Serious Error

T-SQL
When I wrote the book "Dissecting SQL Server Execution Plans" I knew I was going to get things wrong. Several people have pointed out things over the last couple of months. They've all been in the details. None of them were serious errors of fact. Andy Warren just found a huge one. In the section on Table Hints I detail how to apply an INDEX() hint. It's on page 123 in the electronic version or 124 of the first print version. I state that index number starts at 0 with the clustered index. That's just flat wrong. A clustered index is always 1. A 0 indicates a heap. Other indexes will have values greater than 1. If you were to supply a 0 to the INDEX() hint, as shown in the book, it forces either…
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Code from "Dissecting SQL Server Execution Plans"

T-SQL
This is the complete code listing from the book "Dissecting SQL Server Execution Plans." You need a copy of AdventureWorks. Please note, AdventureWorks changes. It changed three times while I wrote the book. These changes can be very subtle causing variations in statistics which will make some of the queries generate execution plans in a different manner than what was published in the book. Some of these changes can be pretty radical causing the queries to not work at all. Also, the book went through quite a few edits including rearranging the order in which sections appear. This listing is the order in which things were written and it might vary from the book. In other words, caveat emptor, your mileage may vary, keep your hands and feet inside the vehicle…
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Constant Scan in Execution Plans

SQL Server, T-SQL
I see a lot of searches from people apparently trying to find out what having a Constant Scan in their execution plan means. I can understand why. Here's the definition from the Books Online: The Constant Scan operator introduces one or more constant rows into a query. A Compute Scalar operator is often used to add columns to a row produced by a Constant Scan operator. OK. Very precise and yet, unless you know what the sentence means, reading it can be pretty confusing. The key is to see what Compute Scalar means: The Compute Scalar operator evaluates an expression to produce a computed scalar value. This may then be returned to the user, referenced elsewhere in the query, or both. An example of both is in a filter predicate…
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Breaking Down Complex Execution Plans

SQL Server, T-SQL
Peter Ward, the editor at SQL Server Performance, has published an article of mine on Breaking Down Complex Execution Plans. I go way beyond the blog entry below and show how the estimated costs in execution plans can mess you up, how to use the XML in execution plans to search through them for costly operations or operations that have mismatched estimated rows & actual rows and some other tips and tricks. Hopefully it's worth a read.
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Why are Subqueries Dangerous?

T-SQL
If you go around to the various forums, you'll see postings, including some I've put up, that say using subqueries, especially correlated subqueries in the SELECT statements of queries is bad because it effectively acts as a cursor. I got called on it. So I had to do a bit of research. Books Online talks about it right off when describing subqueries. That's not proof though. I did some more looking around. Adam Machanic does a nice job of slicing up Functions, which are basically the same thing. But it's not exactly the same. I kept looking. This blog entry is just flat wrong, but the second comment points out the fallacy. Jeff Moden would also like this example since it shows the disparity between the actual cost of queries…
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Operations Manager 2007 SQL Collection Article

T-SQL, Tools
It took me a while to find all the little pieces to put together a custom rule that used data collected through TSQL. In fact, because it took so much work to pull all the little pieces together, I wrote an article describing how I did it. This isn't a wonderful new invention. It's just the publication of a bunch of research. If you're an Operations Manager expert, this isn't for you. If you're like I am, trying to find your way through the forest, this little bit of map will hopefully provide some assistance. Available at SQL Server Central.
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Top Vs Max

SQL Server, T-SQL
I finished a set of research and tests. I put it all together and gave it to Chuck Heintzleman at SQL Server Standard (that web site can use some help). Hopefully he likes it and publishes it. I understand that the upcoming issue will be on performance tuning. If he doesn't like it, I'll get it over to Peter Ward at SQL Server Performance or to Steve Jones at SQL Server Central. I'm posting this because I see so many web searches looking for information comparing TOP to MAX. Here are some short answers for those interested. Data to support these suggestions is included with the article. ROW_NUMBER works best with smaller data sets. TOP has more consistent operation over a range a data. MAX frequently resolves as a TOP operation, but sometimes…
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Internet Searches

T-SQL
I like to look at the searches that pop up as links into the blog just for ideas on posts. What the heck do I do with this one: "nested loop joins" "sql server 2008" Yes, SQL Server 2008 has Nested Loop Joins. They do the same thing in 2008 that they did in 2005 and 2000.
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Break Down Complex Execution Plans

T-SQL
I've seen this question posted in a lot places and I've seen the search come in to the blog: "How do you break down a complex execution plan?" The short answer; just like eating an elephant, one bite at a time. The longer answer... First you have to define what you mean by a complex execution plan. For example, I've seen 16 page stored procedures that consist of about 50 or more different statements executed in series (and I 've heard of worse). No single statement was very big, but trying to dig through all the statements to identify which may be causing slow execution was a problem. This is the easiest type of complex execution plan to solve. In SQL Server 2005 and 2008, when you turn on a graphical…
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