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1 Answers

ICSM Computer
ICSM Computer 05 Apr 2026
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An Index in SQL Server is a database object that improves the speed of data retrieval from a table—just like an index in a book helps you quickly find a topic without scanning every page.

In Microsoft SQL Server:

An index creates a sorted structure (like a tree) that allows fast searching, filtering, and sorting of data.

Why Use Indexes?

  • Faster SELECT queries
  • Efficient searching and filtering
  • Improves JOIN, ORDER BY, GROUP BY performance

But:

  • Slows down INSERT, UPDATE, DELETE
  • Takes extra storage

Types of Indexes in SQL Server

1. Clustered Index

  • Physically sorts and stores the data in the table
  • Only one clustered index per table
  • Data rows are stored in order of the index key

Example:

CREATE CLUSTERED INDEX IX_Employee_Id
ON Employee(Id);

Use Case:

  • Primary key columns
  • Frequently sorted or range queries

2. Non-Clustered Index

  • Does not change physical order of table
  • Creates a separate structure with pointers to data
  • You can have multiple non-clustered indexes

Example:

CREATE NONCLUSTERED INDEX IX_Employee_Name
ON Employee(Name);

Use Case:

  • Frequently searched columns
  • Filtering (WHERE Name = 'John')

3. Unique Index

  • Ensures all values in the column are unique
  • Prevents duplicate entries

Example:

CREATE UNIQUE INDEX IX_Email
ON Users(Email);

4. Composite Index (Multi-Column Index)

Index on multiple columns

Example:

CREATE INDEX IX_Name_Department
ON Employee(Name, Department);

Use Case:

  • Queries using multiple columns in WHERE clause

5. Filtered Index

  • Index on a subset of data
  • Smaller and faster than full index

Example:

CREATE INDEX IX_ActiveUsers
ON Users(IsActive)
WHERE IsActive = 1;

6. Full-Text Index

  • Used for searching text data (large text fields)
  • Supports complex queries like word matching

7. Columnstore Index

  • Stores data column-wise instead of row-wise
  • Best for analytics and reporting

Use Case:

  • Large datasets
  • Data warehousing

8. XML Index

Used for querying XML data efficiently

9. Spatial Index

Used for geographic or spatial data

Clustered vs Non-Clustered (Quick Comparison)

Feature Clustered Index Non-Clustered Index
Data Storage Physically sorted Separate structure
Count One per table Multiple allowed
Speed Faster for range queries Faster for lookups

When to Use Indexes

  • Columns used in WHERE, JOIN, ORDER BY
  • Frequently queried columns
  • Large tables

When NOT to Use

  • Small tables
  • Frequently updated columns
  • Too many indexes (can reduce performance)

Simple Analogy

  • Clustered Index → Like a dictionary (data is sorted A–Z)
  • Non-Clustered Index → Like a book index (points to page numbers)
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