Jul 19 2010

SQL University: Introduction to Indexes, Part the First

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, you’re in the wrong room.

Indexes… indexes…. There are, of course, different kinds of indexes. I’m sure that some of you, glancing ahead in your books, are already thinking, “yeah, two.” And you would, of course, be ABSOLUTELY WRONG! That’s why you’re in this class, because you don’t know. There are a large number of different kinds of indexes. Most people think of the standard indexes, of which there are two, clustered and non-clustered. But when pressed they can usually come up with the Full-Text index and possibly even the XML index. But that leaves out Spatial indexes, filtered indexes… more. Microsoft’s documentation lists eight different indexes:

  • clustered
  • non-clustered
  • unique
  • indexes with included columns
  • Full-Text
  • Spatial
  • Filtered
  • XML

But I’ve seen other people count them other ways and arrive at varying amounts. Is a compound index a different kind of index? If it’s not, is unique really a different kind of index? Things to think about.

Why so many? What the heck is an index good for? They must be useful critters or Microsoft wouldn’t have put so many different sorts (however many that is) into SQL Server. I started off talking about the Dewey Decimal System for a reason. An index, any of the indexes we’re going to talk about, is primarily meant, like the DDS, as a mechanism to make finding things easier. That’s all it is. Pretty simple, right? Wrong. You clearly haven’t spent time with SQL Server indexes or the DDS. It’s really complicated. But, just like the DDS, learning how indexes work will make using them much easier.

Remember, the main purpose of a database, despite what your DBA may secretly feel in his or her heart, is not to keep, store and protect data. No, the main purpose of a database is to feed that data up to your business users, whoever they may be, in a timely and accurate fashion. That’s where indexes come in. They will help your queries get the data out to your users faster. Think about your data like a really huge library and your data like a bunch of books. The index acts like the DDS as a mechanism to speed you through the library and quickly and easily retrieve the book that you want.

Enough comparisons, since this is introductory, I just wanted to get the idea of indexes into your head. In the next installment I’ll take on two (or four, depends on how you count them) different kinds of indexes, starting with the standard two that you expected me to cover, clustered and non-clustered indexes. I’ll also introduce the concept of a heap and we’ll talk about what the heck a B-Tree is.

See you next class, probably. Be careful crossing the quad, I’ve heard Wilbur Whately is back on campus and we all remember what happened last time.

6 Comments

  • By Thomas LeBlanc, July 27, 2010 @ 9:30 pm

    Grant, can you explain the following: You say the non-clustered index will have MORE info…than a clustered index???

    This is because, again, depending on the size of the keys and any INCLUDE columns, a non-clustered index will have more information stored on the page than a clustered index will and so fewer pages will have to be read, making the operation faster.

Other Links to this Post

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  2. SQL University: Introduction to Indexes, Part the Second « Home of the Scary DBA — July 21, 2010 @ 5:03 am

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