Well, it's been a crazy few weeks! Paul and I have been booked solid with client engagements, Immersion Events, conferences and other things (like chapter editing for the upcoming SQL Server Deep Dives Volume 2) and so I've been a bit behind with blogging. I have a ToDo list of items to blog about but I've got one that's fairly urgent. I'll mention a few issues here but above all - don't trust anyone.

OK, I know that sounds a bit paranoid. Not meaning to be but this is a post that I'm adding to my "Just because" series as well as my "Are you kidding me?" series. That's how bad this is... almost as bad as these "stupid bacon related tatoos" - really, someone has this (forever!):

As a DBA (and/or SQL Architect/Database Designer/Developer) we're often tasked with making changes to databases and ideally, we want to use tools to make it easier. While I do agree that many tools help tremendously, I've been shocked lately by a few that have done the wrong thing. Paul blogged about a third party tool that didn't analyze indexes correctly here: Beware of advice from 3rd-party tools around dropping indexes (ultimately, the recommendations for what to drop - were wrong). And, recently, I've been shocked to learn that some of the schema change options in the database designer (in SSMS) are less than optimal when changing index definitions (especially those that handle changes to the clustering key).

Specifically, we had a customer that was doing everything right by testing the changes on a secondary system but SSMS was doing everything wrong (the order in which it made the changes and how it made the changes - were horrible). The end result is that if you're going to make schema changes, you really need to get more insight into what the application is doing. And, many tools have an option to script out the changes rather than execute them. So, my "quick tip" is to do that and see what's going on.

And, if you're going to make schema changes in ANY tool - then work through what those changes are going to do. Most important - consider re-arranging the SQL (especially wrt indexes). If you're ever changing the CL index you need to do the following:

1. Drop nonclustered indexes first
2. Drop the clustered next
   • IMPORTANT NOTES: If you're completely changing it you definitely need to drop the nonclustered and then drop the CL. BUT, if you're keeping the name and only changing the definition (and it's NOT a constraint-based index) then you can use WITH DROP_EXISTING. If you use DROP_EXISTING you do NOT need to drop the nonclustered indexes first.
3. Create the new clustered
4. Recreate the nonclustered indexes

Some of the weird things that we've seen:

1. Some changes to table structure (changing from a heap to a clustered or a clustered to a heap) cause disabled nonclustered to get rebuilt (and re-enabled). That's not even SSMS - that's a general SQL Server problem. In my opinion this means that disabling indexes is somewhat broken. So... gotta use drop/create (which is a bummer!).
2. Changes to the clustering key in SSMS are sometimes done by dropping the clustered and then recreating the clustered. This will cause your nonclustered indexes to be rebuilt TWICE!
3. If you have col1 that's currently clustered and col2 is currently nonclustered but you want to swap them... you can do that in the UI and then save but here's what they do:
   1. They drop the nonclustered
   2. They drop the clustered (this causes all of the remaining nonclustered to get rebuilt)
   3. They create the nonclustered
   4. They create the clustered (this causes all of the remaining AND the new nonclustered to get rebuilt... again)

This is only a short list of some of the crazy things that we've seen. I'm working on a more comprehensive overview of all of these things but I wanted to get a few of them out there. BEWARE OF THE TOOLS and always use scripting and testing to make sure that things are doing what you think. If you're EVER going to make changes to a critical system - this is exactly what I'd do:

1. Script out *JUST* the objects and their definitions from the production environment
2. Take *just* the schema and then go through and make your changes in the designer. Instead of saving the changes (which would immediately implement them), script out the changes and review the SQL.
3. Run through the SQL and see what it does (but, this is an empty database so time isn't going to be as much of a factor here). If you think there's something wrong - ask around (colleagues/twitter/forums)...
4. Then, once you feel you have a good version of the script THEN backup/restore the production database to your test system (I hope that you have this??) and THEN see how long it takes. If you have new concerns then ask around again!
5. Next, before you consider if for production - you need to thoroughly test your applications. Are they affected by these changes?
6. Then, and only when you've thoroughly tested it - you can consider it for production.

Picky yes... surprises NO. On a critical system you CANNOT afford surprises that create downtime - or worse, data loss.

Thanks for reading!!
kt

OK, I've talked about the clustering key many times. Here, I thought I'd bring together one final series of details (and links) to help you really understand why I'm so adamant about DESIGNING a clustering key and not just letting SQL Server pick it for you (for example when it defaults to making the primary key clustered). Just because SQL Server defaults to making the primary key clustered doesn't actually mean it's a GOOD clustering key!

The key things that I've always recommended about a GOOD clustering key is that it is: unique, narrow, static and ever-increasing. For more details on the reasons behind this, check out these posts:

• Ever-increasing clustering key - the Clustered Index Debate..........again!
• The Clustered Index Debate Continues...
• GUIDs as PRIMARY KEYs and/or the clustering key
• Disk space is cheap...
• How much does that key cost?
  

And, today, there are two additional items that I want to add to this list: your clustering key should be non-nullable and fixed-width!

First, why non-nullable?

In a regular data row you will ALWAYS have a null block. This is 2 bytes for a column count (in that row) and 1 bit per column (to store the actual null values). If you want to get more details on the internals of a data row, see Paul's post: Inside the Storage Engine: Anatomy of a record. However, the btree of the clustered index and the nonclustered index leaf/non-leaf levels do NOT have to have this "null block" of information if the columns in the index do NOT allow nulls. While this may be only 3 or 4 bytes (depending on the number of columns), it's still 3 or 4 bytes that would have to be added to EVERY nonclustered index's leaf level (for every row). And, it just doesn't have to be there. Why waste space when you don't have to!

Second, and along the same lines, is the need to use fixed-width columns!

Just like in data rows, index rows only have a variable block (offsets + end of row) when there are variable-width columns. If a clustered index is chosen that has one or more variable-width columns then you are adding at least 4 bytes to every row that might not have otherwise needed to be there. In fact, often people forget that uniquifiers are stored in the variable block portion of the row and as a result, a uniquifier really needs 8 bytes (4 bytes for the int + 2 bytes in the variable block for the offset + 2 bytes for the end of row marker).

To show you this, I've created four tables each with the same 3 columns DATA TYPES (table 4 has col1 as an int that's nullable and you can't do that with an identity so I used INSERT/SELECT to copy the data over):

col1 int identity not null,
col2 datetime2(7) not null default sysdatetime(),
col3 datetime2(7) null default sysdatetime()

And, each of the tables has these four nonclustered indexes:

• Non-unique nonclustered on col2
• Unique nonclustered on col2
• Non-unique nonclustered on col3
• Unique nonclustered on col3

The difference is solely within the definition of the clustering key as well as weather or not the values are unique (or nullable)!

CLTable1 is clustered and non-nullable but NOT defined as unique (the values are unique)

CLTable2 is clustered and non-nullable but there are duplicate values in the clustering key

CLTable3 is defined as a unique clustered index

CLTable4 is clustered, nullable and there are duplicate values in the clustering key

The end result is the sizes of all of the keys and the wasted space from different keys!

For CLTable1, CLTable2 and CLTable3 - all of the minimums are the same. For CLTable1 and CLTable3 - the maximums are the same as the minimums. In CLTable3 it's because the records MUST be unique. In CLTable1 it's because the records just happen to be unique right now (meaning there's no overhead unless there are actual dupes).

For CLTable2 the maximums are each 8 bytes larger (highlighted in yellow) EXCEPT where in the btrees of UNIQUE nonclustered indexes where SQL Server does NOT need to duplicate the clustering key (pale pink):

NOTE: If you're not familar with index internals and when/why SQL Server duplicates the clustering key in nonclustered indexes, check out my Index Internals chapter from the SQL Server 2008 Internals title (here's the Amazon link: http://www.amazon.com/Microsoft-SQL-Server-2008-Internals/dp/0735626243/ref=sr_1_1?ie=UTF8&qid=1291001371&sr=8-1) and my companion content from Chapter 6 here: Companion content for Chapter 6 (Index Internals) of SQL Server 2008 Internals. And, if you really want to see what's in your indexes, check out my updated versions of sp_helpindex here: http://www.sqlskills.com/BLOGS/KIMBERLY/category/sp_helpindex-rewrites.aspx.

Finally, CLTable4 has the most interesting results...

Nonclustered indexes 2 and 3 (and the clustered index's btree) are all 3 bytes larger... why? Because the clustering key allows nulls and each of these structures has only 2-3 columns. As a result, the space needed for the null block is 3 bytes (2 bytes for the column count [NCol] and 1 byte for the actual null bitmap [less than 9 columns]).

The btree for index id 3 only needs the nonclustered key column (col2) which does not allow NULLs and the index is unique so the tree is the same as before.

Nonclustered indexes 4 and 5 already had a nullable column and therefore already had a null block.

My point - it all adds up and if it's not truly necessary, then avoid it!

Thanks for reading!
kt

. 

THAT'S NOT THE POINT!!!

. 

The simple point is that bad (or sloppy/lazy) design cannot be tuned. If you think that data type choice, nullability, keys - don't really matter - you won't scale. It is possible that you may completely fail because of this. Have you ever heard (or possibly said?), let's just get this done - we'll worry about performance later? If you haven't heard it, I'm surprised! I hear this all the time...

Yesterday I gave a lecture at SQLPASS about GUIDs. It wasn't the most well attended (under 200 people) but I suspect that's because of two things: first, our good friend Bob Ward was speaking at the same time (and there were actually a bunch of really good sessions!) AND the simple fact that GUIDs aren't sexy (I agree!). Also, I think that a few folks may have presumed that what I was going to talk about (maybe even solely talk about?) was fragmentation. And, while fragmentation is the most outwardly visable problem with GUIDs - it's by NO MEANS the only problem. And, so I thought I'd blog a few things to think about/remember when trying to design/architect a SQL Server system. Clearly there's got to be a balance between the amount of time you're going to spend on design vs. just "getting it done" but that doesn't mean that NOTHING MATTERS or that you can just do anything with a table and "SQL Server will just 'handle' it." OK, I take that back - SQL Server won't have a choice other than to "just handle it" but some things it just CANNOT handle well. Perform and scalability will suffer and again, your application may fail.

One of the resounding principles of my session (and most of my design talks in general ;-), is that design matters. In fact, in my summary, I said that 3 things really matter in terms of good SQL Server database design/archictecture:

1. Know your data - this helps you make the right decisions in terms of data types/nullability and churn helps with long term maintenance goals (and initial maintenance plans) 
2. Know your workload - this helps you make the right decisions about locking/isolation, optimizing procedural code and indexing strategies (and these are the KEY to a FAST and scalable system)
3. Know how SQL Server works  - this is the one that's often overlooked. And, without information such as "the primary key is enforced by a clustered index and the clustering key is added to ALL nonclustered indexes" then you may inadvertently create a database that grows faster and larger than anticipated where performance slows to a crawl and even management/maintenance becomes a [really HUGE] challenge.

So, while I could go on for ages here I just want to expand on that last point: Know how SQL Server works. Specifically, I want to tie together the PK -> CL Key -> NC Indexes along with the "disk space is cheap" mantra that I also hear ALL THE TIME.

OK - so let's break this down a bit... No matter what your clustered index is - it should be narrow. I do not choose my clustered index for range queries and my choice for the clustering key is NEVER accidental.

Why - because it has a HUGE impact on overall performance. To prove the point (and this was the concept around which my session focused), I created 3 different versions of the SAME "Sales" database. I wanted to show ALL of the impacts of a poorly chosen key - both as CL and really just as a size issue. It's only 12 more bytes than an int, right? What harm can it cause... just wait!

So - to start, I loaded all three databases with roughly 6.7 million rows... and, I made sure everything was clean and contigious so that I'd have the same starting point for all of the tables. I actually strategically started things in one filegroup and then moved things over to another filegroup with 2 files so that I could get some benefits from having multiple files as well (see Paul's excellent post on why a RW filegroup should generally have 2-4 files here: Benchmarking: do multiple data files make a difference?). So, at the initial start I have three databases:

SalesDBInts (inital size with Sales at 6.7 million rows = 334MB):

• Customers - has an ever-increasing identity (int) PK (4 bytes)
• Employees - has an ever-increasing identity (int) PK (4 bytes)
• Products - has an ever-increasing identity (int) PK  (4 bytes)
• Sales - has an ever-increasing identity (int) PK and FKs to Customers, Employees and Products (row size = 27 bytes)

SalesDBGUIDs (inital size with Sales at 6.7 million rows = 1000MB):

• Customers - has a randomly generated (using the NEWID() function) GUID PK (16 bytes)
• Employees - has a randomly generated (using the NEWID() function) GUID PK (16 bytes)
• Products - has a randomly generated (using the NEWID() function) GUID PK (16 bytes)
• Sales - has a randomly generated (using the NEWID() function) GUID PK (16 bytes) and FKs to Customers, Employees and Products (row size 75 bytes)

SalesDBSeqGUIDs (inital size with Sales at 6.7 million rows = 961MB):

• Customers - has a sequentially generated (using the NEWSEQUENTIALID() function) GUID PK (16 bytes)
• Employees - has a sequentially generated (using the NEWSEQUENTIALID() function) GUID PK (16 bytes)
• Products - has a sequentially generated (using the NEWSEQUENTIALID() function) GUID PK (16 bytes)
• Sales - has a sequentially generated (using the NEWSEQUENTIALID() function) GUID PK (16 bytes) and FKs to Customers, Employees and Products (row size 75 bytes)

OK, so here's where the session really starts... I run 10K inserts into the Sales table in each database and then I check and see what happens:

• 10K rows in SalesDBInts takes 00:17 seconds
• 10K rows in SalesDBGUIDs takes 05:07 minutes
• 10K rows in SalesDBSeqGUIDs takes 01:13 minutes

This is already SCARY and should go down into the "Are you kidding me category?" but I also have to add that the hardware and setup for these first few tests are just highlighting a whole myriad of problems. First, I was running with a somewhat crummy setup - a dual-core laptop with only 3GB of memory and this database was on an external USB drive. Certainly not enterprise storage but also not an enterprise size either. For the size of the db we should have been able to do better... wait, we did - with the int-based database things went really well. Only the other two really stunk and the sequential GUID based database definitely faired better than the random (of course - fragmentation, right?). And, yes, that's a part of it... but there's more. And, I thought... no, this can't be right. Let me try again... run 2:

Well, that seems pretty consistent but wow - the random GUID db is really NOT fairing very well... let's try it again:

OK, so you have GOT to be wondering why things are going so horribly wrong? The fragmentation is leading to more page IOs and those also have be put in cache so we're needing a larger and larger cache to handle our GUID database... none of this is good and means you need bigger machines and/or something else to help you out. With the ever-increasing patterns created by the other database we're requiring fewer pages to be read and fewer pages to be cached - these databases are performing somewhat consistently...

OK - so what can we do... let's try FIRST dealing with the fragmentation. To keep it simple, I went to the Books Online for sys.dm_db_index_physical_stats - example D. Using sys.dm_db_index_physical_stats in a script to rebuild or reorganize indexes. This is pretty good but since these databases had never seen a REBUILD (and definitely not a FILLFACTOR setting, I had to tweak the script slightly to include a generic 90% fillfactor). Here's the line that I modified:

SET @command = N'ALTER INDEX ' + @indexname + N' ON ' + @schemaname + N'.' + @objectname + N' REBUILD WITH (FILLFACTOR = 90)'

I ran this in ALL three databases but there wasn't much to do in any of them except for the GUID-based database:

SalesDBInts (5 seconds)
Executed: ALTER INDEX [ProductsPK] ON [dbo].[Products] REBUILD WITH (FILLFACTOR = 90)

SalesDBGUIDs (7:51 minutes)
Executed: ALTER INDEX [SalesPK] ON [dbo].[Sales] REBUILD WITH (FILLFACTOR = 90)
Executed: ALTER INDEX [IX_SalesToProductsFK] ON [dbo].[Sales] REBUILD WITH (FILLFACTOR = 90)
Executed: ALTER INDEX [CustomersPK] ON [dbo].[Customers] REBUILD WITH (FILLFACTOR = 90)
Executed: ALTER INDEX [ProductsPK] ON [dbo].[Products] REBUILD WITH (FILLFACTOR = 90)

SalesDBSeqGUIDs (9 seconds)
Executed: ALTER INDEX [ProductsPK] ON [dbo].[Products] REBUILD WITH (FILLFACTOR = 90)
Executed: ALTER INDEX [CustomersPK] ON [dbo].[Customers] REBUILD WITH (FILLFACTOR = 90)

Then, I ran my inserts again...

The first hardware change that I made was that I moved these to an internal SSD drive... and, ran my test again. Let's get rid of all the random IO problems. That's got to help, eh?

• 10K more rows in SalesDBInts takes 00:04 seconds
• 10K more rows in SalesDBGUIDs takes 01:15 minutes
• 10K more rows in SalesDBSeqGUIDs takes 01:02 minutes

WOW - THAT's AWESOME... killing it with iron brings it VERY close to the speed of the Sequential GUIDs as we're completely eliminating the random IOs. This makes our backups faster, etc. but it still doesn't reduce the memory required because of the pages that are going to be required on insert. And, if you have a very large table with a lot of historical data that wouldn't have otherwise needed to be brought into cache this is a BIG problem especially for much larger tables.

I had quite a few more stuff in my demos but it brings us to a really good point... what are our options and what should we consider? First and foremost, how much control do you have? Did you design/architect your system and if so - how much work are you willing to put into it from here? Or, is this an application over which you have no control? Let's take the obvious...

If this is an application over which you have no control then you really have only 2 options:

1. MAINTENANCE (with a good FILLFACTOR)
2. Consider killing it with iron where the most obvious gains are going to be disk IOs (ie. SSDs for the data portion) and memory...

If this is a system over which you do have control... then, I'd suggest changing the CL key at a minimum. Then, I'd make sure you have good maintenance setup for your nonclustered indexes because those will most certainly be fragmented. Then, I'd slow consider changing over your FKs to use the CL key (identity ideally) and then maybe - eventually - you can remove those GUIDs altogether. But, this is NOT an easy thing to do...

If your CL key is a PK then here are your steps:

1. Take the database offline (sorry, I’m just the messenger!!)
2. Disable the FKs
3. Disable the nonclustered indexes
4. Drop the clustered PK (alter table)
5. Optionally, add an identity column?
6. Create the new clustered index
7. Create the PK as nonclustered
8. Enable the nonclustered indexes (alter index…rebuild)
9. Enable the FKs with CHECK (this is very important)
10. Bring the database online

And, there are certainly derivitives of this but the long story short is that it's going to be a painful process. And, I know some of you have work to do... so, I'll end this post here as well as give you a few links to help you continue learning about these issues. The more you know about SQL Server the better your database design and the longer your database will be healthy and scalable.

I've spoken about this in many posts and many of our SQLMag Q&A posts:

• GUIDs as PRIMARY KEYs and/or the clustering key
• Where does that clustering key go?
• Changing the Definition of a Clustered Index (this can help with generating the code above)
• Isn’t the Clustering Key Redundant?
• Should I index my foreign key columns?
• How many indexes should I create?
• How to choose a good index fill factor (this one is a SQLMag post by Paul)
• Is It a Bad Idea to Rebuild All Indexes Every Night? (also by Paul)

However, as much as I've used that - I've always been frustrated by its output. And, so, over the months+, I wrote sp_helpindex3 (this solely added the index_id as the first column and then ordered the output; I've always hated the weird way in which sp_helpindex/2 ordered the output). Then, I started writing sp_helpindex4 which listed the clustering key columns with each index so that I could more easily identify columns on large tables without a lot of scrolling. Ultimately, I didn't like this because the clustering key (or some of the columns in a multi-column clustering key - might already be in the nonclustered index and so I still had to figure things out more manually). And, so I finally sat down and created what's jokingly being called sp_helpindex8.

Geeky note: Er, you might wonder with how many geeky SQL people I've joked about sp_helpindex8 but in fact, in my last few events (both onsite with a couple of customers as well as when teaching the Indexing Strategies and Performance Tuning portion of the Microsoft Certified Masters - SQL Server course (two weeks ago)), I've actually joked - and promised - that I'd have this blogged before this particular 3 week MCM rotation ends. And, well, that's this week. So, I've actually joked with quite a few folks on this and I'm close to the end of time on blogging it. So, here I am - sitting in a hotel room in Houston, after having presented at the Houston Area User Group (7-9pm) (yes, with Paul) and after having been onsite all day with a customer - blogging this darn thing. I just have to get it out there. ;-)

Having said that - I'm pretty happy with this version but it was pretty painful to write. And, before I get into what it does - and why I wanted this version - let me explain what I didn't do and why. There are a lot of other things that could be added. And, quite a few folks have said - why don't you do this or why don't you do that... Specific suggestions have been made such as adding index usage stats (from the current state shown by the DMV) or add index operational stats (to show if an index is suffering from latch IO waits, for example) or add index physical stats (to show if an index is fragmented), etc. And while many of these suggestions would make for lots of interesting uses of this output, I've stayed away from these. Why? Some are expensive to run - such as physical stats - even a limited scan can be painful on a really large table. Or, the results might not be able to be truly trusted  - such as with index_operational_stats. Why? Because the information from that DMV might only cover a short period of time. The information help by them is cleared whenever SQL Server is restarted (or if a database is taken offline or restored, etc.). So, when were those usage or operational stats last cleared? Moreover, is this single "look" at the DMV information really the best on which to base our decisions (even just in general).

In the end, I think that physical stats, operational stats and even usage stats - should be used wisely and carefully (definitely used but not by this). In fact, we often recommend that the best way to use these is by studying them over time and then analyzing their trends not just the point in time values. And, there are LOTS of posts that can help you with understanding this information; here's just a couple to get you started:

• Craig Freedman's (blog) post: What is the difference between sys.dm_db_index_usage_stats and sys.dm_db_index_operational_stats?
• Paul's blog: Indexes From Every Angle: How can you tell if an index is being used?
• Brent Ozar's (blog | twitter) SQLServerPedia post + video: http://sqlserverpedia.com/wiki/Find_Indexes_Not_In_Use

So, having said that - I've kept my sp_helpindex8 more pure ;-). What I want to see is EXACTLY what's stored in that index. In fact, this has always been my main goal. The first version (sp_helpindex2) added INCLUDED columns (2005+) as well as filter definitions (2008) to the output. So, what was missing? The added columns that SQL Server might have to add if a nonclustered is nonunique and/or when the clustering key is nonunique. The reason this is so important to me is that there are many things coming together and recommending indexes (DTA, the missing index DMVs, application tools, etc.) and while many of these tools are solely there to "help" us get better performance and "help" us create more effective indexing strategies - these tools sometimes recommend the same index over and over again (see Paul's recent blog post: Missing index DMVs bug that could cost your sanity... about the bug in the missing index DMVs) OR the tools might recommend similar indexes to what we already have.

Additionally, the output from sp_helpindex/2 only shows the columns that are EXPLICITLY defined in the key and in the leaf (using INCLUDE) but they don't show what SQL adds for you. Most folks know that SQL Server adds the clustering key to the nonclustered indexes and when you have a simple single-column clustering key it's not difficult to know that this IS "there" in your index even though it doesn't show up. But, even with that, I think there are still a few things that are lesser known. For example, if your nonclustered index is nonunique - did you know that the clustering key is not only added to the leaf level of the index (like INCLUDE does) but that the clustering key is added into the tree (as part of the key and ordering?). And, if have one index that lists this clustering key explicitly and another that doesn't - are you actually going to be able to easily tell that these are in fact the SAME. Ultimately, I want you to be able to better recognize what's REALLY in that index. What you might end up seeing is that two indexes are more similar than you first thought and as a result - maybe you can drop one of them. Or, you might be able to combine them to reduce your overall indexes (I call this index consolidation). And, all of this becomes significantly more challenging when you have more complex clustering keys (and, yes, they do exist!). The other thing I did is delimit the columns and index names properly using brackets. This is also nice because you might have identifiers that make the output of sp_helpindex/2 hard to read. This should help. However, I do have to admit that you probably have other problems if your index/column names have commas, spaces or brackets in them. But, this should still help/work.

And, that's why I wrote "sp_helpindex8" it's a MUCH better version of sp_helpindex but it requires a few other components. And, it was a lot more challenging to write. Basically, this information is more internal to SQL Server and not directly exposed anywhere. So, we had to do a bit of digging. Paul started the process (thanks!) but, I ended up spending about 5 hours getting it exactly the way I wanted it. And, I also ended up breaking it up into two parts: one sp that gets column definitions and the other that builds the final output. And, I've actually named it with a longer name (sorry!). But, I rarely type in the name anyway. What I typically do is go to SSMS, Tools | Options, then under Environment | Keyboard, I set the keyboard shortcut for Ctrl+F1 to my sp_SQLskills_SQL2008_helpindex.

Now, when I want to see the true defintions of indexes on a table - I highlight the tablename and hit Ctrl+F1. But, if you want to rename this to sp_helpindex8 - I won't be offended. ;-)

So, how do you create this?

1. You need to create sp_SQLskills_ExposeColsInIndexLevels (sp_SQLskills_ExposeColsInIndexLevels.sql (6.73 kb)). This is what gives us the tree/leaf definitions. And, this works for both SQL Server 2005 and SQL Server 2008.
2. You need to create the version specific sp_helpindex8. These are named: sp_SQLskills_SQL2005_helpindex (sp_SQLskills_SQL2005_helpindex.sql (10.50 kb)) and  sp_SQLskills_SQL2008_helpindex (sp_SQLskills_SQL2008_helpindex.sql (10.73 kb)). Again, I know they're long names but using the keyboard shortcuts can help. And, if you do rename these make sure you do a replace ALL. There are multiple places where there are comments and/or statements that need to execute to get everything to work (for example the sp_MS_marksystemobject execution).

NOTE: The very first version of the 2005 script had a bug in it (thanks Calvin!). Funnily one I already knew about and thought I had caught (it was fixed in the 2008 version). Anyway, as of 3:15pm CT on May 19, it's fixed in the scripts here. There could be others but so far - it's looking good!

Enjoy!
kt

Expanding on the topic of "are you kidding me"... one of the MOST PREVALENT problems I see today is the dreaded "GUIDs as PKs" problem. However, just to be clear, it's not [as much of a] problem that your PRIMARY KEY is a GUID as much as it is a problem that the PRIMARY KEY is probably your clustering key. They really are two things BUT the default behavior in SQL Server is that a PRIMARY KEY uses a UNIQUE CLUSTERED INDEX to enforce entity integrity. So, I thought I'd take this post to really dive into why this is a problem and how you can hope to minimize it.

Relational Concepts - What is a PRIMARY KEY? (quick and basic reminder for what is what and why)

Starting at the very beginning... a primary key is used to enforce entity integrity. Entity integrity is the very basic concept that every row is uniquely identifiable. This is especially important in a normalized database because you usually end up with many tables and a need to reference rows across those tables (i.e. relationships). Relational theory says that every table MUST have a primary key. SQL Server does not have this requirement. However, many features - like replication - often have a requirement on a primary key so that they can guarantee which row to modify on a related database/server (like the subscriber in a replication environment). So, most people think to create one. However, not always...

What happens when a column(s) is defined as a PRIMARY KEY - in SQL Server?

The first thing that SQL Server checks is that ALL of the columns that make up the PRIMARY KEY constraint do not all NULLs. This is a requirement of a PRIMARY KEY but not a requirement of a UNIQUE KEY. They also check to make sure (if the table has data) that the existing data meets the uniqueness requirement. If there are any duplicate rows, the addition of the constraint will fail. And, to check this as well as to enforce this for [future] new rows - SQL Server builds a UNIQUE index. More specifically, if you don't specify index type when adding the constraint, SQL Server makes the index a UNIQUE CLUSTERED index. So, why is that interesting...

What is a clustered index?

In SQL Server 7.0 and higher the internal dependencies on the clustering key CHANGED. (Yes, it's important to know that things CHANGED in 7.0... why? Because there are still some folks out there that don't realize how RADICAL of a change occurred in the internals (wrt to the clustering key) in SQL Server 7.0). It's always (in all releases of SQL Server) been true that the clustered index defines the order of the data in the table itself (yes, the data of the table becomes the leaf level of the clustered index) and, it's always been a [potential] source of fragmentation. That's really not new. Although it does seem like it's more of a hot topic in recent releases but that may solely because there are more and more databases out there in general AND they've gotten bigger and bigger... and you feel the effects of fragmentation more when databases get really large.

What changed is that the clustering key gets used as the "lookup" value from the nonclustered indexes. Prior to SQL Server 7.0, SQL Server used a volatile RID structure. This was problematic because as records moved, ALL of the nonclustered indexes would need to get updated. Imagine a page that "splits" where half of the records are relocated to a new page. If that page has 20 rows then 10 rows have new RIDs - that means that 10 rows in EACH (and ALL) of your nonclustered indexes would need to get updated. The more nonclustered indexes you had, the worse it got (this is also where the idea that nonclustered indexes are TERRIBLY expensive comes from). In 7.0, the negative affects of record relocation were addressed in BOTH clustered tables and heaps. In heaps they chose to use forwarding pointers. The idea is that the row's FIXED RID is defined at insert and even if the data for the row has to relocate because the row no longer fits on the original page - the rows RID does not change. Instead, SQL Server just uses a forwarding pointer to make one extra hop (never more) to get to the data. In a clustered table, SQL Server uses the clustering key to lookup the data. As a result, this puts some strain on the clustering key that was never there before. It should be narrow (otherwise it can make the nonclustered indexes UNNECESSARILY wide). The clustering key should be UNIQUE (otherwise the nonclustered indexes wouldn't know "which" row to lookup - and, if the clustering key is not defined as unique then SQL Server will internally add a 4-byte uniquifier to each duplicate key value... this wastes time and space - both in the base table AND the nonclustered indexes). And, the clustering key should be STATIC (otherwise it will be costly to update because the clustering key is duplicated in ALL nonclustered indexes).

In summary, the clustering key really has all of these purposes:

1. It defines the lookup value used by the nonclustered indexes (should be unique, narrow and static)
2. It defines the table's order (physically at creation and logically maintained through a linked list after that) - so we need to be careful of fragmentation
3. It can be used to answer a query (either as a table scan - or, if the query wants a subset of data (a range query) and the clustering key supports that range, then yes, the clustering key can be used to reduce the cost of the scan (it can seek with a partial scan)

However, the first two are the two that I think about the most when I choose a clustering key. The third is just one that I *might* be able to leverage if my clustering key also happens to be good for that. So, some examples of GOOD clustering keys are:

• An identity column
• A composite key of date and identity - in that order (date, identity) 
• A pseudo sequential GUID (using the NEWSEQUENTIALID() function in SQL Server OR a "homegrown" function that builds sequential GUIDs - like Gert's "built originally to use in SQL 2000" xp_GUID here: http://sqldev.net/xp/xpguid.htm

But, a GUID that is not sequential - like one that has it's values generated in the client (using .NET) OR generated by the newid() function (in SQL Server) can be a horribly bad choice - primarily because of the fragmentation that it creates in the base table but also because of its size. It's unnecessarily wide (it's 4 times wider than an int-based identity - which can give you 2 billion (really, 4 billion) unique rows). And, if you need more than 2 billion you can always go with a bigint (8-byte int) and get 2⁶³-1 rows. And, if you don't really think that 12 bytes wider (or 8 bytes wider) is a big deal - estimate how much this costs on a bigger table and one with a few indexes...

• Base Table with 1,000,000 rows (3.8MB vs. 15.26MB)
• 6 nonclustered indexes (22.89MB vs. 91.55MB)

So, we're looking at 25MB vs 106MB - and, just to be clear, this is JUST for 1 million rows and this is really JUST overhead. If you create an even wider clustering key (something horrible like LastName, FirstName, MiddlieInitial - which let's say is 64bytes then you're looking at 427.25MB *just* in overhead..... And, then think about how bad that gets with 10 million rows and 6 nonclustered indexes - yes, you'd be wasting over 4GB with a key like that.

And, fragmentation costs you even more in wasted space and time because of splitting. Paul's covered A LOT about fragmentation on his blog so I'll skip that discussion for now BUT if your clustering key is prone to fragmentation then you NEED a solid maintenance plan - and this has it's own costs (and potential for downtime).

So............... choosing a GOOD clustering key EARLY is very important!

Otherwise, the problems can start piling up!

kt