Category: Query Tuning

Greg Gonzalez with the announcement of the week:

Which brings me to today, one of the most exciting days in the history of SQL Sentry. I would like to introduce to you the new unified Plan Explorer v3! This edition includes all features of Plan Explorer PRO, plus two new and incredibly powerful features: Index Analysis and Performance Profiling.

These new features were originally going to be part of yet a 3rd edition called Plan Explorer ULTIMATE, and at an even higher price point. However, we ultimately came to our senses and realized that this would be counter to the mission, and that the only way ahead was to make all features of Plan Explorer freely available to everyone.

Andy Mallon goes in-depth with two of the new features:

If you’ve used Live Query Profile in Plan Explorer (or Live Query Statistics in SSMS), then you’re familiar with seeing your query plan play live as it executes. Performance Profiling adds some special sauce to that feature. Its like a DVR for the live query profile, and also displays the performance metrics as a graph below the plan diagram. The graph gives performance metrics by time, rather than by operator. In complex plans, you might have many operators doing work at the same time, and you’ll be able to see that better by looking at the graphs. Adding a time axis to the plan is a pretty ingenious way to give a new way to look at performance.

You can then replay the “DVRed” query, and re-watch the profile without re-running the query. If you save the plan as a .pesession file, the DVR’ed query with performance profile is included in that file. You can even change the speed to watch in slow-mo or fast-forward.

If you don’t already use Plan Explorer, you’re missing out on the single best query tuning add-in.

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.