Designing, evaluating, maintaining, and troubleshooting indexes and statistics.
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SQL Server has this fancy feature called constraints. Database constraints are used to restrict the domain (the range of allowed values) of a given attribute. That’s just a funny way of saying: through a set of carefully crafted rules, we control the shape of our universe.
Our Test Table
It was a dark and stormy… Oh, wrong story. It was actually a warm, sunny afternoon in Charlotte, NC. I was presenting “Index Methods You’re Not Using” at PASS Summit. In this talk, I discussed how indexed views, filtered indexes, and compressed indexes can improve your query performance by reducing I/O.
From stage right, an intrepid audience member raised his hand and asked, “Can you think of an example of when you would use filtered indexes instead of partitioning?”
Views are logical objects in SQL Server databases that present you with a “virtual table”. Views are typically created for one of three reasons: security, simplification, or aggregation. Security: we create views so that a user can read specific columns out of certain tables, but not all the data. Simplification: sometimes, it’s easier to write…
In Michigan where I grew up, we pull the boats out of the water at the beginning of the fall. It's a bit of a sad time, realizing we're done having fun on the water, and the seasons are about to change.
To prepare a boat for a few months of storage, we drain some fluids, replace others, give it a good cleaning, do some basic maintenance, and put on a tight waterproof cover.
I’ve been told that an attendee at the PASS Summit pre-conference event asked about using dynamic SQL to get around some of the problems with filtered indexes. I’m not entirely sure what the question was, but it did give me the opportunity to play around with filtered indexes and write some simple demo code to illustrate just the kind of shenanigans that you can get up to.
When you write a query, you (usually) don’t want to return all the rows from the table(s) involved – so you add a WHERE clause to the statement. This ensures that fewer rows are returned to the client - but doesn't reduce the amount of I/O done to get the results. When you create a…
What Developers Need to Know About Designing Databases
https://twitter.com/SQLintheWild/status/393386854000492545
One of my favorite tips, and I never thought of it that way. Code is agile, and databases are brittle. It seems easy to refactor the database in the beginning, but as your app grows and more stuff interfaces with it, life gets a lot harder.
Let me ask you a question: do you want to boss the SQL Server query optimizer around?
If you answered no: good. You’re willing to let the query optimizer do its job, which is to find the least expensive way to run a query, as quickly as possible.
This question came up in the pre-show chat for our weekly webcast: when you rebuild a nonclustered index offline, does it impact only the nonclustered index? Or does it impact the entire table? Showing is more fun than telling! Let's take a look. First I restore AdventureWorks to slow storage Sometimes slower storage is handy.…
In the e-discovery business, kCura Relativity is the big gorilla, and I've been working with kCura and their clients since 2011. I think it's one of the most interesting businesses I've ever worked with - up there with StackOverflow - and like Stack, they're really open with their customers. Today marks my second year being…
Are you still rebuilding all of your indexes every week - or heaven forbid, every night - and still having performance problems? Odds are, you're not actually making things better - you're making them worse! I explain why in this video:
https://www.youtube.com/watch?v=fD1CZVc6oUk
Editors Note: this post was updated in June 2014 to link to an online index rebuild bug that can cause corruption. Once up on a time, there was a database server with 500GB of data and a heavy read workload of dynamic queries. Data was updated frequently throughout the day and index tuning was a…
I'm going to tell you a secret. Index tuning is complicated, but it's something you can become great at. You just need to practice it regularly.
Here's that one step: stop thinking index tuning is a problem for Future You.
Recently, Jes asked the team an index tuning question: "If a query has an index hint in it, will the optimizer ever suggest a missing index for that query?"
I immediately loved the question because I'd never really thought about it before. I typically think of index hints as being a very risky game and avoid them whenever I can-- after all if someone drops the index you've hinted, any query hinting a non-existent index will start to fail. (That's a really bad day!)
sp_BlitzIndex® just can't keep a secret. That's a feature, not a bug. I wrote sp_BlitzIndex® specifically so that it would rat out some things that SQL Server is doing behind the scenes to secretly help you out-- because it's really better if you know about it. What are secret columns? SQL Server needs a unique…
Some query hints sound too good to be true. And, unfortunately, usually they aren't quite as magical as they might seem.
Frustration with unpredictable execution times
People often learn about parameter sniffing when query execution times stop being predictable. Occasionally you'll hear about a stored procedure taking much longer than normal, but the next time you look, it might be faster.
When it comes to indexes, SQL Server is really helpful. It lets you see what indexes queries are asking for both in execution plans, and missing index dynamic management views ("DMVs"). I like to look at the DMV missing index requests using sp_BlitzIndex®.
When you look at missing index requests, it's always important to remember one of the biggest things: these missing index requests won't ever ask for or recommend a specific clustered index.
Key length matters in SQL Server indexes.
It's a best practice to keep your index keys as narrow as possible, and SQL Server enforces a maximum key length of 900 bytes on most "normal" clustered and nonclustered indexes.
I'll never think "I've seen it all" when it comes to SQL Server-- there's just always someone waiting around the corner who's found a weird new thing to do with it. But there are some things I really wish I could stop finding.One of those things I hate finding is bad fillfactor settings. Fillfactor can…
Taking care of your health is incredibly important (says the marathon runner). Eating right, exercising, and getting plenty of rest help you stay healthy and perform optimally. Your database server’s health is important to your company’s health. You need to take regular backups, check for consistency, and keep indexes tuned for optimal performance. Here are…
