30Nov 2009 by Grant Fritchey 6 Comments

SQL Server XQuery For Idiots

SQL Server, T-SQL
I'm still struggling with learning XQuery. My latest little revelation was small, but vital. The difference between: @inXML.nodes('/rss/channel/item') and @inXML.nodes('/rss[1]/channel[1]/item') Is the difference between a 10 minute query and a 4 second query. Kind of shocking really. My understanding is that XQuery assumes there are multiple possible paths that look like "/rss/channel" so it searches over and over again through the XML to see if it can find them. But by identifying it as a root, showing that only one possible path is available, it stops stumbling around in the dark and simply reads the data. It's a little thing, but it made an enormous difference. I've still got a long way to go in learning how to use XPath within XQuery. UPDATED: I modified the title so that it's…
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28Oct 2009 by Grant Fritchey 6 Comments

Snags With Profiler GUI

SQL Server, T-SQL, Tools
Running the Profiler GUI against a production server is not something you should do. I've outlined my research into exactly why in the past. But I hit another little issue with the Profiler GUI as part of work I'm doing on a Microsoft PSS call (more on that in another post). We have a procedure on one of our systems that is erroring out, but only on a particular server and only when called from the application, not when it's called, on the same server, from SQL Server Management Studio. I needed to capture some trace events and do it quickly, so I decided to use the GUI, just this once. I put filters on it so that I would only collect certain data, and the database I was collecting…
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21Oct 2009 by Grant Fritchey 1 Comment

Characters

SQL Server, T-SQL, Tools
No, I'm not talking about a Dickens novel. I'm talking about the number of characters in a string. I had a painful time recently because of the word "characters."  If you take a look at the dynamic management view sys.dm_exec_sql_text you can get the queries that have been run on your system that are still in the cache. It's a great utility. Better still, you can get specific statements from the code that are actively running through sys.dm_exec_requests or ones that have run through sys.dm_exec_query_stats. To do this is very simple. Each of these DMV's has a pair of columns, statement_start_offset and statement_end_offset. These columns, and I'm quoting directly from books online measure the "number of character" offset from the beginning of the SQL string and from the end of…
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21Oct 2009 by Grant Fritchey No Comments

Free Training RIGHT NOW

SQL Server, T-SQL
Quest Connect 2009 is occurring even as I type this. Get on over there if you're interested in some free training. I recorded a session on understanding execution plans. But even better, there are live sessions with some great people. Stop reading this, click the link, get yourself some free training.
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13Oct 2009 by Grant Fritchey 5 Comments

Table Variables Are Only in Memory: Fact or Myth

SQL Server, T-SQL
I keep seeing these concepts that have long been disproven, posted again and again on newsgroups as if they were valid information. The latest? Table variables are better because they're only in memory where as temporary tables write to the disk through tempdb. This one is abjectly wrong. I'm not even going to hedge with "it depends." From a macro point of view, there are only a few differences between temporary tables and table variables, statistics being the biggest. Temporary tables have 'em and table variables don't. Other than that, both will reside completely in memory or will swap out to the disk through tempdb, depending on their size. Some of the minor differences, and why you might want to use table variables over temporary tables, table variables won't cause a statement recompile while temporary tables will, table…
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05Oct 2009 by Grant Fritchey 15 Comments

Ad Hoc Queries Don’t Reuse Execution Plans: Myth or Fact

SQL Server, T-SQL
Another frequently heard story is that stored procedures get and reuse execution plans, but ad hoc queries do not. A lot of people believe this and say as much online and in the real world. Unlike my last myth, this time, I'm going to give you the DBA answer to this question. It depends. There are ad hoc queries and there are ad hoc queries. The classic ad hoc query looks like this: DECLARE @sql NVARCHAR(MAX), @value int; SET @value = 43668; SET @sql = 'SELECT * FROM Sales.SalesOrderHeader AS soh '; SET @sql = @sql + 'JOIN Sales.SalesOrderDetail AS sod '; SET @sql = @sql + 'ON soh.SalesOrderID = sod.SalesOrderID '; SET @sql = @sql + 'WHERE soh.SalesOrderID = ' + CAST(@value AS NVARCHAR); EXEC (@sql); And as ad…
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30Sep 2009 by Grant Fritchey 27 Comments

Pre-Compiled Stored Procedures: Fact or Myth

SQL Server, T-SQL
There's a very common belief among DBA's: Stored procedures are pre-compiled. I had someone tell me all about it just last week. There are lots of sources that say what I'm getting ready to say, so this really shouldn't come as news to anyone,but it will. No, they're not. Stored procedures are not pre-compiled. When you create or update a stored procedure on the server, it gets a syntax check. But there is no compile process, let alone a pre-compile process. That stored procedure exists exactly as it is named, a procedure that is stored on the server. It's just a collection of TSQL code, possibly with some parameters, waiting to be called. Until it's called, it is not ready to be executed. Don't believe me? I'm not a huge…
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22Sep 2009 by Grant Fritchey No Comments

Gila Monster on Estimated vs. Actual Row Counts

SQL Server, T-SQL
I don't generally do lots of blog aggregation and cross post linking & stuff. It's just not something I'm that into. However, this time is an exception. Gail Shaw, Gila Monster to those who hang on out SQL Server Central, has posted an excellent explanation of times when Estimated and Actual row counts vary for a reason. I'm one of those who emphasises that differences between estimated & actual is an indication of... something. It could be out of date or missing statistics or it could be caused by query language like multi-statement table valued functions, but it's usually and indication of a problem. Except when it's not. Read Gail's explanation for more.
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21Sep 2009 by Grant Fritchey 6 Comments

Identifying Frequently Running Queries

SQL Server, T-SQL
It's not enough to look at the longest running query. What if you have two queries, one runs for 5 minutes and the other runs for 5 seconds. Which do you tune? The first one, of course. But, let's add a little information to our thought experiment. The one that runs for 5 minutes is called 2AM, your local time, when there are almost no users on the system and the one that runs for 5 seconds is called 40 times a minute all day long. Now, which one do you tune? That's right, suddenly that 5 second query takes on a HUGE significance based on how often it's called. But how do you know how often a query is called? The easiest way to gather that information is not…
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15Sep 2009 by Grant Fritchey 2 Comments

More Free Training

SQL Server, T-SQL
Quest Connect 2009, taking place in October 21 for 24 hours, looks like it's going to have 64 different sessions, live and recorded, by a variety of the names in the industry. It's another chance to dig in and learn the details on a variety of topics from some of the top names in the business. Can you say Tom LaRock? How about Tim Ford? I know you want to hear from Brent Ozar. Those are just some of the featured speakers. There are a whole slew of others, it's worth pursuing, and did I mention, the price is right. I recorded a session for them last night. It's on the basics of understanding execution plans.
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