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📄 Contents

  1. SQL Server Reference Guide
  2. Introduction
  3. SQL Server Reference Guide Overview
  4. Table of Contents
  5. Microsoft SQL Server Defined
  6. SQL Server Editions
  7. SQL Server Access
  8. Informit Articles and Sample Chapters
  9. Online Resources
  10. Microsoft SQL Server Features
  11. SQL Server Books Online
  12. Clustering Services
  13. Data Transformation Services (DTS) Overview
  14. Replication Services
  15. Database Mirroring
  16. Natural Language Processing (NLP)
  17. Analysis Services
  18. Microsot SQL Server Reporting Services
  19. XML Overview
  20. Notification Services for the DBA
  21. Full-Text Search
  22. SQL Server 2005 - Service Broker
  23. Using SQL Server as a Web Service
  24. SQL Server Encryption Options Overview
  25. SQL Server 2008 Overview
  26. SQL Server 2008 R2 Overview
  27. SQL Azure
  28. The Utility Control Point and Data Application Component, Part 1
  29. The Utility Control Point and Data Application Component, Part 2
  30. Microsoft SQL Server Administration
  31. The DBA Survival Guide: The 10 Minute SQL Server Overview
  32. Preparing (or Tuning) a Windows System for SQL Server, Part 1
  33. Preparing (or Tuning) a Windows System for SQL Server, Part 2
  34. Installing SQL Server
  35. Upgrading SQL Server
  36. SQL Server 2000 Management Tools
  37. SQL Server 2005 Management Tools
  38. SQL Server 2008 Management Tools
  39. SQL Azure Tools
  40. Automating Tasks with SQL Server Agent
  41. Run Operating System Commands in SQL Agent using PowerShell
  42. Automating Tasks Without SQL Server Agent
  43. Storage – SQL Server I/O
  44. Service Packs, Hotfixes and Cumulative Upgrades
  45. Tracking SQL Server Information with Error and Event Logs
  46. Change Management
  47. SQL Server Metadata, Part One
  48. SQL Server Meta-Data, Part Two
  49. Monitoring - SQL Server 2005 Dynamic Views and Functions
  50. Monitoring - Performance Monitor
  51. Unattended Performance Monitoring for SQL Server
  52. Monitoring - User-Defined Performance Counters
  53. Monitoring: SQL Server Activity Monitor
  54. SQL Server Instances
  55. DBCC Commands
  56. SQL Server and Mail
  57. Database Maintenance Checklist
  58. The Maintenance Wizard: SQL Server 2000 and Earlier
  59. The Maintenance Wizard: SQL Server 2005 (SP2) and Later
  60. The Web Assistant Wizard
  61. Creating Web Pages from SQL Server
  62. SQL Server Security
  63. Securing the SQL Server Platform, Part 1
  64. Securing the SQL Server Platform, Part 2
  65. SQL Server Security: Users and other Principals
  66. SQL Server Security – Roles
  67. SQL Server Security: Objects (Securables)
  68. Security: Using the Command Line
  69. SQL Server Security - Encrypting Connections
  70. SQL Server Security: Encrypting Data
  71. SQL Server Security Audit
  72. High Availability - SQL Server Clustering
  73. SQL Server Configuration, Part 1
  74. SQL Server Configuration, Part 2
  75. Database Configuration Options
  76. 32- vs 64-bit Computing for SQL Server
  77. SQL Server and Memory
  78. Performance Tuning: Introduction to Indexes
  79. Statistical Indexes
  80. Backup and Recovery
  81. Backup and Recovery Examples, Part One
  82. Backup and Recovery Examples, Part Two: Transferring Databases to Another System (Even Without Backups)
  83. SQL Profiler - Reverse Engineering An Application
  84. SQL Trace
  85. SQL Server Alerts
  86. Files and Filegroups
  87. Partitioning
  88. Full-Text Indexes
  89. Read-Only Data
  90. SQL Server Locks
  91. Monitoring Locking and Deadlocking
  92. Controlling Locks in SQL Server
  93. SQL Server Policy-Based Management, Part One
  94. SQL Server Policy-Based Management, Part Two
  95. SQL Server Policy-Based Management, Part Three
  96. Microsoft SQL Server Programming
  97. An Outline for Development
  98. Database
  99. Database Services
  100. Database Objects: Databases
  101. Database Objects: Tables
  102. Database Objects: Table Relationships
  103. Database Objects: Keys
  104. Database Objects: Constraints
  105. Database Objects: Data Types
  106. Database Objects: Views
  107. Database Objects: Stored Procedures
  108. Database Objects: Indexes
  109. Database Objects: User Defined Functions
  110. Database Objects: Triggers
  111. Database Design: Requirements, Entities, and Attributes
  112. Business Process Model Notation (BPMN) and the Data Professional
  113. Business Questions for Database Design, Part One
  114. Business Questions for Database Design, Part Two
  115. Database Design: Finalizing Requirements and Defining Relationships
  116. Database Design: Creating an Entity Relationship Diagram
  117. Database Design: The Logical ERD
  118. Database Design: Adjusting The Model
  119. Database Design: Normalizing the Model
  120. Creating The Physical Model
  121. Database Design: Changing Attributes to Columns
  122. Database Design: Creating The Physical Database
  123. Database Design Example: Curriculum Vitae
  124. NULLs
  125. The SQL Server Sample Databases
  126. The SQL Server Sample Databases: pubs
  127. The SQL Server Sample Databases: NorthWind
  128. The SQL Server Sample Databases: AdventureWorks
  129. The SQL Server Sample Databases: Adventureworks Derivatives
  130. UniversalDB: The Demo and Testing Database, Part 1
  131. UniversalDB: The Demo and Testing Database, Part 2
  132. UniversalDB: The Demo and Testing Database, Part 3
  133. UniversalDB: The Demo and Testing Database, Part 4
  134. Getting Started with Transact-SQL
  135. Transact-SQL: Data Definition Language (DDL) Basics
  136. Transact-SQL: Limiting Results
  137. Transact-SQL: More Operators
  138. Transact-SQL: Ordering and Aggregating Data
  139. Transact-SQL: Subqueries
  140. Transact-SQL: Joins
  141. Transact-SQL: Complex Joins - Building a View with Multiple JOINs
  142. Transact-SQL: Inserts, Updates, and Deletes
  143. An Introduction to the CLR in SQL Server 2005
  144. Design Elements Part 1: Programming Flow Overview, Code Format and Commenting your Code
  145. Design Elements Part 2: Controlling SQL's Scope
  146. Design Elements Part 3: Error Handling
  147. Design Elements Part 4: Variables
  148. Design Elements Part 5: Where Does The Code Live?
  149. Design Elements Part 6: Math Operators and Functions
  150. Design Elements Part 7: Statistical Functions
  151. Design Elements Part 8: Summarization Statistical Algorithms
  152. Design Elements Part 9:Representing Data with Statistical Algorithms
  153. Design Elements Part 10: Interpreting the Data—Regression
  154. Design Elements Part 11: String Manipulation
  155. Design Elements Part 12: Loops
  156. Design Elements Part 13: Recursion
  157. Design Elements Part 14: Arrays
  158. Design Elements Part 15: Event-Driven Programming Vs. Scheduled Processes
  159. Design Elements Part 16: Event-Driven Programming
  160. Design Elements Part 17: Program Flow
  161. Forming Queries Part 1: Design
  162. Forming Queries Part 2: Query Basics
  163. Forming Queries Part 3: Query Optimization
  164. Forming Queries Part 4: SET Options
  165. Forming Queries Part 5: Table Optimization Hints
  166. Using SQL Server Templates
  167. Transact-SQL Unit Testing
  168. Index Tuning Wizard
  169. Unicode and SQL Server
  170. SQL Server Development Tools
  171. The SQL Server Transact-SQL Debugger
  172. The Transact-SQL Debugger, Part 2
  173. Basic Troubleshooting for Transact-SQL Code
  174. An Introduction to Spatial Data in SQL Server 2008
  175. Performance Tuning
  176. Performance Tuning SQL Server: Tools and Processes
  177. Performance Tuning SQL Server: Tools Overview
  178. Creating a Performance Tuning Audit - Defining Components
  179. Creating a Performance Tuning Audit - Evaluation Part One
  180. Creating a Performance Tuning Audit - Evaluation Part Two
  181. Creating a Performance Tuning Audit - Interpretation
  182. Creating a Performance Tuning Audit - Developing an Action Plan
  183. Understanding SQL Server Query Plans
  184. Performance Tuning: Implementing Indexes
  185. Performance Monitoring Tools: Windows 2008 (and Higher) Server Utilities, Part 1
  186. Performance Monitoring Tools: Windows 2008 (and Higher) Server Utilities, Part 2
  187. Performance Monitoring Tools: Windows System Monitor
  188. Performance Monitoring Tools: Logging with System Monitor
  189. Performance Monitoring Tools: User Defined Counters
  190. General Transact-SQL (T-SQL) Performance Tuning, Part 1
  191. General Transact-SQL (T-SQL) Performance Tuning, Part 2
  192. General Transact-SQL (T-SQL) Performance Tuning, Part 3
  193. Performance Monitoring Tools: An Introduction to SQL Profiler
  194. Performance Tuning: Introduction to Indexes
  195. Performance Monitoring Tools: SQL Server 2000 Index Tuning Wizard
  196. Performance Monitoring Tools: SQL Server 2005 Database Tuning Advisor
  197. Performance Monitoring Tools: SQL Server Management Studio Reports
  198. Performance Monitoring Tools: SQL Server 2008 Activity Monitor
  199. The SQL Server 2008 Management Data Warehouse and Data Collector
  200. Performance Monitoring Tools: Evaluating Wait States with PowerShell and Excel
  201. Practical Applications
  202. Choosing the Back End
  203. The DBA's Toolbox, Part 1
  204. The DBA's Toolbox, Part 2
  205. Scripting Solutions for SQL Server
  206. Building a SQL Server Lab
  207. Using Graphics Files with SQL Server
  208. Enterprise Resource Planning
  209. Customer Relationship Management (CRM)
  210. Building a Reporting Data Server
  211. Building a Database Documenter, Part 1
  212. Building a Database Documenter, Part 2
  213. Data Management Objects
  214. Data Management Objects: The Server Object
  215. Data Management Objects: Server Object Methods
  216. Data Management Objects: Collections and the Database Object
  217. Data Management Objects: Database Information
  218. Data Management Objects: Database Control
  219. Data Management Objects: Database Maintenance
  220. Data Management Objects: Logging the Process
  221. Data Management Objects: Running SQL Statements
  222. Data Management Objects: Multiple Row Returns
  223. Data Management Objects: Other Database Objects
  224. Data Management Objects: Security
  225. Data Management Objects: Scripting
  226. Powershell and SQL Server - Overview
  227. PowerShell and SQL Server - Objects and Providers
  228. Powershell and SQL Server - A Script Framework
  229. Powershell and SQL Server - Logging the Process
  230. Powershell and SQL Server - Reading a Control File
  231. Powershell and SQL Server - SQL Server Access
  232. Powershell and SQL Server - Web Pages from a SQL Query
  233. Powershell and SQL Server - Scrubbing the Event Logs
  234. SQL Server 2008 PowerShell Provider
  235. SQL Server I/O: Importing and Exporting Data
  236. SQL Server I/O: XML in Database Terms
  237. SQL Server I/O: Creating XML Output
  238. SQL Server I/O: Reading XML Documents
  239. SQL Server I/O: Using XML Control Mechanisms
  240. SQL Server I/O: Creating Hierarchies
  241. SQL Server I/O: Using HTTP with SQL Server XML
  242. SQL Server I/O: Using HTTP with SQL Server XML Templates
  243. SQL Server I/O: Remote Queries
  244. SQL Server I/O: Working with Text Files
  245. Using Microsoft SQL Server on Handheld Devices
  246. Front-Ends 101: Microsoft Access
  247. Comparing Two SQL Server Databases
  248. English Query - Part 1
  249. English Query - Part 2
  250. English Query - Part 3
  251. English Query - Part 4
  252. English Query - Part 5
  253. RSS Feeds from SQL Server
  254. Using SQL Server Agent to Monitor Backups
  255. Reporting Services - Creating a Maintenance Report
  256. SQL Server Chargeback Strategies, Part 1
  257. SQL Server Chargeback Strategies, Part 2
  258. SQL Server Replication Example
  259. Creating a Master Agent and Alert Server
  260. The SQL Server Central Management System: Definition
  261. The SQL Server Central Management System: Base Tables
  262. The SQL Server Central Management System: Execution of Server Information (Part 1)
  263. The SQL Server Central Management System: Execution of Server Information (Part 2)
  264. The SQL Server Central Management System: Collecting Performance Metrics
  265. The SQL Server Central Management System: Centralizing Agent Jobs, Events and Scripts
  266. The SQL Server Central Management System: Reporting the Data and Project Summary
  267. Time Tracking for SQL Server Operations
  268. Migrating Departmental Data Stores to SQL Server
  269. Migrating Departmental Data Stores to SQL Server: Model the System
  270. Migrating Departmental Data Stores to SQL Server: Model the System, Continued
  271. Migrating Departmental Data Stores to SQL Server: Decide on the Destination
  272. Migrating Departmental Data Stores to SQL Server: Design the ETL
  273. Migrating Departmental Data Stores to SQL Server: Design the ETL, Continued
  274. Migrating Departmental Data Stores to SQL Server: Attach the Front End, Test, and Monitor
  275. Tracking SQL Server Timed Events, Part 1
  276. Tracking SQL Server Timed Events, Part 2
  277. Patterns and Practices for the Data Professional
  278. Managing Vendor Databases
  279. Consolidation Options
  280. Connecting to a SQL Azure Database from Microsoft Access
  281. SharePoint 2007 and SQL Server, Part One
  282. SharePoint 2007 and SQL Server, Part Two
  283. SharePoint 2007 and SQL Server, Part Three
  284. Querying Multiple Data Sources from a Single Location (Distributed Queries)
  285. Importing and Exporting Data for SQL Azure
  286. Working on Distributed Teams
  287. Professional Development
  288. Becoming a DBA
  289. Certification
  290. DBA Levels
  291. Becoming a Data Professional
  292. SQL Server Professional Development Plan, Part 1
  293. SQL Server Professional Development Plan, Part 2
  294. SQL Server Professional Development Plan, Part 3
  295. Evaluating Technical Options
  296. System Sizing
  297. Creating a Disaster Recovery Plan
  298. Anatomy of a Disaster (Response Plan)
  299. Database Troubleshooting
  300. Conducting an Effective Code Review
  301. Developing an Exit Strategy
  302. Data Retention Strategy
  303. Keeping Your DBA/Developer Job in Troubled Times
  304. The SQL Server Runbook
  305. Creating and Maintaining a SQL Server Configuration History, Part 1
  306. Creating and Maintaining a SQL Server Configuration History, Part 2
  307. Creating an Application Profile, Part 1
  308. Creating an Application Profile, Part 2
  309. How to Attend a Technical Conference
  310. Tips for Maximizing Your IT Budget This Year
  311. The Importance of Blue-Sky Planning
  312. Application Architecture Assessments
  313. Transact-SQL Code Reviews, Part One
  314. Transact-SQL Code Reviews, Part Two
  315. Cloud Computing (Distributed Computing) Paradigms
  316. NoSQL for the SQL Server Professional, Part One
  317. NoSQL for the SQL Server Professional, Part Two
  318. Object-Role Modeling (ORM) for the Database Professional
  319. Business Intelligence
  320. BI Explained
  321. Developing a Data Dictionary
  322. BI Security
  323. Gathering BI Requirements
  324. Source System Extracts and Transforms
  325. ETL Mechanisms
  326. Business Intelligence Landscapes
  327. Business Intelligence Layouts and the Build or Buy Decision
  328. A Single Version of the Truth
  329. The Operational Data Store (ODS)
  330. Data Marts – Combining and Transforming Data
  331. Designing Data Elements
  332. The Enterprise Data Warehouse — Aggregations and the Star Schema
  333. On-Line Analytical Processing (OLAP)
  334. Data Mining
  335. Key Performance Indicators
  336. BI Presentation - Client Tools
  337. BI Presentation - Portals
  338. Implementing ETL - Introduction to SQL Server 2005 Integration Services
  339. Building a Business Intelligence Solution, Part 1
  340. Building a Business Intelligence Solution, Part 2
  341. Building a Business Intelligence Solution, Part 3
  342. Tips and Troubleshooting
  343. SQL Server and Microsoft Excel Integration
  344. Tips for the SQL Server Tools: SQL Server 2000
  345. Tips for the SQL Server Tools – SQL Server 2005
  346. Transaction Log Troubles
  347. SQL Server Connection Problems
  348. Orphaned Database Users
  349. Additional Resources
  350. Tools and Downloads
  351. Utilities (Free)
  352. Tool Review (Free): DBDesignerFork
  353. Aqua Data Studio
  354. Microsoft SQL Server Best Practices Analyzer
  355. Utilities (Cost)
  356. Quest Software's TOAD for SQL Server
  357. Quest Software's Spotlight on SQL Server
  358. SQL Server on Microsoft's Virtual PC
  359. Red Gate SQL Bundle
  360. Microsoft's Visio for Database Folks
  361. Quest Capacity Manager
  362. SQL Server Help
  363. Visual Studio Team Edition for Database Professionals
  364. Microsoft Assessment and Planning Solution Accelerator
  365. Aggregating Server Data from the MAPS Tool

As I mentioned in my last overview on Stored Procedures, the two main categories of database objects are those that store data and those that access or provide access to data. Tables are the only objects that actually store data; all other objects in the database provide access to data.

It seems obvious that objects such as views and stored procedures provide access to data, but in this article I’ll describe another object that provides a round-about way of data access: indexes. In this overview, I’ll explain index theory, index structures, and how indexes are implemented. To put these theories into practice, we have other articles here at InformIT regarding maintain indexes. In other words, this article is more theory; the "how to" portions are in the sections I'll reference at the bottom of this overview.

Indexes Defined

So just what is an index? An index is a structure in the database that SQL Server can use to find and access data quickly.

Think about a fairly thick book. If I asked you to find a particular topic or even a word in that book, you’d turn to the index. The index in the book is very similar to what a SQL Server Index.

In the case of the book’s index, you can find the page numbers where a word or phrase is located. You simply find the word (which is listed in alphabetical order) and the page or pages where that word or phrase appears is listed just to the right. In fact, most indexes are even broken out into alphabetical groupings in a bold heading, which is a visual cue for you to locate that section quickly. In addition to alphabetical order, sometimes books have additional indexes based on content type or another grouping.

If you didn’t have an index or table of contents in a book when you were asked to locate a word, you’d have to read each and every page to find it. And even when you found it, you’d have to keep reading to make sure that it didn’t occur again. You would have to do this each and every time I asked you to find a word or phrase, since you probably can't memorize every location of something as you look it up. Each time you're asked where a word is, you have to scan the entire book.

In the case of SQL Server, you have the same issue. You have a lot of data, and it may surprise you to learn that by its very definition, the data in a relational database isn't stored in any particular order. It's just scattered all over the data pages (which are 8K blocks on the hard drive) with no particular order other than what was quickest to insert at the time. That makes the job of finding the data very difficult.

In fact, without some sort of help, the server’s Query Optimization engine has to do a "scan" operation, which is the same as what you and I would do without an index in a book — it would have to read every page of data to locate the field you're asked for. With tables under a thousand rows or so, this isn't really a problem, and in fact is the default behavior in SQL Server. But with larger tables, those with thousands or millions of rows, this takes a lot of time. It also has some other bad side-effects. Because all of those rows don't fit into memory, the server has to read from the I/O system, which is among the slowest components on your server. While data are being read from the I/O system, the database can't write to the tables while it is reading from them. That slows down not only reads but writes.

It gets worse. In a database the data is changing all the time. Imagine that you’re asked to find a word in a book, but someone keeps writing new words in the book, and changing where the old ones were.

What you can do to mitigate this scanning problem is to create an Index. The index in SQL Server works the same as the index in a book. It stores the words you're interested in along with a number (which is really a pointer to a block on the I/O system) where the word or phrase is found. Now all SQL Server has to do when you ask for data is to look up the word or phrase in the index, which is much smaller, and find where it lives on the hard drive.

The physical arrangement of indexes in SQL Server is just a table-structure in the database, and on the hard drive. When you create an index, a new physical structure is created and new rows of data are put into that table. You can tell SQL Server when you create the index which fields in the database table you want to track, just like you would mark words in a book that need to be tracked for its index. You don't put all data into an index, since it that would create the same problem you're trying to solve. You only index the fields that users are likely to look up, or fields that are useful for your programs to use. I'll explain more about this later.

The physical arrangement of indexes fall into two types: Clustered and Non-Clustered. There are important differences in these two types, and which one you choose should be carefully considered during the design phase.

A clustered index actually changes the way the data is stored in the table, and the index. In fact, if you create a clustered index on a table, the table itself simply becomes the index.

Let’s take a look at a very simple table:

A

D

C

B

E

One tenet of a relational database is that the data isn’t stored in any particular order, and SQL Server holds to that tenet as you can see here. Tables where the data is in no particular order and with no indexes are called heap tables. Once you apply a clustered index to the columns, however, the table changes physically on the hard drive to this:

A

B

C

D

E

As you can see, the data can only be physically arranged one way. You can’t have the same data arranged physically as A-F and also F-A. Since the data is arranged this way, the server doesn't need to store another table for the index pointing to the data — it just calls the table the index, and the index the table.

The advantage here is that data access is extremely fast. It's like rearranging a book to the point where every word is in alphabetical order. That wouldn't help the storyline much, but you would be able to find anything quickly. Come to think of it, there is a book like that — the dictionary.

Of course, this speed comes at a price. Since the table is physically ordered by the index fields, each time you insert a new row of data the I/O subsystem has to rearrange all the data in the table. So if there are a lot of data changes, the system can slow down to a crawl. If the data doesn't change that often or is arranged in large groups, a clustered index might be the best solution.

The other type of index in SQL Server is called Non-Clustered. This type doesn’t physically re-arrange the data; it just makes another table-like structure which points to where the data is physically located.

So if we continue the book analogy, a clustered index is similar to the table of contents, since the data is arranged that way. A non-clustered index would be similar to a book’s index in the back.

Let's examine our table again, but this time I'll add some more data and show a hidden column: the block address of each row:

AEF1

A

BFE2

D

AEF1

C

BFE2

B

BFE2

E

BFE2

A

You'll notice that there are two "A" values, stored at different locations. At this point if you asked for all of the "A" values the engine would have to read the entire table structure, stored on all these block fragments. If there were thousands of blocks and only two values of "A", the engine would still read them all just to make sure. That would take a lot of time, and create lots of locks as it did so.

Now we'll create an index on the table, which makes another internal table structure that looks like this:

AEF1, BFE2

A

BFE2

B

AEF1

C

BFE2

D

BFE2

E

Now when you look up all the "A" values the engine sees that there is an index, uses that index to find the data, and quickly calls up just those two blocks where the data lives. That's a faster access process, and takes far fewer locks on the rows or tables.

Types of Indexes

There are many types of indexes in SQL Server. The one that you'll work with most often is the user-defined index. This is the Index object type in the database, and they are stored in one of the physical arrangements I just explained.

But there are other types of indexes as well. You see, even if you don't specify an index on a table, the SQL Server Query Optimizer wants to be able to find data quickly. So it will "watch" what you are querying on and make temporary indexes of its own, called "statistical indexes" or more commonly "statistics". You can tell SQL Server to create and maintain these statistics automatically, or you can control that manually. You can make those changes on the database settings in the Properties tab for the database. I recommend that you allow the server to create and maintain statistics, unless the database is quite large and you'll run this process manually. I'll cover this topic more in the other articles referenced at the end of this overview.

Another type of index is the Full-Text index. This structure is used for Full-Text fields and isn't stored in SQL Server at all, and I've covered that information in more depth here.

Finally, in SQL Server 2005, you can even index views, computed columns, and create a special XML index. I'll cover those in other overviews as well.

Using Indexes Effectively

The basic premise in any of these situations is that you need to create an index on the columns that the users access the most for searching. For instance, in an address table, the last name, city, and state are normally prime candidates for an index. In that case, you might make an index for each of those three fields. You might also make a single index that has all three of these fields. I’ll describe that further in a moment. First, take a look at this table:

1

1

1

1

2

2

2

2

3

Now look at this table:

1

2

3

4

5

See the difference? If you have an index on first field of the second table, you'll end up with the same values in the index as in the table itself. In the first filed of the first table, you have a lot of the same values, and they will potentially end up all over the drive. If users need to look up that data frequently, that's the column you should index. We'll cover these decisions more in the other tutorials I've mentioned.

You’ll normally want to create indexes on the fields in a query’s predicate (the WHERE clause). You may include more than one column in an index. Including more than one column in an index is called a composite index. If your users are including the AND predicate, then you may need to include all those columns in a single index. Don’t go crazy with that, as anything above five or so columns will usually cost you.

To follow on with that concept, sometimes you’ll create a compound index that has all of the fields a user might select at one time. Creating an index on all those fields is called a covering index. What this means is that all of the data you're query needs is included in the index table itself, so it never has to use the pointer at all – it's kind of like having the advantages of a clustered index without as much of the expense. In some cases that can really speed up the query.

If your database is used in an application that performs many inserts, edits, and deletes, then speedy access isn’t always the driving factor. Databases that are used in transaction-oriented solutions, such as a sales database, fit this model. In a database where sales are recorded, there aren’t a lot of look-up activities, other than perhaps checking inventory. These types of database systems are called On-Line Transaction Processing systems, or OLTP. The decisions regarding the use of indexes is important in OLTP databases, because adding an index slows down INSERT operations, as the index is kept up to date when the data changes. So indexes on this type of database are less useful, and sometimes can impact performance. That isn't to say you shouldn't have any indexes, because even insert operations are sometimes based on lookups.

The second type of database is more reporting or look-up oriented. Databases that match this description are those used at a public library, or those used to provide reporting information. When those systems provide multidimensional data lookups they are called On-Line Analytical Processing systems, or OLAP. It’s easier to design indexes for these databases, since the data doesn’t change as often and you’re usually familiar with the columns the user is searching on.

The next type is the hybrid transaction/reporting database. This type of database is used for both entry and reporting. This is the most difficult situation for designing indexes, because you need them to speed up access for reporting but you don’t need them because they will slow down access for data entry. Unfortunately, this is also the type you’ll encounter most often.

Creating indexes is more difficult to do with OLAP databases than with OLTP databases. As a matter of fact, it’s usually best to minimize indexes for OLTP. Of course, that’s assuming your database is used purely for transactions and not also for reporting. Unfortunately, that’s not normally the case.

The next question is how to determine which kind of physical index to use — clustered or non-clustered. There are some other good indicators that the index you’re making should be clustered. Since the table is physically re-arranged if it’s clustered, then it’s best if the data doesn’t change a lot. If the table with a clustered index on the first field looks like this:

1

3

4

6

7

Then to insert the values "2" and "5" into the table (which you'll remember is the index as well) everything has to move around to accommodate the insertion. It’s better if the data that will be inserted comes after the data that’s already there — at least on the columns where a clustered index is defined. That way no movement happens at all.

So now you can see that perhaps a great candidate for the one (and only) clustered index on a table is the primary key — the fields you have set as the ones that uniquely identify the rows in the table. And you also can see that picking a "meaningful" value like an identification number for a person is such a bad idea for a Primary Key. If the number changes or the person leaves, the whole table would need to be rearranged physically on the drive. If you pick a meaningless, incrementing number for the Primary Key it becomes a natural target for a clustered index, since so many lookups are based on the Primary Key.

Another consideration for making the index clustered is the length of the data. If the data length is short, and even better if it’s made up of integers, the insert operations are quick and the system can move the data around easily. It’s also a good idea if the data is unique. When the values are unique, there is less chance that you'll have to move the data to insert it.

You should also consider a clustered index if the data is within a range that the user will search on. Queries such as:

SELECT au_lname 
FROM authors 
WHERE au_lname like ’[A-L]%’

Work well with a clustered index on the au_lname column.

If the order of the data is similar to the order in which the user wants to get results, it’s also a great candidate for a clustered index.

If a column isn’t a candidate for a clustered index after all these considerations, then of course you’re looking at creating a non-clustered index. Luckily, this is the default option.

Check the references section that follows for more strategies on how you can create and maintain good indexes. What I've shown here are more guidelines than hard and fast rules. You mileage will vary.

Informit Articles and Sample Chapters

An overview of the mechanics of implementing effective indexes is here.

To learn more about index operations, see this tutorial.

SQL Server: Optimizing Database Performance Through Indexes, by Kevin Kline, Baya Pavliashvili.

Online Resources

SQL Server Performance asks: Will the SQL Server Query Optimizer will use your carefully crafted indexes?

Create and Modify SQL Server Indexes Properly — Learn how to determine the state of your indexes and understand whether that state should be changed. See how evaluating the construction and location of your system’s indexes and deciding whether to adjust their fill factors and padding can improve the system’s performance, by Drew Georgopulos.

Here are some tips on optimizing your SQL Server indexes.

Microsoft explains the architecture of indexes here.

Here is another general description of Indexes.

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Last Update: November 17, 2020