Does The Join Order of My Tables Matter?

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I had a great question submitted to me (thank you Brandman!) that I thought would make for a good blog post:

…I’ve been wondering if it really matters from a performance standpoint where I start my queries. For example, if I join from A-B-C, would I be better off starting at table B and then going to A & C?

The short answer: Yes.  And no.

More of a watcher than a reader?  Watch this week’s episode on YouTube!

Table join order matters for performance!

Disclaimer: For this post, I’m only going to be talking about INNER joins.  OUTER (LEFT, RIGHT, FULL, etc…) joins are a whole ‘nother animal that I’ll save for time.

Let’s use the following query from WideWorldImporters for our examples:

Note: with an INNER join, I normally would prefer putting my ‘USA’ filter in the WHERE clause, but for the rest of these examples it’ll be easier to have it part of the ON.

The key thing to notice is that we are joining  three tables – Orders, OrderLines, and StockItems – and that OrderLines is what we use to join between the other two tables.

We basically have two options for table join orders then – we can join Orders with OrderLines first and then join in StockItems, or we can join OrderLines and StockItems first and then join in Orders.

In terms of performance, it’s almost certain that the latter scenario (joining OrderLines with StockItems first) will be faster because StockItems will help us be more selective.

Selective?  Well you might notice that our StockItems table is small with only 227 rows.  It’s made even smaller by filtering on ‘USA’ which reduces it to only 8 rows.

Since the StockItems table has no duplicate rows (it’s a simple lookup table for product information) it is a great table to join with as early as possible since it will reduce the total number of rows getting passed around for the remainder of the query.

If we tried doing the Orders to OrderLines join first, we actually wouldn’t filter out any rows in our first step, cause our subsequent join to StockItems to be more slower (because more rows would have to be processed).

Basically, join order DOES matter because if we can join two tables that will reduce the number of rows needed to be processed by subsequent steps, then our performance will improve.

So if the order that our tables are joined in makes a big difference for performance reasons, SQL Server follows the join order we define right?

SQL Server doesn’t let you choose the join order

SQL is a declarative language: you write code that specifies *what* data to get, not *how* to get it.

Basically, the SQL Server query optimizer takes your SQL query and decides on its own how it thinks it should get the data.

It does this by using precalculated statistics on your table sizes and data contents in order to be able to pick a “good enough” plan quickly.

So even if we rearrange the order of the tables in our FROM statement like this:

Or if we add parentheses:

Or even if we rewrite the tables into subqueries:

SQL Server will interpret and optimize our three separate queries (plus the original one from the top of the page) into the same exact execution plan:

Basically, no matter how we try to redefine the order of our tables in the FROM statement, SQL Server will still do what it thinks it’s best.

But what if SQL Server doesn’t know best?

The majority of the time I see SQL Server doing something inefficient with an execution plan it’s usually due to something wrong with statistics for that table/index.

Statistics are also a whole ‘nother topic for a whole ‘nother day (or month) of blog posts, so to not get too side tracked with this post, I’ll point you to Kimberly Tripp’s introductory blog post on the subject: https://www.sqlskills.com/blogs/kimberly/the-accidental-dba-day-15-of-30-statistics-maintenance/

The key thing to take away is that if SQL Server is generating an execution plan where the order of table joins doesn’t make sense check your statistics first because they are the root cause of many performance problems!

Forcing a join order

So you already checked to see if your statistics are the problem and exhausted all possibilities on that front.  SQL Server isn’t optimizing for the optimal table join order, so what can you do?

Row goals

If SQL Server isn’t behaving and I need to force a table join order, my preferred way is to do it via a TOP() command.

I learned this technique from watching Adam Machanic’s fantastic presentation on the subject and I highly recommend you watch it.

Since in our example query SQL Server is already joining the tables in the most efficient order, let’s force an inefficient join by joining Orders with OrderLines first.

Basically, we write a subquery around the tables we want to join together first and make sure to include a TOP clause. 

Including TOP forces SQL to perform the join between Orders and OrderLines first – inefficient in this example, but a great success in being able to control what SQL Server does.

This is my favorite way of forcing a join order because we get to inject control over the join order of two specific tables in this case (Orders and OrderLines) but SQL Server will still use its own judgement in how any remaining tables should be joined.

While forcing a join order is generally a bad idea (what happens if the underlying data changes in the future and your forced join no longer is the best option), in certain scenarios where its required the TOP technique will cause the least amount of performance problems (since SQL still gets to decide what happens with the rest of the tables).

The same can’t be said if using hints…

Query and join hints

Query and join hints will successfully force the order of the table joins in your query, however they have significant draw backs.

Let’s look at the FORCE ORDER query hint.  Adding it to your query will successfully force the table joins to occur in the order that they are listed:

Looking at the execution plan we can see that Orders and OrderLines were joined together first as expected:

The biggest drawback with the FORCE ORDER hint is that all tables in your query are going to have their join order forced (not evident in this example…but imagine we were joining 4 or 5 tables in total).

This makes your query incredibly fragile; if the underlying data changes in the future, you could be forcing multiple inefficient join orders.  Your query that you tuned with FORCE ORDER could go from running in seconds to minutes or hours.

The same problem exists with using a join hints:

Using the LOOP hint successfully forces our join order again, but once again the join order of all of our tables becomes fixed:

A join hint is probably the most fragile hint that forces table join order because not only is it forcing the join order, but it’s also forcing the algorithm used to perform the join.

In general, I only use query hints to force table join order as a temporary fix.

Maybe production has a problem and I need to get things running again; a query or join hint may be the quickest way to fix the immediate issue.  However, long term using the hint is probably a bad idea, so after the immediate fires are put out I will go back and try to determine the root cause of the performance problem.

Summary

  • Table join order matters for reducing the number of rows that the rest of the query needs to process.
  • By default SQL Server gives you no control over the join order – it uses statistics and the query optimizer to pick what it thinks is a good join order.
  • Most of the time, the query optimizer does a great job at picking efficient join orders.  When it doesn’t, the first thing I do is check to see the health of my statistics and figure out if it’s picking a sub-optimal plan because of that.
  • If I am in a special scenario and I truly do need to force a join order, I’ll use the TOP clause to force a join order since it only forces the order of a single join.
  • In an emergency “production-servers-are-on-fire” scenario, I might use a query or join hint to immediately fix a performance issue and go back to implement a better solution once things calm down.

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How to Search Stored Procedures and Ad-Hoc Queries

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Have you ever wanted to find something that was referenced in the body of a SQL query?

Maybe you need to know what queries you will have to modify for an upcoming table rename.  Or maybe you want to see how many queries on your server are running  SELECT *

Below are two templates you can use to search across the text of SQL queries on your server.

Prefer learning by watching?  Check out this week’s post on YouTube.

1. Searching Stored Procedures, Functions, and Views

If the queries you are interested in are part of a stored procedure, function, or view, then you have to look no further than the  sys.sql_modules view.

This view stores the query text of every module in your database, along with a number of other properties.

You can use something like the following as a template for searching through the query texts of these database objects:

For example, I recently built a query for searching stored procedures and functions that might contain SQL injection vulnerabilities.

Using the starting template above, I added some filtering in the WHERE clause to limit my search to queries that follow common coding patterns that are vulnerable to SQL injection:

2. Searching Ad-Hoc SQL Queries

Searching across ad-hoc queries is a little tougher.  Unless you are proactively logging the query texts with extended events or some other tool, there is no way to definitively search every ad-hoc query text.

However, SQL Server does create (or reuse) an execution plan for each query that executes.  Most of those plans are then added to the execution plan cache.

Execution plans are eventually removed from the cache for various reasons, but while they exist we can easily search their contents, including searching through that plan’s query text.

As a starting point, you can use the following code to retrieve SQL query texts that are currently stored in the plan cache:

Although the template above searches for the query texts in our execution plans, you can also use it to search for other query plan elements, such as elements that indicate if you have non-sargable query.

I used this technique recently to search for ad-hoc queries that might be vulnerable to SQL injection.  I modified the template above to search the input parameter values instead of the query texts, flagging any values that look like they might have some injection code in them:

So while using this technique won’t allow you to search across 100% of ad-hoc queries, it should be able to search the ones that run most frequently and appear in your plan cache.

Intrigued by how I’m searching query texts for SQL injection vulnerabilities? Attend my online webcast on Tuesday November 14, 2017 at 1PM Eastern at the PASS Security Virtual Group to learn about these queries and how protect yourself from SQL injection.

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4 Common Misconceptions About SQL Injection Attacks

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Interested in learning more about SQL injection attacks, including how to prevent them?  Attend my online webcast on Tuesday November 14, 2017 at 1PM Eastern at the PASS Security Virtual Group.

SQL injection continues to be one of the biggest security risks that we face as database professionals.

Every year, millions of users’ personal information is leaked due to poorly written queries exploited by SQL injection.  The sad truth is that SQL injection is completely preventable with the right knowledge.

My goal today is to cover four common misconceptions that people have about SQL injection in an effort to dissuade any illusions that an injection attack is not something that can happen to you.

Prefer watching me get angry about these common misconceptions?  You can watch this content on my YouTube channel.

1. “My database information isn’t public”

Let’s see, without me knowing anything about your databases, I’m guessing you might have some tables with names like:

  • Users
  • Inventory
  • Products
  • Sales
  • etc…

Any of those sound familiar?

You might not be publicly publishing your database object names, but that doesn’t mean they aren’t easy to guess.

All a malicious user needs is a list of common database table names and they can iterate over the ones they are interested in until they find the ones that match in your system.

2. “But I obfuscate all of my table and column names!”

Oh jeez.  I hope you don’t do this.

Some people do this for job security (“since only I can understand my naming conventions, I’m guaranteeing myself a job!”) and that’s a terrible reason in and of itself.

Doing it for security reasons is just as horrible though.  Why?  Well, have you ever heard of some system tables like sys.objects and sys.columns?

A hacker wanting to get into your system can easily write queries like the ones above, revealing your “secure” naming conventions.

Security through obscurity doesn’t work.  If you have table names that aren’t common, that’s perfectly fine, but don’t use that as your only form of prevention.

3. “Injection is the developer’s/dba’s/somebody else’s problem”

You’re exactly right.  SQL injection is a problem that should be tackled by the developer/dba/other person.

But it’s also a problem that benefits from multiple layers of security, meaning it’s your problem to solve as well.

Preventing sql injection is hard.

Developers should be validating, sanitizing, parameterizing etc…  DBAs should be parameterizing, sanitizing, restricting access, etc..

Multiple layers of security in the app and in the database are the only way to confidently prevent an injection attack.

4. “I’m too small of a fish in a big pond – no one would go out of their way to attack me”

So you run a niche business making and selling bespoke garden gnomes.

You only have a few dozen/hundred customers, so who would bother trying to steal your data with SQL injection?

Well, most SQL injection attacks can be completely automated with tools like sqlmap.  Someone might not care about your business enough to handcraft some SQL injection code, but that won’t stop them from stealing your handcrafted garden gnome customers’ data through automated means.

No app, big or small, is protected from the wrath of automated SQL injection tooling.

Interested in learning more about SQL injection attacks, including how to prevent them?  Attend my online webcast on Tuesday November 14, 2017 at 1PM Eastern at the PASS Security Virtual Group.

Thanks for reading. You might also enjoy following me on Twitter.

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The Quickest Way To Get SQL Command Help

Formula One …. F1 …. Photo by Jp Valery on Unsplash

Every once in a while I discover a SQL Server Management Studio trick that’s apparently been around forever but is completely new to me.

Today I want to point out one of those features that had me thinking “how did I not know about this before”:

The F1 keyboard shortcut.

Prefer video?  Watch this week’s tip on my Youtube channel.

To use it, highlight a command or function that you want to know more information about and then press F1.  Simple as that.

Pressing F1 brings up the Microsoft online documentation for that keyword/function, making it the fastest way of getting to Microsoft’s online documentation.  You’ll solve your own questions faster than a coworker can tell you “to google it.”
Most recently I’ve been using the F1 shortcut in the following scenarios:
  • Can’t remember the date/time style formats when using CONVERT?  Highlight CONVERT and press F1: BOOM! All date and time style codes appear before you.
  • Need to use some option for CREATE INDEX and don’t remember the syntax?  Just highlight CREATE INDEX and press F1!  Everything you need is there.
  • Do you remember if BETWEEN is inclusive or exclusive?  F1 knows.  Just press it.

You get the idea.

Assuming you use the online Microsoft docs 10 times per day, 250 days a year, and each time it takes you 10 seconds to open a browser and search for the doc…

( 10/day * 250/year * 10 sec ) / 60 sec / 60 min = 6.94 hours saved.  Your welcome.

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10 Questions To Spark Conversation At Your Next SQL Event

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Here’s a word for word transcription of a conversation I’ve had a hundred times over:

“Hi I’m Bert.  What do you do?”

“I’m ____ and I’m a SQL developer.”

“That’s cool, me too.”

*crickets*

*I look down at phone because I don’t know what to talk about*

Sound familiar?

In the next few weeks, you might find yourself at a conference like PASS Summit or SQLintersection.  If not a conference, then maybe a local user group, meetup, or SQL Saturday.

Inevitably you will find yourself surrounded by strangers.  Strangers who you know share common interests with you (SQL, duh!).

But if you are like me, starting a meaningful conversation with those strangers can be uncomfortable.  Most people have interesting stories to share, the challenge is to get them to talk about them.

The good news is that I’ve developed an easy way to get a conversation started with the people you just met:

Come prepared with interesting open-ended questions.

Prefer watching on YouTube?  Go ahead!  Otherwise, keep reading below.

I keep a memorized list of open-ended questions that I can ask whenever I don’t know how to keep the conversation going.  Try asking any of these questions the next time you don’t know what to say (and reciprocate by sharing your own fun story); I guarantee these will spark some interesting conversations.

1. “What’s your best SQL Server war story?”

We’ve all been in the trenches and have had to play the hero.

2. “What are your thoughts on EntityFramework/ORMs?”

If you ever want to get a table full of SQL DBAs going, this will do it.

3. “What’s the oldest version of SQL Server you are still stuck supporting?”

Although this one elicits a one-word response, the next easy follow-up is “why/how!?”

4. “What was your biggest “oops” moment?”

Backups were corrupt?  Yeahhhhh….

5. “What’s the most recent feature you started using in SQL Server 2014/2016/2017? How is it?”

I love hearing people’s answers to this because it’s a good way to figure out what new features really add value and which ones are over-hyped/limited in functionality.

6. “Are you using <feature you are interested in learning>?  How is it?”

Similar to #5, this is a great way to get real-world feedback about certain features.

7. “What’s your favorite session from today/this week?  What did you like most about it?”

I love finding out what sessions other people found useful – once again, real world reviews on what I should check out in the future.

8. “Have you been to <city> before? Do you have any recommendations for what I should do/see/eat?”

Great way to get to know the surrounding area without having to read reviews online.

9. “Do you use PowerShell or any other software to automate/do dev ops?”

PowerShell is the future.  Start learning how others are incorporating it into their environments, what struggles they’ve had implementing automated processes, etc…

10. “Are there any other events going on tonight?”

Especially great if talking to people who have attended the event before.  Find out what’s worth going to, if it’s better to show up early or late, is there a “best seat” in the house, etc…

I hope this list of questions encourages you to become better acquainted with your fellow conference goers.  And if I see you at PASS Summit…don’t be surprised if you hear me ask you one of these questions!

Thanks for reading. You might also enjoy following me on Twitter.

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How to Make SQL Server Act Like A Human By Using WAITFOR

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You probably tune your queries for maximum performance.  You take pride in knowing how to add indexes and refactor code in order to squeeze out every last drop your server’s performance potential.  Speed is usually king.

That’s why you probably don’t use SQL Server’s WAITFOR command regularly – it actually makes your overall query run slower.

However, slowness isn’t always a bad thing.  Today I want to show you two of my favorite ways for using the WAITFOR command.

You can also watch this week’s content on my YouTube channel.

1. Building A Human

Modern day computers are fast.  CPUs perform billions of actions per second, the amount of RAM manufactures can cram onto a stick increases regularly, and SSDs are quickly making disk I/O concerns a thing of the past.

While all of those things are great for processing large workloads, they move computers further and further away from “human speed”.

But “human speed” is sometimes what you want.  Maybe you want to simulate app usage on your database or the load created by analysts running ad hoc queries against your server.

This is where I love using WAITFOR DELAY – it can simulate humans executing queries extremely welll:

Throw in some psuedo-random number generation and some IF statements, and you have a fake server load you can start using:

2. Poor Man’s Service Broker

Service Broker is a great feature in SQL Server.  It handles messaging and queuing scenarios really well, but requires more setup time so I usually don’t like using it in scenarios where I need something quick and dirty.

Instead of having to set up Service Broker to know when some data is available or a process is ready to be kicked off, I can do the same with a WHILE loop and a WAITFOR:

Fancy? No.  Practical? Yes.

No longer do I need to keep checking a table for results before I run a query – I can have WAITFOR do that for me.

If you know there is a specific time you want to wait for until you start pinging some process, you can incorporate WAITFOR TIME to make your checking even more intelligent:

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How NOLOCK Will Block Your Queries

lock
Photo by James Sutton on Unsplash

Note: the problem described below applies to all SELECT queries, not just those adorned with NOLOCK hints.  The fact that it applies to NOLOCK queries was a huge surprise to me though, hence the title.

Lots of people don’t like NOLOCK (i.e. the read uncommitted isolation level) because it can return inaccurate data.  I’ve seen plenty of arguments cautioning developers from retrieving uncommitted reads because of how they can return dirty data, phantom reads, and non-repeatable reads.

I’ve known about all of those above problems, but there’s one problem that I’ve never heard of until recently: NOLOCK can block other queries from running.

Watch this week’s post on YouTube

Let’s step back and understand why I’ve so often used NOLOCK in the past.  A fairly typical instance of when I use NOLOCK is when I want to let a query run overnight to return some large set of data.  I’m okay with some inconsistencies in the data (from dirty reads, etc…).  My primary concern is that I don’t want the long running query to get in the way of other processes.

I always thought NOLOCK was a perfect solution for this scenario because it never locks the data that it reads – the results might not be perfect, but at least the query won’t negatively impact any other process on the server.

This is where my understanding of NOLOCK was wrong: while NOLOCK won’t lock row level data, it will take out a schema stability lock.

A schema stability (Sch-S) lock prevents the structure of a table from changing while the query is executing.  All SELECT statements, including those in the read uncommitted/NOLOCK isolation level, take out a Sch-S lock.  This makes sense because we wouldn’t want to start reading data from a table and then have the column structure change half way through the data retrieval.

However, this also means there might be some operations that get blocked by a Sch-S lock.  For example, any command requesting a schema modification (Sch-M) lock gets blocked in this scenario.

What commands request Sch-M locks?

Things like an index REBUILD or sp_recompile table.  These are the types of commands running in my nightly maintenance jobs that I was trying to avoid hurting by using NOLOCK in the first place!

To reiterate, I used to think that using the NOLOCK hint was a great way to prevent blocking during long running queries.  However, it turns out that my NOLOCK queries were actually blocking my nightly index jobs (all SELECT queries block in this example, but I find the NOLOCK to be particularly misleading), which then caused other SELECT statements to get blocked too!

Let’s take a look at this in action.  Here I have a query that creates a database, table, and then runs a long running query with NOLOCK:

Now, while that billion row read is occurring, we can verify that the query took out a Sch-S lock by looking at sys.dm_tran_locks:

Sch-S lock granted

While that’s running, if we try to rebuild an index, that rebuild is blocked (shown as a WAIT):

rebuild is blocked

Our index rebuild query will remain blocked until our billion row NOLOCK SELECT query finishes running (or is killed).  This means the query that I intended to be completely unobtrusive is now blocking my nightly index maintenance job from running.

Even worse, any other queries that try to run after the REBUILD query (or any other commands that request a Sch-M lock) are going to get blocked as well!  If I try to run a simple COUNT(*) query:

chained blocks

Blocked!  This means that not only is my initial NOLOCK query causing my index REBUILD maintenance jobs to wait, the Sch-M lock placed by the REBUILD maintenance job is causing any subsequent queries on that table to get blocked and be forced to wait as well.  I just derailed the timeliness of my maintenance job and subsequent queries with a blocking NOLOCK statement!

Solutions

Unfortunately this is a tough problem and there’s no one-size-fits-all remedy.

Solution #1: Don’t run long running queries

I could avoid running long queries at night when they might run into my index maintenance jobs.  This would prevent those index maintenance jobs and subsequent queries from getting delayed, but it means my initial billion row select query would then have to run earlier, negatively impacting server performance during a potentially busier time of day.

Solution #2: Use WAIT_AT_LOW_PRIORITY

Starting in 2014, I could do an online index rebuild with the WAIT_AT_LOW_PRIORITY option set:

This query basically gives any blocking SELECT queries currently running 1 minute to finish executing or else this query will kill them and then execute the index rebuild.  Alternatively we could have also set ABORT_AFTER_WAIT = SELF and the rebuild query would kill itself, allowing the NOLOCK billion row SELECT to finish running and not preventing any other queries from running.

This is not a great solution because it means either the long running query gets killed or the index REBUILD gets killed.

Solution #3: REBUILD if no Sch-S, REORGANIZE otherwise

A programmatic solution can be written that tries to REBUILD the index, but falls back to REORGANIZE if it knows it will have to wait for a Sch-M lock.

I’ve created the boiler plate below as a starting point, but the sky is the limit with what you can do with it (e.g. create a WHILE loop to check for the lock every x seconds, create a timeout for when the script should stop trying to REBUILD and just REORGANIZE instead, etc…)

This solution is my favorite because:

  1. Ad hoc long running queries don’t get killed (all of that time spent processing doesn’t go to waste)
  2. Other select queries are not blocked by the Sch-M lock attempt by REBUILD
  3. Index maintenance still occurs, even if it ends up being a REORGANIZE instead of a REBUILD

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3 Tips You Need To Know When Using PowerShell with SQL Server

Watch this week’s interview with Drew on YouTube.

Have you ever had to perform repetitive tasks in SQL Server?

Maybe you’ve had to manually verify backups, script out all of a server’s logins/groups/permissions, or refresh a dev environment with data.  With PowerShell, you can automate all of these manual tasks…and more!

This week I had the opportunity to interview PowerShell expert Drew Furgiuele and learn his three favorite tips for using PowerShell with SQL Server.

Whether you are just getting started with PowerShell or have already written some automation scripts, you’ll want to be sure you are following Drew’s advice.

So if you haven’t already, go grab the SqlServer module and get busy scripting in PowerShell today!

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Clustered vs Nonclustered: Index Fundamentals You Need To Know

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How many times have you known that adding an index would improve query performance but you weren’t exactly sure which type of index to add?

This happened to me all the time in my first few years (and maybe an extra year or two after that) of working with SQL Server.

Today I want to help alleviate some of that confusion by comparing two of the most common index types: clustered and nonclustered rowstore indexes.

Watch the this post on YouTube, or continue reading below if that’s more your style.

Clustered Indexes

Every table’s data has some natural order to it.

If the order is random and not explicitly defined then that table is known as a heap.  With the exception of a few special cases, we generally don’t want to have heaps.  Heaps don’t perform well for the majority of queries becauase SQL Server has no meta knowledge about where data is stored within a heap.

If we don’t have a random heap, that means we have defined the order that data should be stored in a table.  The physical storage order of our data is defined by our clustered index.

Every table can have exactly one clustered index because the data in a table can only be stored in one order i.e. you can’t have that table’s data physically stored on the disk in more than one order.

What are the benefits of a clustered index?

The data in a clustered index is stored in order.  That means:

  1. Finding the data you need in your clustered index is a matter of knowing where to look in our alphabetical list of data.  Computers are really good at doing this.
  2. If your data needs to be outputted in the same order that it’s stored in – presto! – SQL doesn’t need to do any additional sorting.

A lot of people like to put the clustered index on their table’s primary key (PK).  This is usually fine because a lot of the time our primary key is likely to be our most used field for joins, where statements, etc…

Some people think they can ONLY put their clustered index on their PK.  That’s not true! Often times it can be much more beneficial to put your clustered index on something that isn’t your PK, like a different column that is getting more use than our PK.  For example, you might have an identity column set as your PK, but every query against your table is filtering and sorting on a datetime2 column.  Why store your table in PK order if you are always going to be filtering and returning data on that datetime2 column?  Put that clustered index on the datetime2 column!

The downside to having data stored in this order is that actions like inserts and updates take long because SQL has to put them into the correct sorted order of the table pages – it can’t just quickly tack them onto the end.

Another major benefit of a clustered index is that we don’t have to “include” any additional data in our index.  All of the data for our row exists right beside our indexed columns.  This is not necessarily true of other index types (see nonclustered indexes below).

Pretend our clustered index is like the white pages of a phone book (note to future SQL developers in 2030 who have no idea what a phonebook is: it’s something that stores the names, addresses, and landline phone numbers in your area.  What’s a landline?  Oh boy…)

The phone book stores every person’s name in alphabetical order, making it easy to look up certain individuals.  Additionally, if we look someone up, we immediately have their address and phone number right their next to their name – no additional searching necessary!

This is a great feature of clustered indexes – if you ever need to retrieve many or all columns from your table, a clustered index will usually be efficient because once it finds the indexed value you are searching on, it doesn’t need to go anywhere else to get the remaining data from that row.

Nonclustered Indexes

If a clustered index is like a phone book, a nonclustered index is like the index in the back of a chemistry text book.  The chemistry text book has some natural order to it (“Chapter 1: Matter, Chapter 2: Elements, Chapter 3: Compounds, etc…”).  However, this order doesn’t help us if we want to look up the location of something specific, like “noble gases”.

So what do we do?  We go to the index in the back of the textbook which lists all topics in alphabetical order, making it easy to find the listing for “noble gases” and the page number they are discussed on.  Once we know the page number for noble gases from our index, we can flip to the correct page and get the data we need.

This book index represents our nonclustered index.  A nonclustered index contains the ordered data for the columns specified in that index, with pointers (book page numbers) that tell us where to go to find the rest of the data from that row (flip to the right book page).  That means unlike a clustered index where all data is always present, using a nonclustered index often is a two step process: find the value of interest in the index and then go look up the rest of that row’s data from where it actually exists on disk.

What are the benefits of a nonclustered index?

We can have as many nonclustered indexes on our tables as we want (well, we max out at 999).  That’s great! Create an index for every column!

Well, no, don’t do that.  There’s overhead in creating nonclustered indexes.  Essentially, every time you index some column(s), you are duplicating the unique values in those column(s) so that they can be stored in sorted order in your index.  We get speed and efficiency in our data lookups, but with the cost of losing disk space.  You need to test and see for each table and set of queries what the optimal number of indexes is.  Adding an additional index can absolutely destroy performance, so always test your changes!

Additionally, using a nonclustered index to find an indexed column’s value is fast (SQL is just going through the ordered index data to find the value it needs – once again, something computers are really good at doing).  However, if you need other columns of data from the row that you just looked up, SQL is going to have to use those index pointers to go find the rest of that row data somewhere else on disk.  This can really add up and slow down performance.

If those additional lookups are hurting performance, what you can do is INCLUDE your nonindexed columns in your nonclustered index.  What this basically does is in addition to storing the sorted values of your indexed column(s), the index will also store whatever additional values you want to include as part of the index itself.  Once again, you’ll probably get better performance because SQL won’t have to go to somewhere else on disk to find the data it needs, but you lose storage space because you are creating duplicates of that data as part of your index.

Example Usage Scenarios

Note: I want to clarify that the above definitions and below examples don’t cover lots of corner cases (blob values, fragmentation, etc…).  I wanted this post to be a simple starting point when people don’t know what index type they should try adding first, because this was the paralysis that I had when starting out.

Every statement in this article can probably have an asterisk appended to the end of it, pointing out some example where a recommendation I wrote is 100% wrong.  ALWAYS test your index changes, because what might improve one query may hurt another one already running on that table, and over time you will learn about all of those edge cases and how they affect index performance.

Alright let’s take a look at a few common scenarios and what the best index for them might be.  After reading each scenario, take a guess about what kind of index you would add and then click on the answer to reveal what I would do in that scenario.  Assume no indexes exist yet on these tables unless otherwise noted.

  • You have OLTP data that’s used only for transactional reads and writing new rows. You know the primary key is an identity integer column.  What type of index would you create for the primary key?
    Answer

    Clustered index – Your queries are probably always going to be looking up by PK to return data.  If you store the data in the table ordered by that PK, SQL will be able to do this very quickly.  New row additions to the table will always get put at the end because of the auto-incrementing identity column, not creating any overhead for having to insert data in a specific location in the ordered data.

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  • You have a query that wants to return most or all of the columns from a table.  What type of index would make this the most efficient?
    Answer

    Clustered index – Since all of the column values are stored in the same location as your indexed fields, SQL won’t have to go do any additional lookups to get all of the data you are requesting from it.  If you created a nonclustered index you would have to INCLUDE all nonindexed columns, which would take up lots of space since you are essentially duplicating your entire table’s data.

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  • You have a table that is constantly having values updated.  These updated values are used as in your JOINs and WHERE clauses.  What type of index would you add?
    Answer

    Nonclustered index – If our values are constantly changing, SQL only has to update the index and pointers while putting the actual data wherever it has available space on disk.  Compare this to a clustered index where it has to put the inserted/updated data in the correct order, meaning potentially lots of operations to shift the data around if available free space doesn’t exist at that location.

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  • You have a table that already has a clustered index, but it doesn’t cover columns in JOINs and WHERE clauses.  What do you do?
    Answer

    Nonclustered index – since the clustered index already exists, your only option is to add a nonclustered index.  Depending on the queries hitting this table however, you may want to consider changing your clustered index to a nonclustered index if you think your JOINs and WHERE clauses will be improved by having those fields be part of the clustered index.  Test it out!

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  • You have a small staging table that you will always read all rows from and then truncate.  You don’t care about the order.  Do you add an index?
    Answer

    No, leave it as a heap – This is one scenario where not adding an index can give you better performance since there is no overhead in SQL having to store things in a sorted order or update indexes to specify the order.  If you truly don’t care about the order, and you will always be reading all rows from a table and then truncating the table, then it’s better not to have the overhead of having indexes on the table.

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How To Make Your Queries Perform Like They Used To

Photo by Chad Kirchoff on Unsplash

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In the ideal world, you fully test how your SQL Server will handle upgrading to the latest version.  You’re able to catch and mitigate any performance surprises early on.

In the real world, you might not be able to perform a good test before upgrading.  You only learn about poor performance once you upgrade in production.

Does this scenario sound familiar?  It’s what I’ve been thinking about recently as I have to deal with query performance issues after upgrading to SQL Server 2016.

Coming from SQL Server 2012, I know the new cardnality estimator added in 2014 is a major source of the problems I’m experiencing.  While the new cardinality estimator improves performance of some queries, it has also made made some of my queries take hours to run instead of seconds.

Long-term, the solution is to revisit these queries, their stats, and the execution plans being generated for them to see what can be rewritten for better performance.

But ain’t nobody got time for that (at least when facing performance crippling production issues).

Short-term, put a band-aid on to stop the bleeding.

I could change the compatibility mode of the database to revert back to SQL Server 2012 (before the new cardinality estimator was introduced), but then I miss out on being able to use all of the new SQL Server 2016 improvements just because a few queries are misbehaving.

I could enable trace flag 9481 to have my queries use the old cardinality estimator, however as a developer I probably don’t have access to play with trace flags (and for good reason).

Starting with 2016 SP1, what I can do is use the legacy cardinality estimator query hint:

This hint is great because it doesn’t require developers to have any special permissions.  It also allows SQL to use the old cardinality estimator for poor performing queries only – the rest of the server gets to benefit from the improvements brought on by the new cardinality estimator.

With time, I can revisit the queries that are using this hint to try to improve their performance, but in the mean time it’s a great way to fix regressed query performance due to the new cardinality estimator.

Thanks for reading. You might also enjoy following me on Twitter.

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