Reinforcing the Importance of Statistics on Row Estimate

SQL Server, SQL Server 2016, T-SQL
I recently wrote an introductory post about the importance of statistics. I just received a reinforcement on how important they are during my own work. Bad Estimate I hit a weird problem while I was setting up a query to illustrate a point (blog to be published next week). Let's take the basis of the problem and explain it. I wanted data with distribution skew, so I ran this query to find out if there was a wide disparity between the top and bottom of the range: SELECT i.BillToCustomerID, COUNT(i.BillToCustomerID) AS TestCount FROM Sales.Invoices AS i GROUP BY i.BillToCustomerID ORDER BY TestCount ASC; Sure enough, the bottom of the range returned three (3) rows and the top returned 21,551. If I then run a query to retrieve just a few rows…
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Avoiding Bad Query Performance

There’s a very old saying, “When you find yourself in a hole, stop digging.” And my evidence today is: That’s certainly not the hairiest execution plan I’ve seen. In some ways, it’s not all that horrible. But it sure is evidence that someone was down in a hole and they were working that shovel hard. If you’re interested, most of the operators are scans against a table variable that’s 11 million rows deep. There are also table spools chugging away in there. And the select statement only returns 1500 rows. Please, stop digging.
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SQL Azure Query Tuning

SQL Azure is still SQL Server at the end of the day. This means it is entirely possible to write queries against SQL Azure that really… what’s a good word… stink. So what do you do? It’s all in the cloud. You couldn’t possibly tune the queries, right? Wrong. Many of the same tools that you have available to you, such as execution plans and dynamic management objects, are still available in SQL Azure. Let’s talk DMOs for a second. First off, don’t make the mistake I did of trying to run these outside the context of a specific database on SQL Azure. You’ll get extremely inconsistent results, trust me on this. Anyway, I did a quick run-down on some of the most used DMOs for performance tuning, the sys.dm_exec_*…
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Reason for Early Termination of Statement

SQL Server, T-SQL
Wouldn't you like to know why the optimizer stopped tuning the execution plan you're looking at? It's actually possible and simple to get this information. I talked about this a little more than a year ago, but I left out some information that might be useful. Let's take a very simple query: SELECT * FROM Person.Address AS a; This generates a simple execution plan, in fact a trivial plan: I know that this is a trivial plan one of two ways. The hard way is to look at the XML (BTW, this is the type of thing you can't see it in STATISTICS PROFILE, which I understand a lot of people are using). Right at the top is the SELECT element. You can see the value for StatementOptmLevel property is…
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