks and deadlocks are and how to identify them once you suspect they re occurring Once identified, you need to know how to kill them, or in other words be able to stop the processes causing the lock Thankfully, SQL Server 2005 has many tools that can be used to help you track down and manage locks These include:
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What Are Locks
Locks occur when a process needs to prevent access to data by another process Let s consider a simple example where data is updated without locks:
EmpID 101 FName Sally LName McGirk Phone 555-1212 Email Sally@mcitpcom
Sally gets married, changes her last name to Johnson, and her phone number changes to 555-3434 Now let s say that one person executes an UPDATE statement to change her last name to Johnson At the same time, another person enters an UPDATE statement to change her phone number Since each person is updating only one column of data, all the other data should stay the same with a regular UPDATE However, what s not apparent is that when an UPDATE is issued, the entire row (all columns) is rewritten, not just the changed column When the statement is executed to change her name, it would result in the following row Notice that only the last name has changed
EmpID 101 FName Sally LName Johnson Phone 555-1212 Email Sally@mcitpcom
When the second statement is executed at the same time to change her phone number, it would result in the following row Remember, both statements were executed at the same time, but let s say that the second statement took a millisecond longer so it was saved last Notice that since the original last name of McGirk existed when the phone UPDATE started, McGirk is retained The update of the last name is lost
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