What Is The Preferred Join Operator in SQL Server?

SQL Server 2005, SQL Server 2008, SQL Server 2012, SQL Server 2014, SQL Server 2016, SQL Server 2017, TSQL
I schedule many of my blog posts at least 2-3 weeks out, so I have time to adjust them, change, them, insert new things into the schedule, what have you. So, as you read this, I'll have written it at least two weeks ago. As I write this, I'm recovering from a very mild, but irritating, cold. I needed to write a blog post, but my stuffed head wasn't coming up with ideas. So, I half-jokingly asked Twitter for any execution plan questions so I could write something. First response, also a joke, was the question at the title of this post: What is the preferred operator when joining tables: Hash Match, Nested Loops or Merge? While my immediate response to this question is, yes. Meaning, they're all preferred, situationally.…
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Plan Metrics Without the Plan: Trace Flag 7412

SQL Server 2014, SQL Server 2016, SQL Server 2017, TSQL
I place a lot of emphasis on capturing actual execution plans because of the runtime metrics, but with Trace Flag 7412, we don't need the plan. This is great news, because capturing execution plans, even using extended events, is an expensive proposition. However, using either the query_thread_profile event, or, Trace Flag 7412, we can get the runtime metrics without the plan. Trace Flag 7412 Here's how it works.You can either be running the extended event, query_thread_profile (a debug event, but one documented and supported by Microsoft) or, enable the Trace Flag 7412. I like to use the extended event in a targeted fashion to easily see behaviors on a query without having to capture the plan. You can even capture the information and then combine it with an estimated plan…
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Extended Events, the system_health Session, and Waits

SQL Server 2008, SQL Server 2012, SQL Server 2014, SQL Server 2016, SQL Server 2017, TSQL
I advocate for, use, document, teach, and just downright love, Extended Events. They are so much better than the old Trace Events (aka, Profiler) that it's sometimes difficult to keep from just gushing. Let's talk about a common situation that you're going to run into on your servers all the time and how you can put Extended Events to work to help you, without actually doing any work at all. What's that? Be lazy and get rewards? Yes. The Extended Events system_health Session On your servers, any of them that are SQL Server 2008 or newer, right now, unless you've performed actions to prevent this, you're running the Extended Events system_health session. It's just happening, currently, on all your servers. Nothing you need to do about it at all. I'll…
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Query To Retrieve Statistics Data: dm_db_stats_histogram

SQL Server 2017, TSQL
Starting with SQL Server 2016 Sp1 CU2, a new way of directly querying statistics, specifically the histogram, has been introduced: dm_db_stats_histogram. We've always been able to query the statistics using DBCC SHOW_STATISTICS. However, the output, three different result sets with three different structures, made automating access to statistics information a pain. Now, we have more capability through dm_db_stats_histogram. dm_db_stats_histogram To access the information in dm_db_stats_histogram, you just have to pass in the object_id and the statistics_id values for the statistics you're interested in like this: [crayon-5b51bfc0157d9733789430/] It's very straight forward to use. The results look like this: Handy right? Now you can query the histogram directly. Yeah, I hear a few of you saying... and this helps me... how? Here's an example. This query will quickly find the rang_hi_key value…
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Adaptive Joins

TSQL
I was surprised to find out that a lot people hadn't heard about the new join type, Adaptive join. So, I figured I could do a quick overview. Adaptive Join Behavior Currently the adaptive join only works with columnstore indexes, but according to Microsoft, at some point, they will also work with rowstore. The concept is simple. For larger data sets, frequently (but not always, let's not try to cover every possible caveat, it depends, right), a hash join is much faster than a loops join. For smaller data sets, frequently, a loops join is faster. Wouldn't it be nice if we could change the join type, on the fly, so that the most effective join was used depending on the data in the query. Ta-da, enter the adaptive join.…
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Database Fundamentals #7: Create a Table Using T-SQL

Database Fundamentals, TSQL
The syntax for creating a table logically follows many of the same steps that you did when using the GUI, but it will all be done with the statements. This script will exactly replicate everything that you did with the GUI: [crayon-5b51bfc020e2d143066331/] Breaking the script into separate lines, it’s easy to see how the TSQL commands perform the actions defined in the GUI (it also makes it easier to read). The CREATE TABLE statement in this context is self-explanatory.  After that you’re defining the schema and the table name. Within the parenthesis you define each of the columns. First is the name of the column followed by the data type and, for most columns, the ability of the column to store NULL values. The table will be stored on the…
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There Is a Magic Button, a Rant

SQL Server 2005, SQL Server 2008, SQL Server 2012, SQL Server 2014, SQL Server 2016, TSQL
OK guys. I think it's way past time. A bunch of us have been keeping a secret from the rest of you. We know something that you don't. I don't think we should hide this secret from the world any more. Illuminati? Incompetents. Free Masons? I am one, so I already know all those secrets. Bilderbergers, Cthulhu Cultists, MKUltra, New World Order, Rotarians? All of these are nothing compared to the vast conspiracy that I'm about to reveal. We need to just unveil the magic "Run Really Fast" button. We've been keeping that sucker a secret forever. It's been tough. Every so often some unauthorized person almost finds it or a "query tuning expert" (as if that was a real thing) tries to reveal it. But we've kept it secret…
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Database Fundamentals #4: Create a Database

Database Fundamentals, TSQL
SQL Server provides multiple ways to perform most functions. In order to maximize your understanding of how SQL Server works and have as many different mechanisms as possible for getting work done, you’ll use the GUI and TSQL to learn how to create and drop databases. You can then use whichever mechanism works best for you at the moment. Using  each method, we’ll first create a database using the least amount of work possible, just so you can see how easy it is create a database. We’ll go over how to remove databases from the system, getting rid of the database you just created. From there we’ll drill down to create another database, exploring some of the different mechanisms you can use to change how databases get created. Then we’ll…
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The Three T’s of Backups

TSQL
I just read several more horror stories that include, among other things, failed backups. I've said it before (at volume, extreme volume), and evidently I have to say it again. Simply creating a backup file is not enough to ensure the protection of your information. In order to attempt to reinforce the importance of this idea, I'm going to introduce a new concept. Maybe it'll help. I'm calling it "The Three T's of Backups." Take 'Em First, and most important, you have to Take backups. That's your first "T". No, disk redundancy through RAID or a SAN or some other setup is not adequate to protect your information. You must take backups. They have to be created and they have to be run regularly. You absolutely should automate this. Test…
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Does Query Store Pre-Allocate Space

SQL Server 2016, TSQL
I love the questions I get while I'm presenting because they force me to think and learn. The question in the title is one I received recently. The answer, now that I'm not standing in front of people, is easy. Of course the space is not pre-allocated. Query Store tables are just system tables. They have a limit on how big they can grow (100mb by default), but that space isn't going to be pre-allocated in any way. The space will just get used as and when it's needed, just like any other system table. However, don't take my word for it, let's prove that. The Test Testing whether or not enabling Query Store is straight forward. Here's a query that should give us information rather quickly: [crayon-5b51bfc0286f3138097431/] The results come back…
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