What Is an Index?

In Chapter 5, you learned about tables, which are, in essence, repositories that hold data and information about data—what it looks like and where it is held. However, a table definition is not a great deal of use in getting to the data quickly. For this, some sort of cross-reference facility is required, where for certain columns of information within a table it should be possible to get to the whole record of information quickly.

If you think of this book, for example, as a table, the cross-reference you would use to find information quickly is the index at the back of the book. You look in the book index for a piece of information, or key. When you find the listing for that information in the index, you’ll find it’s associated with a page number, or a pointer, which directs you to where you can find the data you’re looking for. This is where an index within your SQL Server database comes in.