15Sep 2009 by Grant Fritchey 30 Comments

No Join Predicate

SQL Server, T-SQL
You could be looking at an execution plan on a query and see this message: Warning, No Join Predicate. With a very few exceptions (very few), that's not a warning, like "Mind the gap" or "Your Mileage May Vary" or "Never fight a land war in Asia." No. It's a screaming shout saying "BY ALL THE GODS IN VALHALA, YOU DON'T HAVE ANY JOIN PREDICATES IN THIS QUERY!" Seriously, that's exactly what it says. But, you might be asking yourself, that's what it says, but what does it mean? Please allow me to explain. The tables in your system, whether it's a classic BI star schema, or a highly normalized OLTP system, or even (shudder) ORM objects, are related to one another. Usually they're related through the use of primary…
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11Sep 2009 by Grant Fritchey No Comments

LaRock on Monitoring

SQL Server, T-SQL
Since I just spent a bit more than half of my 24 Hours of PASS presentation on tuning queries talking about monitoring performance, you could probably tell that I think that the two are inextricably linked. I'm not alone. Tom LaRock has put a post on why it's important to understand the extent of your problem prior to attempting to fix it. It's absolutely spot-on and a must read. Remember, if someone says performance on a system you manage is slow, you have to ask, as compared to what? You need to understand what "normal" performance is on your system in order to arrive at the ability to identify a particular process as performing in an abnormal manner. That's not to say that you couldn't tune a query in complete…
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09Sep 2009 by Grant Fritchey 8 Comments

Spools in Execution Plans

SQL Server, T-SQL
I got the question the other day, when are you likely to see a spool in an execution plan? Easy, whenever SQL Server needs to walk through the data multiple times, usually in JOIN operations... Yeah, well, once again, my flip answers are not quite the entire story. Spool operations are temporary storage of the data for later reuse in a query plan. There are two types of spool operations, eager spool and lazy spool. A spool is basically a temporary table created within the execution of the query that is used when it's likely that data will be needed again, and again during the execution of the query. This is not an explicit #temp temporary table, but a work table for operations within the processing necessary for a given query's…
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02Sep 2009 by Grant Fritchey 3 Comments

24 Hours of PASS

PASS, SQL Server, T-SQL
This was a pretty cool event. The advertising leading up to it seemed to really cover the community very well. There was even a link to 24 Hours of PASS in my team's local web site. Unfortunately, I'm not an iron man like Tom LaRock, watching all 24 hours. I only hit a few sessions. The first session I hit was Allen White's PowerShell for SQL Server. I have attended a couple of  his presentations at the PASS Summit. I think he's done a great job of gleaning better and better examples out of his presentations. This one was good. I loved the way he put together the backup processes for automating backups. It looked good, took into account common issues, it just worked. I was really happy I got…
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21Aug 2009 by Grant Fritchey 15 Comments

What happens when you use WITH RECOMPILE

SQL Server, T-SQL
I saw this question and my immediate thought was "Well, duh, the execution plan is recreated by a recompile." But, someone a bit smarter than myself suggested that, in fact, no plan was stored in cache, forcing a new plan to be created each time the query was run. So, which is it? Does a plan get added to cache and then removed every time the procedure is called, or do you get nothing in cache and the "recompile" is actually a compile every time? As Mr. Owl says, let's find out. I have a small script that looks like this: CREATE TABLE [dbo].[Test]( [col] [varchar] (10) NULL ); CREATE TABLE [dbo].[Test2] ( [col] VARCHAR(10) NULL ); INSERT INTO dbo.Test (col) VALUES ('Val1'), ('Val2'),   ('Val3') ; INSERT INTO dbo.Test2…
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12Aug 2009 by Grant Fritchey 3 Comments

Excellent Blog List

SQL Server, T-SQL
For the one or two you that are not reading Buck Woody's blog (and why aren't you), you may not have seen that he's posted a recommended reading list of the blogs that he reads on a regular basis. This list must be reviewed by you, now. Go on, I'll wait here till you get back.
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11Aug 2009 by Grant Fritchey 3 Comments

Some Statement Start Times!

SQL Server, T-SQL
I thought I had captured statement start times within the DMV sys.dm_exec_sessions. I was absolutely wrong. However, this has sparked a bit of discussion, as you can see in the link to my correction. John Vanda (I couldn't find a blog by him, but I think this is his LinkedIn profile), suggested possibly joining between the sys.dm_exec_requests and sys.dm_tran_active_transactions through the transaction_id available in the requests DMV. He ran a series of tests that showed it to be promising. I re-ran my original test, a few long-running statements within a batch; BACKUP, CHECKDB, etc.. This time I captured the transaction information along with the other DMV's. The results were interesting. Rather than pull together any particular set of data as I was testing, I just collected all three DMV's once…
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28Jul 2009 by Grant Fritchey 7 Comments

When did this statement start?

SQL Server, T-SQL
UPDATE: This post is incorrect. Adam nailed it in the comments. I explain my mistake here. A question came up over at SQL Server Central where someone was wondering if it was possible to know when a given statement within a batch started. Immediately I thought, oh yeah, that's easy, use the sys.dm_exec_requests dynamic management view (DMV). Done. Wrong. The original poster pointed out that I had assumed that the values present in the DMV represented statement level values, but they show the batch. While the DMV shows a start_time, that time is the start of the current batch, not the statement within the batch. Now the question was, where else might I get this data? I next tried sys.dm_exec_sessions because it has the last_request_start_time value. Sure enough this worked. Don't…
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20Jul 2009 by Grant Fritchey No Comments

MS Field Engineer’s on Performance Troubleshooting

SQL Server, T-SQL
Do you want to get a glimpse into how the Microsoft Field Engineers would go about troubleshooting performance issues on your server? Then go and read this blog entry by Denzil Ribeiro. Not only is this an excellent how-to on troubleshooting performance problems, but Mr. Ribeiro provides multiple links that describe the concepts he's dealing with further, making it a great piece of documentation. The MS Field Engineer blog is not terribly active, but what gets posted there is worth reading. If you don't have it on your feed list, you should.
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17Jul 2009 by Grant Fritchey 15 Comments

Execution Plan Compile Termination

SQL Server, T-SQL
Recently I've been seeing a lot of people with bad execution plans, desperately trying to tune them, but they were unable to explain why they had such bad plans. More often than no these were larger queries, with a number of derived tables, CTE's, CROSS APPLY, etc. In most cases the statistics appeared to be fine (this is usually checked by comparing estimated & actual rows within the operations in the execution plan) and the plans themselves didn't look crazy, but the execution plans were flat out, not good. If you're looking at a plan and it doesn't make much sense, one option that most people don't check... SQL Server didn't have enough time to complete optimization. The optimizer is a pretty amazing bit of code. The scary volume of…
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