This is in part a response to a comment by Paul White (@SQL_Kiwi) to my previous post on the performance impact of enabling hyperthreading (HT) on OLTP queries, and in part due to my desire to capture a more complete set of test data for future investigation on this very topic. I’m posting below the results of re-running the same exact test as ...

My previous post focuses on the performance impact of enabling hyperthreading (HT) on a machine with four Intel Westmere-EX processors on reporting queries. Let’s turn our attention to OLTP queries. To oversimplify it, reporting queries are generally processed by scanning a large number of pages, whereas quick index seeks are the hallmark of OLTP ...

In commenting on my previous post, Greg Linwood and GrumpyOldDBA raised questions about various implications of parallelism. In this post, I’ll look at the impact of different degrees of parallelism on the query performance. I’ll limit my examination on the same query that uses a Cartesian product temp table, as discussed in the previous two posts ...

Although Cartesian product as a concept is essential to the relational database theory, it is often a dirty phrase – something that is associated with bad performance and therefore should be avoided. But there are cases where a Cartesian product is highly desirable. This post shows you an example query—from the real world although presented here ...

In an earlier post, I highlighted that linked server security configuration can have a huge performance impact, and complained that this was not properly documented in SQL Server Books Online and filed a Connectitem for this. Good news is that in Books Online for SQL Server 2008 R2,  Microsoft has revised the documentation, and included the ...

Whether or not you specify it explicitly, ANSI_PADDING setting is there when you create a table, and can have an impact on the performance of some queries. If you are not careful, it can even hurt performance big time!   Let’s demonstrate that with an extremely simple example.   Assume that you have this simple view with a UNION ALL ...

In SQL Server, it is rather handy to retrieve data from a different SQL Server instance and use the result locally in another SQL statement for further processing. In theory and in the set purists’ fantasy land, it shouldn’t matter where you get your data or even how you get the data as long as you can use them to further compose a set-based ...

If you change the security configuration of a linked server from using login UserA to login UserB, would you expect that to significantly impact the performance of a query that uses the linked server, assuming that both users have permission to select from the remote table referenced in the query?   Many people probably would answer no. ...

All the previously posted results (May 25th and May 29th) on this exercise were obtained with query parallelism disabled (i.e. the sp_configure ‘max degree of parallelism’ option was set to 1).   Since the following test query is sensitive to query parallelism, we need to see what impact query parallelism may have.   DBCC ...

This is another follow-up on the T-SQL exercise.   So the test query below is rather simple:   DBCC DROPCLEANBUFFERS go SELECT COUNT(*) FROM dbo.test;   But beneath its simple appearance, many factors are at play and interact in a complex way to influence the query performance. In other words, trying to predict its ...