Not everyone has the opportunity to call Microsoft Premier Support. For those who have not yet had this experience, I'll document my most recent, non-critical, call. Critical calls are a different critter entirely. We were experiencing a very odd error on one server in our system. When a particular procedure was run with a particularly large set of data, it would produce an error, but only when called from the application. The exact same error with exactly the same data called from SSMS did not produce an error. We went through a very extensive set of tests and were unable to fix the problem on the server. After moving the production system that was experiencing the issue to a different server, we decided to contact PSS. 8:48 Am, Tuesday: I made the…
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."Â 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…
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…
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…
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…
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'), Â ('Val3') ; INSERT INTO dbo.Test2…
I keep thinking I've got a handle on the way execution plans are dealt with in SQL Server. I had a pretty simplistic view of things, there's the estimated plan which comes out of the optimizer and there's the actual plan which comes out of the data engine. The one shows the best guess for what will happen based on the query and the underlying statistics. The other shows what actually happened based on the query and the real data, load on the server, available memory, you get the idea. This model is easy to explain and understand. Too bad it's not quite the way things work. If you query the dynamic management function sys.dm_exec_query_plan, you can see a query plan. Once you drill down on the XML, or browse…
In my previous post, Spatial Data Hurts My Brain, I showed how a query used the spatial index, but hurt performance. Several people jumped in to try to help out. Thanks to Bob Beauchamin, Isaac Kunin, Gail Shaw, and Valerie Yakich (via Twitter). I've definately learned a bit more than I knew previously about spatial data and spatial indexes. I've posted about it before, but it bears repeating, if you're just starting out, I can't recommend Alistair Aitchison's book, Beginning Spatial with SQL Server 2008, enough. Here's where I'm at. I think I was confusing/confused/confounded/something about what to expect from a spatial index. I'm going to present two queries and two indexes and try to explain why each works well, or not together, mainly as a means for enabling my…
I'm still barely scratching the surface working with spatial data in SQL Server 2008. We've ported some of the data into a table where we built a geography spatial data column and we're begginning to work with point data. The requirements from the developers are, so far, very simple. They'll feed me a point and I find all the locations "close" to it. We had to go round & round on what defines "close" but finally settled on, I think, 15km. The query to answer a question like this is ridiculously simple (a few object names have been changed): SELECT ebe.[Location].STDistance(@Location) AS Distance, ebe.[InterestId], ebe.[Location].Lat AS Latitude, ebe.[Location].Long AS Longitude, ebe.[OrgId] FROM dbo.[ebe] AS ebe WHERE ebe[OrgId] = @OrgId AND ebe.[Location].STDistance(@Location) < @CloseDistance I'm not even hard-coding the "close" value…
You read that correctly, three kinds of execution plans. You may have thought that all you had to deal with are estimated and actual, but there is one more. The estimated plan is the plan that comes out of the optimizer. It's based on statistics and indexes and known objects within the system. The actual plan is the plan that was used to execute the query and will show all the actual number of rows processed, etc. It might be different than the estimated plan because the stastics were off or for any number of other reasons. Those were the ones you knew about. There is also the plan that gets stored in the plan cache, the compiled plan. I lied. The compiled plan and the estimated plan are the…