A Fine Slice Of SQL Server
Kendra Little has a new video involving indexing:
Recently, I was thinking about nonclustered indexes in SQL Server, and how included columns are stored. Is SQL Server smart enough to optimize the storage for small indexes with includes? Find out in this free seven minute video.
It’s a short video, well worth your time.
Comments closedKendra Little provides a T-SQL script to script out all indexes on a database:
Sometimes you need to script out all the indexes in a database.
Maybe you’re concerned something has changed since they were last checked in.
Or maybe the indexes aren’t checked into source control, and you’re working on fixing that. (Important!)
Either way, sometimes you need to do it, and it’s not fun through the GUI. I needed to write some fresh demo code for this recently, and I needed it to give the details for partitioned tables using data compression, and I thought I’d share.
The fact that the built-in Generate Scripts does not include compression is annoying, but Kendra’s script does. For bonus points, use Powershell to update scripts automatically with index changes and check them into your source control system of choice.
Comments closedSQL Server 2016 and Azure SQL Database have increased the maximum size for index keys with nonclustered indexes. The new maximum key size for nonclustered indexes is 1700 bytes. The maximum key size for clustered indexes remains 900 bytes.
95-99% of the time, we don’t want to get anywhere near 900 bytes (much less 1700 bytes), but we all have that one edge case.
Comments closedNiko Neugebauer talks about extended events relating to columnstore indexes in SQL Server 2016:
In SQL Server 2014 we have had 18 Extended Events and with Service Pack 1 we have received 1 more to be a total of 19 Extended Events for studying the Columnstore Indexes and the Batch Mode processing. In SQL Server 2016 that number has been greatly increased – there are whooping 61 Extended Events, that will give us an important insight into the Columnstore Indexes.
Even more important, Sunil & his team have given an own category inside the Extended Events – a category that is named Columnstore, which will ease the search for the basic columnstore events. Be aware though not all Extended Events related to Columnstore Indexes are included in that category – even including all channels will give you 41 Extended Events, while hiding the other 20 Extended Events, which are sometimes not categorised at all and at other times are stored under different categories, such as Execution or Error, for example. I believe the reason behind not changing the old Extended Events category is quite simple – Microsoft always looks for avoiding breaking existing applications.
There’s a lot here to digest, so read the whole thing.
Comments closedDerik Hammer walks us through heap tables as part of his indexing series:
A heap is a grouping of unsorted pages which are not linked. Page anatomy is out of scope for this series since all types of indexed and non-indexed tables use the same page structure but I do encourage you to check out here and here to learn more.
A heap is comprised of one or more index allocation map (IAM) pages which point to the data pages which make up the heap. The only exception to this is when you have a row which has been updated and could not fit in its page anymore. In that case, you get a forwarding pointer to the row which has been moved to an existing page with space or a new page. It is possible for you to produce a chain of forwarding records if the row continues to need relocation by further operations.
This is a good introduction to heaps and will feed into later work on how different types of indexes work.
Comments closedDerik Hammer is starting a new series on index types and has started with the venerable B-tree:
On more than one occasion I have been told that the B-Tree index is the index structure which was designed after the A-Tree index. Another common misconception is that it stands for Binary-Tree. As logical as those may seem, they are false. The ‘B’ in B-Tree does not actually have any specific meaning. Check out Ed McCreight’s explanation here (16:08) where he admits that the name discussion was never settled.
In its most basic form, the B-Tree index is a hierarchy of data pages (page structures lightly touched on in the next post of this series). The lowest level is called the leaf level, the highest level is the index root, and all levels in between are the intermediate levels. This structure is an improvement over the Binary Tree index because its balanced nature greatly improved the performance of maintenance operations such as, INSERT, DELETE, and UPDATE.
On the terminological point, I’d always heard that the “B” stood for “Balanced” because of the level flatness—in contrast to a “normal” tree, you wouldn’t have more than a pre-defined number of levels separation (usually one) between leaf nodes.
Comments closedErik Darling shows missing index requests while creating indexes:
So there I was, creating some indexes. Since I was in demo mode, I had execution plans turned on. I’m actually terrible about remembering to turn them off. There have been times when I’ve run loops to create some REALLY big tables, and left them turned on. SSMS crashed pretty quickly. Apparently generating execution plans isn’t free! Keep that in mind when you’re timing/tuning queries.
Since they were there, I took a look at them. They were basically what I expected. The only way to index data is to read it and sort it and blah blah blah. But there was a little something extra!
Yo dawg, I heard you like indexes…
Comments closedDave Mason talks about full-text index maintenance:
My first encounter with full text indexes and degraded performance was related to an enhancement I made to an aspx page years ago. I wanted all of the search fields to use an AutoComplete AJAX extender to mimic the behavior you see when you type a few letters into the search field on Google.com or Bing.com. A traditional non-clustered index wasn’t sufficient for the “Location Address” field, so I settled on a full text index–it worked very well.
After some amount of time (I don’t remember how long), performance slowed considerably. I was surprised to find the full text index for “Location Address” had a large number of fragments. I wish I had kept some notes on my findings. I can’t remember how may fragments there were, but I’m thinking it was in the 15-20 range. If memory serves me, Orange Co., FL has about 400,000 physical location addresses. The underlying table had one row per location address. Knowing me, the indexed column was probably VARCHAR(100) or VARCHAR(128). This does’t seem like a huge amount of data, so I was surprised the full text searches were slow, even with 15-20 fragments. Reorganizing the related full text catalog made a world of difference. Performance improved drastically.
All indexes need maintenance. Dave has a script to help with full-text indexes.
Comments closedI have a new script available to reorganize columnstore indexes:
Note that this script requires SQL Server 2016 (or later) because the database engine team made some great changes to columnstore indexes, allowing us to use REORGANIZE to clear out deleted rows and compact row groups together, as well as its previous job of marking open delta stores as available for compression.
The code is available as a Gist for now, at least until I decide what to do with it. Comments are welcome, especially if I’m missing a major reorganize condition.
As mentioned, comments are welcome.
Comments closedJason Brimhall digs into an error where page-level locking is disabled:
You receive the error message similar to the following:
Msg 2552, Level 16, State 1, Line 1 The index “blah” (partition 1) on table “blah_blah_blah” cannot be reorganized because page level locking is disabled
Immediately, you start double-checking yourself and verifying that it worked the previous night. You even go so far as to confirm that the same index was previously reorganized. How is it possible that it is failing now on this index. What has changed? Has something changed?
There’s an interesting troubleshooting story, but the important message is about setting up a good set of Extended Events so that you can troubleshoot these types of problems.
Comments closed