I came cross this article regarding Hash Joins. It explains the Hash Join/Match in great detail. Download it from here
Nov 142012
Oct 172012
Inserted and deleted tables are 2 pseudo tables preseting the before and after images of modified rows. They can only be accessed from within triggers. Those two have been around for quite long time but it’s hard to find any articles to tell how they are populated. Some rumors said they are retrieved from transaction logs in SQL Server 2000. Recently, I was asked to evaluate the performance impact of using trigger for my customer’s project. I did some investigation and got some interesting results.
Oct 102012
Do you think it’s a good idea to add UPDLOCK hint to the table which will be modified in an update query? Many people will say NO because it seems to be redundant. But the fact is that sometime it removes deadlocks in your application.
Sep 102012
location: AppendOnlyVerStoreMgr.cpp:776 Expression: 0 SPID: 64 Process ID: 4912 Description: Cannot locate version record and page is not allocated. Status = 3 Msg 3624, Level 20, State 1, Line 1 A system assertion check has failed. Check the SQL Server error log for details. Typically, an assertion failure is caused by a software bug or data corruption. To check for database corruption, consider running DBCC CHECKDB. If you agreed to send dumps to Microsoft during setup, a mini dump will be sent to Microsoft. An update might be available from Microsoft in the latest Service Pack or in a QFE from Technical Support. Msg 0, Level 20, State 0, Line 0 A severe error occurred on the current command. The results, if any, should be discarded.
This was the error a Sr. DBA at my client asked me. There error appeared after reading a table from decrypted database which has been encrypted by TDE on SQL Server 2008 R2.
Sep 042012
Come and join SQL PASS Victoria User Group in Victoria, BC and let’s talk about SQL Server. This will be a great opportunity to improve your SQL skills by learning from others and build your professional network. Meetings will be held 3rd or 4th Thursday every month at campus of Camosun College in Victoria, BC. Membership and attendance of sessions are absolutely free. The schedule for each meeting will be
5:00PM – 6:00PM – Networking.
6:00PM – 8:30PM – Presentation(s)
8:30PM – 9:00PM – Door prizes and Question & Answer session
The first meeting will be on Oct 18, 2012. Visit http://victoria.sqlpass.org/ for more details.
Jun 292012
Quite often people need to write a new T-SQL application to replace an existing one for various reasons. For instance, Existing report is generated by a complex T-SQL procedure which pulls data from OLTP directly. The performance of the procedre is poor since everything is calculated while report is running. Company decided to pre-calculate and save the result in a denormalized table, and the report runs off that denormalized table. Obviously, the original code need to be re-written. Let’s say you are new to the company. Before starting your work, you may want to know what databases, tables, and columns are referenced by the procedure. It will not be hard if the procedure code block only has few lines of code. However, if the procedure includes few or tens thousand lines, accomplishing such work might takes days. The worth thing is you might miss some refereces and finally you might abandon your work result at the end due to the inaccuracy.
Jun 262012
With Snapshot (read committed snapshot and snapshot isolation level) enabled, when data reading on a table takes place, if any records are being modified at the same time, the data will be read from the rows in version store rather than wait until data modification complete. Such behavior lets prgrammers think that reader process will NEVER ever be blocked by writer process. In most of the scenario, this is correct, however, when reader process is indirectly issued and running against a table, the isolation level for that reader process is always read committed lock isolation regardless the transacitoin isolation level of the parent statement.
Jun 202012
I run into a bug recently with SQL Server 2012. I believe this exists in SQL Server 2008 as well. When a TVP is passed to a SQL function/procedure/script through an CLR function, the second execution will cause error which will be written to SQL Server log — Process x generated fatal exception c0000005 EXCEPTION_ACCESS_VIOLATION. SQL Server is terminating this process. It has been filed to Microsoft Connect. If you think it’s important, please visit this bug on Microsoft Connect and vote it as important. Please download the code from here.
Jun 182012
Recently I was looking for a solution for dynamic code compiling. I came cross Microsoft Roslyn and had a test. It’s perfect solution for dynamic code compilation and execution. You will have to have .net 4.0, Visual Studio 2010 sp1, Visual Studio 2010 SDK sp1, and Microsoft Roslyn installed in order to make this happen.
May 142012
Expression class under System.Linq.Expressions is a very useful class which includes many static members helping you to create expression tree and execute the tree. Whenever you need to execute a dynamically generated logic in SQL Server through an CLR, you might consider using the classes in Expressions name space. In this post, I am going to give you an very simple example to explain how to create and execute an expression tree.