The (Un)Importance of Index Fragmentation

Tibor Karaszi argues that index fragmentation is a less serious issue than most DBAs think:

You know the story. Every week or so, we defragment the indexes. Many of us uses Ola Hallengren’s great script for this, some uses Maintenance Plans, and there are of course other alternatives as well. But are we just wasting time and effort? Quite probably we are. I’m going to start with some basics, and then do some reasoning, and finally give you some numbers of a very simple test that I ran. The T-SQL code is available. If you give it a try, please let us know your finding for your environment by adding a comment. I will do some generalizations and simplifications, to avid this post being 10 times longer.

Jeff Moden has a couple of great talks on the topic which really pushed me in this direction. Grab his slides from the SQL Saturday site for a much deeper look at this topic.

The Performance Hit From Ignoring Duplicate Keys

Paul White explains why there is a big performance hit when using IGNORE_DUP_KEY on clustered indexes:

The IGNORE_DUP_KEY index option can be specified for both clustered and nonclustered unique indexes. Using it on a clustered index can result in much poorer performance than for a nonclustered unique index.

The size of the performance difference depends on how many uniqueness violations are encountered during the INSERT operation. The more violations, the worse the clustered unique index performs by comparison. If there are no violations at all, the clustered index insert may even perform better.

I use IGNORE_DUP_KEY primarily in cases like queue tables where I might be queuing up changes to migrate to a warehouse and where the chance of collision is low but non-zero. It looks like pushing much beyond that pattern can be devastating for performance.

Important Considerations with Indexed Views

Jason Brimhall takes us through a few important considerations with indexed views:

The creation of this view has chewed up a bunch of storage. It has jumped right up to the number two spot on the biggest objects list within this database. You can see that differences by comparing the highlighted rows to the previous image. The vPerson view is highlighted in red in this second image to help point it out quickly.

Surely this must be a contrived example and people don’t really do this in the real world, right? The answer to that is simply: NO! It DOES happen. I see situations like this all too often. Far too often, large text fields are added to an indexed view to make retrieval faster. I have mimicked that by adding in two XML columns from the Person.Person table. This is definitely overkill because a simple join back to the table based on the BusinessEntityID would get me those two columns. All I have effectively done is duplicated data being stored and I have achieved that at the low low cost of increased storage of 25% for this small database. If you are curious, the column count between the Person.Person table and this new view is 13 columns each.

Jason takes us through a couple more gotchas and provides some important advice you should follow if you think indexed views might be a fit for you.

Included Columns on Filtered Indexes

Rob Farley take a look at included columns on filtered indexes:

First let’s think a little about indexes in general.

An index provides an ordered structure to a set of data. (I could be pedantic and point out that reading through the data in an index from start to end might jump you from page to page in a seeming haphazard way, but still as you’re reading through pages, following the pointers from one page to the next you can be confident the data is ordered. Within each page you might even jump around to read the data in order, but there is a list showing you which parts (slots) of the page should be read in which order. There really is no point in my pedantry except to answer those equally pedantic who will comment if I don’t.)

And this order is according to the key columns – that’s the easy bit that everyone gets. It’s useful not only for being able to avoid re-ordering the data later, but also for being able to quickly locate any particular row or range of rows by those columns.

Rob does a great job of covering some of the nuances of filtered indexes.

Cancelling Resumable Index Creation

Brent Ozar takes us through a couple considerations when using online, resumable index creation:

In SSMS, you’re used to being able to click the “Cancel” button on your query, and having your work rolled back.

You’re also used to being able to kill a query, and have it automatically roll back.

Neither of those are true with resumable index creations. In both cases, whether you kill the index creation statement or just hit the Cancel button in SSMS to abort your request, your index creation statement is simply paused until you’re ready to come back to it. (Or, it’s ready to come back to haunt you, as we saw above.)

There are some good things to keep in mind here.

Finding Missing Index Hints in Query Store

Grant Fritchey shows us another place where we can find missing index hints:

A couple of notes on the query. I cast the query_plan as xml so that I can use the XQuery to pull out the information. It is possible that the plan might be so large that you get an error because of the limit on nesting levels within XML. Also, I aggregate the information from the sys.query_store_runttime_stats. You may want to modify this to only look at limited ranges. I’ll leave that to you as an exercise.

Do read Grant’s warning in the conclusion.

Benefits of Inline Indexes

Phil Factor covers a topic fairly close to my heart:

Usually, the added features of the CREATE TABLE syntax in new releases of SQL Server are esoteric, and unless you are dealing with memory-optimized tables or other esoteric stuff, they aren’t of great interest. However, the Inline INDEX for both a table and column index has just crept in quietly with SQL Server 2014 (12.x). This was interesting because the SQL Server team back-fitted it to all tables rather than just in-memory OLTP tables for which it was, at the time, found necessary. The new syntax was introduced which allows you to create certain index types inline with the table definition. These could be at column level, concerning just that column, or at the table level, with indexes containing several columns.

Why interesting? This affects multi-statement table functions, user-defined table types, table-valued parameters as well as table variables. It was considered a game-change for table variables because, for a start, it allowed non-unique indexes or explicit clustered indexes to be declared on columns for the first time because you can create indexes on table variables as part of the table definition. Of more significance were the table-level indexes that allowed you to specify multi-column indexes. Previous releases had allowed multi-column primary or unique constraints, but not explicitly named indexes. You still cannot declare an index after the table is created, which is a shame as there are good reasons for being able to do so after a table is stocked with data. Any sort of large import of data into a table that is over-indexed or prematurely-indexed is doomed to crawl rather than to run. I’ll show this later on in this article.

Click through for an analysis of inline indexes themselves as well as how they fit on table variables—something I tend not to do much.

The Costs Of Rebuilding Indexes Online

Tibor Karaszi explains that TANSTAAFL (There A’int No Such Thing As A Free Lunch) applies to online index rebuilds:

The time it takes to rebuild the index can be substantially longer for ONLINE. Many of us has other things to do with the database during night-time and/or weekends. Yes, these are the typical window in time where we try to find things such as index rebuilds. Say that you do it night-time and it currently take 4 hours. Wouldn’t it be nice if you could cut that time down to 1.5 hours? That would leave more time for imports, massaging of data, CHECKDB and other things you want to do. Sure, you can do it ONLINE, but it will slow down access during the rebuild. Also the more data you modify during the rebuild, the more space you need in tempdb.

Betteridge’s Law of Headlines applies too, so that’s two important principles in one post.

As far as the post goes, Tibor makes a fair point: there is a trade-off between availability and efficiency with index rebuilds. But having worked with clustered columnstore indexes in 2014, you’ll pry the online operations in subsequent versions out of my cold, dead hands.

The Importance of Cardinality

Bert Wagner shows us why cardinality is important to understand when indexing data:

When building indexes for your queries, the order of your index key columns matters.  SQL Server can make the most effective use of an index if the data in that index is stored in the same order as what your query requires for a join, where predicate, grouping, or order by clause.

But if your query requires multiple key columns because of multiple predicates (eg. WHERE Color = ‘Red’ AND Size= ‘Medium’), what order should you define the columns in your index key column definition?

One of my favorite books for query tuning is a bit long in the tooth at this point but remains quite relevant, and a key point there is to look for ways to drop the largest percent of rows as soon as possible. This applies for good indexes as well: they’ll let you ignore as large a percentage of your irrelevant data as you can, as soon as possible.

On Disabling Indexes

Kenneth Fisher has a few notes on disabling indexes:

Indexes are probably the number one tool we have to improve performance. That said, there are times when we want to put that index on hold. While indexes dramatically improve read performance they do cause a slight dip in write performance. This isn’t significant most of the time but when doing a large load it can frequently be faster to get rid of the existing indexes and then put them back when you are done.

I don’t think that I’ve ever regularly disabled indexes, even during bulk loading. It’s good to know that the option exists, however.


April 2019
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