Indexes help to select and find records efficiently. There are two main types: clustered indexes which determine the physical order of data in a table, and non-clustered indexes which are maintained separately. Index organization involves data being stored in pages that can split when full, and fill factor determines the percentage of each page initially used for data storage. Good indexes have low maintenance columns with infrequent changes and cover commonly used query conditions, while poor indexes involve unused columns or those returning many rows. Unique constraints and indexes validate data uniqueness in the same way.

1. CIS282

2. Indexes
 Help to select and find records
 Serve as a “cross-reference”
 Can be based on one or more fields

3. Types of Indexes
 Clustered: How table is physically ordered
 Only one per table
 Non-Clustered: Index is maintained separately from
table (data)
 Can be stored in a separate file group to speed access
(find pointers using one drive, find records on a second
drive)

4. Index Organization
 Data in SQL Server is organized by ‘pages’
 When an index entry is inserted in a full page the data
is split between two pages
 By default each page is filled to allow two additions

5. Fill Factor
 Allows you to specify how much ‘room’ to leave open
on each page
 Specify the percentage of each page that is used initially
to store data
 Impacts how often page splits occur
 OLTP should have low percentage
 OLAP should have high percentage

6. “Good Index” Characteristics
 Low maintenance columns
 Values don’t change frequently
 Includes columns used frequently in Where clause
 Don’t have to go out to table for data
 Covered indexes include multiple columns where
related data is commonly used
 Help search for ranges of records

7. “Poor Index” Characteristics
 Columns not used in queries (where clause)
 Index doesn’t return one or few rows (selectivity)
 Exception: indexing foreign keys can speed up joins
significantly
 Many columns in index
 Few records in table

8. Unique Constraint v. Unique Index
 No significant differences between creating a unique
constraint and a unique index
 Data validation occurs in the same manner
 Query optimizer does not differentiate between a
unique index created by a constraint or created
manually

9. Unique Constraint or Unique Index?
 A unique index makes sense when uniqueness is a
characteristic of the data
 Know that will always have a different value
 If uniqueness must be enforced to ensure data
integrity, create a unique constraint
 Need to ensure that have a different value
 Unique constraint allows null
 Null treated as a value so can only occur once