Indexing

Indexing

Do you have the key? Do foreign keys hurt the performance of inserts? Okay, we all know that foreign keys do require some work, but the crazy people of the internet have wildly varying guesses as to just how much work is involved. Estimates varied from “barely any” to “as much as 50% slower”. I…

Are Index ‘Included’ Columns in Your Multi-Column Statistics?

April 23, 2015

Internals Cat populates the density vector When you create an index in SQL Server with multiple columns, behind the scenes it creates a related multi-column statistic for the index. This statistic gives SQL Server some information about the relationship between the columns that it can use for row estimates when running queries. But what if…

Online Index Creation and Cruel Defaults

March 31, 2015

Erik Darling

dba

Have you ever been watching queries crawl, feeling sort of helpless because you can’t change them? And even if you could, it’s hitting a new custom column in an ISV table that no one bothered to tell you about. I’ve been there. I’ve been pomade deep there. I’ve also had my Robot tell me: “there’s…

Using “OR” and “IN” with SQL Server’s Filtered Indexes

February 12, 2015

You can’t do everything with filtered indexes in SQL Server. For instance, you can’t create the following index: CREATE INDEX IX_Votes_filter ON dbo.Votes (PostId) WHERE (VoteTypeId = 1 OR VoteTypeId = 2); GO 123 CREATE INDEX IX_Votes_filter ON dbo.Votes (PostId) WHERE (VoteTypeId = 1 OR VoteTypeId = 2);GO If you try, you’ll get the error message:…

Testing ALTER INDEX REBUILD with WAIT_AT_LOW_PRIORITY in SQL Server 2014

January 14, 2015

Index Maintenance, Locking, Blocking, and Isolation Levels, SQL Server

One of the blocking scenarios I find most interesting is related to online index rebuilds. Index rebuilds are only mostly online. In order to complete they need a very high level of lock: a schema modification lock (SCH-M). Here’s one way this can become a big problem: An online index rebuild starts against a large index A…

How to Configure Ola Hallengren’s Maintenance Scripts

December 17, 2014

Index Maintenance, Ola Hallengren's Database Maintenance Scripts

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If you’re a production database administrator responsible for backups, corruption checking, and index maintenance on SQL Server, try Ola Hallengren’s free database maintenance scripts. They’re better than yours (trust me), and they give you more flexibility than built-in maintenance plans. However, the index maintenance defaults aren’t good for everyone. Here’s how they ship: Transact-SQL CREATE PROCEDURE…

Who’s Allowed to Add Indexes to ISV Apps?

December 4, 2014

Indexing, ISV and SaaS Database Design, SQL Server

Performance tuning of independent software vendor (ISV) databases is a little tricky. To understand who does what, let’s think through all of the work required with building and hosting a database application: Who adds indexes and tunes queries? Typically the two parties involved – the vendor and the customer – start at opposite ends of…

When Does Index Fragmentation Matter?

November 18, 2014

Index Maintenance, SQL Server

A while back, we posted about how SQL Server index fragmentation doesn’t matter. That blog post generated a lot of heated discussion and has served to make people aware that fragmentation may not be the source of their woes. Sometimes, though, there are data access patterns where fragmentation really does matter. Perpetual Fragmentation Some data…

Should I Run sp_recompile After I Create An Index?

August 19, 2014

Making index changes in SQL Server is tricky: you immediately want to know if the new index helped your performance and if it’s being used, but SQL Server execution plans and their related statistics can be are insanely confusing. It can be useful to run sp_recompile after you create an index, but not necessarily for the reason you might…

Using Triggers to Automatically Add Indexes

July 30, 2014

Bad Idea Jeans, Indexing, SQL Server

Let’s say you’ve got a vendor application that creates tables, loads them, queries them, and drops them all the time. You can’t change the code. The tables involved desperately need an index in order to go faster, and they’re your biggest performance bottleneck. Enter DDL triggers. Regular DML triggers fire when you insert/update/delete rows in a…

Video: Test Your Index Design Skills (with poll results)

July 22, 2014

You know how to design indexes, but you’re not sure how good your skills really are. In this quiz-packed session you’ll get a chance to test your skills! Kendra Little will walk you through a set of index design challenges. You’ll have time to answer each problem on your own, then find out whether the…

Why Index Fragmentation and Bad Statistics Aren’t Always the Problem (Video)

April 5, 2014

Index Maintenance, SQL Server

Do you rely on index rebuilds to make queries run faster? Or do you always feel like statistics are “bad” and are the cause of your query problems? You’re not alone– it’s easy to fall into the trap of always blaming fragmentation or statistics. Learn why these two tools aren’t the answer to every problem…

How to Add Nonclustered Indexes to Clustered Columnstore Indexes

March 25, 2014

clustered columnstore, columnstore, sql 2014

Bad Idea Jeans, Columnstore Indexes, Indexing, SQL Server

SQL Server 2012 introduced nonclustered columnstore indexes, but I never saw them used in the wild simply because once created, they made the underlying table read-only. Not a lot of folks like read-only tables. (Bad news, by the way – that limitation hasn’t disappeared in 2014.) SQL Server 2014 brings clustered columnstore indexes, and they’re…

UPDATE STATISTICS: the Secret IO Explosion

January 29, 2014

Index Maintenance, SQL Server, Statistics

I first knew something was up when I looked at the job history for a simple maintenance plan. It had two steps: Rebuild all the indexes in the database – this took 10 minutes each night. Update statistics – this took 2-3 hours each night. What was going on? Statistics in SQL Server are small,…

Keep it Constrained

December 10, 2013

constraints, database design

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 We’re going to be testing with…

Filtered Indexes vs. Table Partitioning

November 26, 2013

Jes Schultz

Indexing, Partitioning, SQL Server

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…

What You Can (and Can’t) Do With Indexed Views

November 19, 2013

Jes Schultz

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…

Filtered Indexes and Dynamic SQL

November 13, 2013

Brent Ozar Unlimited Team

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…

What You Can (and Can’t) Do With Filtered Indexes

November 12, 2013