In this post, I show you how to determine exactly which statistics objects were used by the query optimizer to produce an execution plan. Trace Flags We will need three undocumented trace flags.  The first one (3604) is well-known – it redirects trace output to the client so it appears in the SSMS messages tab. The second trace flag is ...

Did you ever wanted to know which execution plans the Query Optimizer would generate for your queries should your tables have millions of records? You can actually generate those plans by using the undocumented ROWCOUNT and PAGECOUNT options of the UPDATE STATISTICS statement. These options can be used on small or empty tables and can be ...

I got a question from a reader of my post How the Query Optimizer Uses Statistics (http://sqlblog.com/blogs/ben_nevarez/archive/2009/09/03/how-the-query-optimizer-uses-statistics.aspx) and I thought that it would be a good idea to post my answer here. Basically the request was to expand the previous example for a predicate with two columns. ...

In case you are not aware, the excellent white paper “Statistics Used by the Query Optimizer in SQL Server” has been updated for SQL Server 2008.   You can find it here   Statistics Used by the Query Optimizer in SQL Server 2008 Writer: Eric N. Hanson and Yavor Angelov Contributor: Lubor ...

  This post shows how the Query Optimizer uses statistics to estimate the selectivity of expressions during query optimization.   You can also use this as a second part of my last post, The Query Optimizer and Parameter Sniffing. Here I will show you the advantage of the use of statistics when the Query Optimizer can “sniff” the ...

  As we all know, the SQL Server Query Optimizer uses statistics to estimate the cardinality and selectivity of predicates of a query to produce an optimal execution plan. The Query Optimizer accomplishes this by first inspecting the values of the query parameters. This behavior is called parameter sniffing and it is a good thing. Getting an ...

In my previous post I talked about contradiction detection as one of the early steps in query optimization. Another interesting step performed during query optimization is the automatic matching of computed columns. Although computed columns have been available in previous versions of SQL Server, the automatic matching feature was introduced until ...