Category: Query Tuning

Kendra Little describes the nested loop operator in execution plans:

One of the first things we often learn when we’re looking at plans is that SQL Server uses estimates. And sometimes, those estimates are wrong. At first glance, this looks really wrong– it estimated 11.5 rows, and actually got 20,825 rows!

Read the whole thing to see how nested loop estimates which seem way off might not actually be.

Brent Ozar has a guide on troubleshooting parameter sniffing:

Parameter sniffing fixes are based on your career progression with databases, and they go like this:

1. Reboot the server! – Junior folks panic and freak out, and just restart the server. Sure enough, that erases all cached execution plans. As soon as the box comes back up, they run rpt_Sales for China because that’s the one that was having problems. Because it’s called first, it gets a great plan for big data – and the junior admin believes they’ve fixed the problem.

2. Restart the SQL Server instance – Eventually, as these folks’ careers progress, they realize they can’t go rebooting Windows all the time, so they try this instead. It has the same effect.

If a reboot can’t fix the problem, I’m out of ideas…

By the way, I second Brent’s recommendation of Erland’s query plan article.  Erland doesn’t publish frequently, but when he does it’s worth the wait.

Brent Ozar has time to kill and a 72-core box on which to kill it:

For example, when I run it with 10,000 joins:

1. Msg 8631, Level 17, State 1, Line 1
2. Internal error: Server stack limit has been reached. Please look for potentially deep nesting in your query, and try to simplify it.

When I drop it down to a much more realistic 5,000 joins:

1. Msg 4074, Level 16, State 1, Line 1
2. Client drivers do not accept result sets that have more than 65,535 columns.

Ah! Okay, that’s fair. (That’s also two error messages I’ve never seen before. Probably a good thing.) Alright, let’s take out the SELECT * and replace it with SELECT p1.* and see what happens:

This is a fun read.

Jack Li explains when you might not get a result back for query_plan in sys.dm_exec_cached_plans:

When you first execute a batch or stored procedure which may contain multiple statements.   Not every statement will be compiled when you first use the procedure.   If there are some code paths that result in the statement not being executed, SQL Server may choose not to compile that statement in a small set of scenarios. In other words, some statements will only be compiled upon first execution.  So far, I have found the following two scenarios that a statement will not be compiled (deferred compilation) if code path result in that statement being skipped.

1. Statement involving temp table.   Data could have been changed when the statement is actually executed.  So it doesn’t make sense to compile right off the beginning.

2. Statement that has option (recompile) attached to it   We need to recompile that statement every time anyway.  If we don’t use, it why compile in advance?

Read the whole thing.

Ginger Grant explains several problems with nested views:

In addition to the performance reasons, there are other reasons not to use nested views. Supporting and maintaining a nested views can be a nightmare. If there is an issue with the accuracy of the data, finding the problem is just that much harder. And what about when you go to fix the problem? Then you need to test all of the places the view is called. I worked with some code recently where a view was created on a set of data just to create 3 new values based up 3 case statements. That view was nested many levels below where the field was actually called. I know people from a development background look at nested views as modular development. Don’t. TSQL is not the same as object oriented code.

Read the whole thing.  She also has helpful links digging further into the topic.