Blog post #7 in support of Tim Ford’s (b|t) #iwanttohelp, #entrylevel. Read about it here. Sooner or later when you're working with SQL Server, someone is going to complain that the server is slow. I already pointed out the first place you should look when this comes up. But what if they're more precise? What if, you know, or at least suspect, you have a problem with a query? How do you get information about how queries are behaving in SQL Server? Choices For Query Metrics It's not enough to know that you have a slow query or queries. You need to know exactly how slow they are. You must measure. You need to know how long they take to run and you need to know how many resources are…
The Common Table Expression (CTE)Â is a great tool in T-SQL. The CTE provides a mechanism to define a query that can be easily reused over and over within another query. The CTE also provides a mechanism for recursion which, though a little dangerous and overused, is extremely handy for certain types of queries. However, the CTE has a very unfortunate name. Over and over I've had to walk people back from the "Table" in Common Table Expression. The CTE is just a query. It's not a table. It's not providing a temporary storage space like a table variable or a temporary table. It's just a query. Think of it more like a temporary view, which is also just a query. Every time I explain this, there are people who don't…
Based on the number of times I see this question on forums, it must be occurring all the time. You have two different servers that, as far as you know, are identical in terms of their options and setup (although not necessarily in terms of power, think a test or pre-production system versus production). On these servers you have a database on each that, as far as you know, is the same as the other in terms of options, objects, maybe even data (although, this does mean that you have unmasked production information in your QA environment, which potentially means you're going to jail, might want to address this, especially now that I've told you about it, mens rea, you're welcome). On each database you run, as far as you know, the exact same query (whether…
While presenting at SQLDay in Wroclaw, Poland, on the Query Store, I was asked a pretty simple question, which takes precedence, the Query Store or a Plan Guide? One of my favorite answers to questions is "I don't know" because it gives me the opportunity to learn. Let's figure this one out together. I'll post the code to recreate this experiment within AdventureWorks at the end of the article. I'm doing this because the code for forcing execution plans using Plan Guides can be pretty doggone long (you may need to generate your own XML from a plan on your own system, fair warning). I have a stored procedure that I use frequently to demonstrate parameter sniffing and bad parameter sniffing, AddressByCity (listed below). This query when passed the value…
I love questions. I recently received one about new plans in the Query Store (available in Azure SQL Database now and in SQL Server 2016 after June 1). Let's say you have selected a plan that you want to force. You set it up. Now, let's say the plan ages out of cache or even goes through a recompile. During the recompile, due to out of date statistics or skew in the statistics, you would, under normal circumstances, get a new plan. However, with Query Store and plan forcing, the plan that's going to be used is the plan that is being forced. But, does that other plan, the one not used, get stored in Query Store? I have no idea. Let's find out. The Setup To start with, a small stored procedure…
Blog post #4 in support of Tim Ford’s (b|t) #iwanttohelp, #entrylevel In SQL Server, in the T-SQL you use to query it, a view looks just like a table (I'm using the AdventureWorks2014 database for all these examples): SELECT * FROM Production.vProductAndDescription AS vpad; SELECT vpad.Name, vpad.Description, vpmi.Instructions FROM Production.vProductAndDescription AS vpad JOIN Production.Product AS p ON p.ProductID = vpad.ProductID JOIN Production.vProductModelInstructions AS vpmi ON vpmi.ProductModelID = p.ProductModelID WHERE vpad.ProductID = 891 AND vpad.CultureID = 'fr'; The above query actually combines two views and a table. This is what is commonly referred to as a "code smell". A code smell is a coding practice that works, but that can lead to problems. In this case, we're talking about performance problems. The performance problems when using views to join to…
THIS IS NOT AN APRIL FOOL POST! Seriously. My Speaker of the Month for April 2016 is Keith Tate (b|t) and his session at SQL Saturday Chicago called Profiler is Dead, Long Live Extended Events. I actually suspected very strongly from the start of the session that it was going to be good. The reason for this, Keith was having issues with his machine, but he started the session anyway. It was an excellent beginning. Then, he started to talk about Extended Events and use his slide deck to emphasize the points he was making, and it was wonderful. For example, as he talked about the way the number of events has grown in each version of SQL Server since 2008, he used larger and larger fonts with the bigger…
This is blog post #2 in support of Tim Ford's (b|t) #iwanttohelp, #entrylevel. If you haven't been working in SQL Server for very long, you may not have got this phone call yet, but you will: Hi, yeah, the server is slow. Thanks. Bye. Let's pretend for a moment that you know which server they're referring to (because just finding out that piece of information can be a challenge). Now what? The list of tools and mechanisms within SQL Server for gathering metrics is extremely long: Performance Monitor Dynamic Management Views & Functions System Views Extended Events Trace Events Activity Monitor Data Collector Execution Plans 3rd Party Tools I'm leaving out lots of stuff in that list. So where do you start when you get this phone call? Where is the server slow?…
I'm sitting in the classroom of SQL Cruise listening to Tim Ford (b|t) explain mechanisms for monitoring indexes. It's a great class. Earlier in the week I got to hear Jes Borland (b|t) talk about extended events and do a session on wait statistics. I was also lucky enough to listen to David Klee (b|t) talking about systems monitoring, especially around VMs. Argenis Fernandez (b|t) and Jason Hall (b|t) are coming up today. In short, I've received some excellent learning while on a boat in the Caribbean. Now, one could argue (and you'd be right) that I'm thinking about positivity because of the nature of the position in which I find myself. Hang on though, I have some additional points. One of the biggest strengths of the SQL Cruise is the intimacy of the…
One of the ways that you take more direct control over your SQL Server instances is through the use of trace flags. There are a number that people recommend you enable by default. Prior to Extended Events for example, I'd say you should turn on trace flag 1222 in order to capture deadlock information on your server (now I just recommend you use the system_health session). I absolutely think you should turn on trace flag 2371 to get better behavior out of your automated statistics updates. There are others that I'll leave to all the systems experts to advise you on. What about Azure SQL Database? I doubt you'll be shocked, but if I try this: DBCC TRACEON (2371,-1); I get the following error: Msg 2571, Level 14, State 3,…