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…
Read More

Index Statistics

The other day a developer showed up at my desk. They were getting time-outs in production on a query that didn't normally give them trouble. With the parameters they provided, I ran the query. It ran for over 30 seconds, the application side timeout, before it returned it's data. So I ran it again with an execution plan. It had a bunch of index scans with loop joins across thousands of rows and even created a table spool with 700 million rows as part of the process. Clearly not good. Next I looked at the query plan. It wasn't too bad, as these things go. It was probably moving too many columns and apparently the business wanted a pivot on the data since they were using an aggregate method to pivot some…
Read More