The CASE Statement and Performance

SQL Server 2005, SQL Server 2008, SQL Server 2012, SQL Server 2014, TSQL
In case you don't know, this query: [crayon-5a868932a3cbb632447868/] Will run quite a bit slower than this query: [crayon-5a868932a3cc5643457340/] Or this one: [crayon-5a868932a3cc9343480701/] That's because the second two queries have arguments in the filter criteria that allow SQL Server to use the statistics in an index to look for specific matching values and then use the balanced tree, B-Tree, of the index to retrieve specific rows. The argument in the first query requires a full scan against the index because there is no way to know what values might match or any path through the index to simply retrieve them. But, what if we do this: [crayon-5a868932a3ccd431095571/] We're avoiding that nasty wild card search, right? So the optimizer should just be able to immediately find those values and retrieve them... Whoa!…
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SQL University: Index Usage

SQL Server 2005, SQL Server 2008, SQLServerPedia Syndication, TSQL
Greetings. Welcome once more to the Miskatonic University branch of SQL University. Does anyone know where to buy some camping equipment, cheap? I’ve been tagged to go an expedition to Antarctica and I need some cold weather gear a bit more substantial than my LL Bean boots. Evidently the last expedition found some caves in some mountains down there. Sounds like the perfect place to get away from all the crazy stuff that goes on here at Miskatonic. I mean, what could happen? Anyway, our last several talks have all been about indexes and indexing. One of the things that we haven’t talked about is how to tell if, how or when your indexes are being used. Starting with SQL Server 2005, and continuing to 2008 and R2, there has…
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SQL University–Recommendations for a Clustered Index

SQL Server 2005, SQL Server 2008, SQLServerPedia Syndication, TSQL
Welcome, SQL University Students to another extension class here at Miskatonic University, home to the Fighting Cephalopods (GO PODS!). Never mind the stains on the floor, or the walls…or those really nasty ones on the ceiling. There was a… oh what did the dean call it… an incident last week when one of the students had a little accident after reading Die Vermiss Mysteriis one too many times. But we’re not here to talk about arcane tomes and unspeakable horrors today. No, today we’re here to talk about clustered indexes. SQL Server storage is really predicated around the idea of clustered indexes. Don’t believe me? Let’s list a few places that require a clustered index: Partitioning. A table in SQL Azure In order to create XML indexes What about the…
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SQL University: Introduction to Indexes, Part the Third

SQL Server 2005, SQL Server 2008, SQLServerPedia Syndication, TSQL
Nice to see most of you have managed to fight your way through the shoggoths outside to attend another lecture at the Miskatonic branch of SQL University. This will be the third and final part of the introduction to indexes lecture. Please, if you're going mad, step out into the hall. Our previous two lectures introduced the concept of indexes and then talked about two types of indexes, clustered and nonclustered. This lecture will cover the concept of statistics as they relate to indexes. If you followed the previous lecture then you know that indexes are stored in a Balanced Tree or B-Tree structure. You know that this storage mechanism is intended to provide fast retrieval of data. But, how can the query engine inside SQL Server know which index…
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SQL University: Introduction to Indexes, Part the Second

SQL Server 2005, SQL Server 2008, SQLServerPedia Syndication, TSQL
Welcome once more to the Miskatonic branch of SQL University. Please try to concentrate. I realize the whipoorwills singing outside the window in a coordinated fashion that sounds almost like laboured breathing can be distracting, but we're talking about indexes here. We left last class with a general idea what an index is, now it's time for some specifics. There are several different kinds of indexes, as we talked about last class. But the two you're probably going to work with the most are clustered, non-clustered. Each of these indexes is stored in a structure called a B-Tree, a balanced tree, not a binary tree. That's a very important distinction. A B-Tree is a double-linked list that is defined by the keys of the indexes on the top and intermediate pages, and…
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SQL University: Introduction to Indexes, Part the First

PowerShell, spatial data, SQL Server 2005, SQL Server 2008, SQLServerPedia Syndication, TSQL
Right, all eldritch tomes are to be closed and Elder Signs are to be put away during this course. Welcome to the History department here at the Miskatonic branch of SQL University. Why the History department? Well, first, because I like history and have frequently thought I would enjoy teaching it. Second, because I needed a hook upon which to hang part of the story I want to tell. What story is that you ask? Why, the story of the Dewey Decimal System. We are interested in studying history and historians must classify our subjects carefully. For advanced students we'll be covering the Library of Congress Classification System and the... Right, I give, this is the introductory class on indexes. If you thought we were covering something exciting and sexy like PowerShell,…
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Spatial Indexes and a Lack of Data

spatial data, SQL Server 2008, SQLServerPedia Syndication, TSQL
I was feeling quite confident about my new-found abilities with spatial indexes so I did a presentation for my team, to share what I had learned. I had also been sharing with one co-worker as I developed the knowledge of spatial indexes. While I was preparing my presentation, he was preparing his. I had focused on finding a set of data that showed it's proximity to a test location and then showing how retrieving that set of data was faster because of the spatial index. He took a different approach. He took the idea of saying, here's a list of different test locations, let's see which one of our internal locations meet the proximity test. At the same time, he tried three different spatial indexes, one with high granularity, one with medium and a final…
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Spatial Data Hurts My Brain

spatial data, SQLServerPedia Syndication, TSQL
I'm still barely scratching the surface working with spatial data in SQL Server 2008. We've ported some of the data into a table where we built a geography spatial data column and we're begginning to work with point data. The requirements from the developers are, so far, very simple. They'll feed me a point and I find all the locations "close" to it. We had to go round & round on what defines "close" but finally settled on, I think, 15km. The query to answer a question like this is ridiculously simple (a few object names have been changed): [crayon-5a868932a7c24822142494/] I'm not even hard-coding the "close" value so it can change when they change their minds. It retrieves exactly what's needed. But... Well, look at the STATISTICS IO & TIME:…
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Tim Ford’s Top 5 Indexing Best Practices

SQL Server 2005, SQL Server 2008, TSQL
All I can really add to this is, yeah, me too. If you want some absolutely great advice on indexes, read this post. It's a must. And might I add, I've been the bad guy in Tim's example. Once, many, many years ago, I was reading from the SQL Server 7.0 documentation. It suggested that compound indexes were no longer needed since the optimizer could build them on the fly using index intersection. I had a performance problem and a consultant was telling me to use a compound index. I swore up and down it wouldn't work because Microsoft said so. He kept pushing and I kept pushing back. Finally, after a rather heated discussion in which I was convinced I had the upper hand, I got off the phone…
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