Power of Index in MS SQL Server: A Comprehensive Guide

Index in MS SQL Server

When we talk about relational databases then speed and efficiency are the most important factors of a SQL Server. The speed of a database depends on many components. The important component in ease of use is retrieving data from the database, here index plays its role. Indexes works as a silent hero in the server which accelerates any query and optimises the performance.

Today in this block post we will talk about indexing in SQL Server Database, empowering you to harness its potential and unlock the power of your SQL server database.

What is an index in MS SQL Server?

At its core, an index is a database object that enhances the speed of data retrieval operations on a database table. It organizes table data based on specific columns, allowing the database engine to locate and retrieve targeted information significantly faster than scanning the entire table. Imagine a library – searching for a specific book by title is much quicker when you have an alphabetized catalog compared to manually browsing every shelf.

  • An index can be described as a pointer to a row on a table.
  • An index can be created on a table or a view.
  • The purpose of an index is to retrieve data from a table efficiently and fast.

How to use an index in an SQL Server?

The CREATE INDEX Command. This command is used for generating the table index in SQL using the build index expression. At the time of creating the table, it will allow you to accept the duplicated data into the table.

How Does Index Work?

Indexing involves creating a data structure (the index) that improves the speed of data retrieval operations on a database table. SQL Server uses a B-tree structure, efficiently organizing and storing index keys. (Index in MS SQL Server)

Read More: MS SQL Server Architecture 3 Layers

How many indexes are there in SQL Server?

Types of Indexes in MS SQL Server: MS SQL Server offers a variety of index types, each catering to specific scenarios and data access patterns. Here’s a breakdown of the most common ones:-

  • Clustered Indexes
  • Non-clustered Indexes
  • Unique Indexes
  • Primary Key Indexes

1) Clustered Index

What is a clustered Index?

These unique indexes physically order the table data based on the indexed column(s). This means the data itself is stored in the same order as the index, offering exceptional performance for queries that involve retrieving data in sorted order or using specific range filters. However, only one clustered index can exist per table, and updates or insertions involving the indexed column(s) can incur additional overhead due to data reorganization.

If a table does not have an index, it is called the ‘Heap‘ table.

2) Non-Clustered Index

What is a non-cluster Index?

This is the most prevalent index type. Unlike clustered indexes, non-clustered indexes maintain a separate structure that maps to the actual table data. They offer excellent performance benefits for queries that filter or sort data based on the indexed column(s) without needing the data in a specific order. Multiple non-clustered indexes can coexist on a single table, providing flexibility in optimizing different query patterns.

A non-cluster index has a separate structure from the data rows. It contains key values and each key-value entry has a pointer to the data row. A non-clustered index does require disk space. For a cluster table, the row locator is the cluster index key. For a heap, a row locator is a pointer to a row.

3) Unique Index

What is a Unique Index? (Index in MS SQL Server)

These indexes enforce uniqueness, ensuring no duplicate values exist in the indexed column(s). This is crucial for maintaining data integrity and preventing invalid entries. Unique indexes can be either clustered or non-clustered.

A Composite Index can be created on both clustered and non-clustered indexes.

4) Primary Key Index

What is a Primary Key Index?

Every table can have only one primary key, which is a unique identifier for each row. By default, SQL Server automatically creates a clustered index on the primary key, ensuring efficient data retrieval and data integrity enforcement.

When to use clustered or non-clustered indexes?

  1. The number of indexes requires on a table, non-cluster if using more than one index.
  2. SELECT operations: clustered index is useful
  3. INSERT/UPDATE operations: non-clustered indexes are faster as sorting is not needed.
  4. Disk Space: Non-clustered index consumes disk space.
  5. Every table should have one clustered index on the column which has unique values.

(Index in MS SQL Server)

When to create an Index?

  1. When the table is large (row count)
  2. When the table is used in queries frequently
  3. When columns are frequently used in the ‘WHERE’ clause
  4. Columns that are frequently referenced in ‘Order by’ and ‘Group by’ sortings
  5. The index should be created on the column which has a high number of unique values in it.

Read More: Important Scripts for MS SQL DBA

When not to use an Index?

  1. Index on small tables
  2. The index should not be used on columns that contain a high number of ‘Null’ values

Tip: Using the SQL Profiler and Database Engine Tuning Advisor tools, you can find where the index is required.

Which index is better in SQL Server?

While indexing offers substantial benefits, it’s essential to adopt a strategic approach to avoid potential drawbacks. Here are some key considerations when deciding whether or not to create an index, and which type to choose:

  • Query Patterns: Analyze your most frequently executed queries and identify the columns involved in filtering, sorting, and joining operations. These are prime candidates for indexing.
  • Selectivity: The selectivity of an index refers to the percentage of rows that match a specific search condition. Indexes are most beneficial for columns with high selectivity, where they can significantly reduce the data scanned by the engine.
  • Insert, Update, and Delete (DML) Operations: Creating and maintaining indexes involves overhead. Frequent data modifications involving indexed columns can impact performance. Evaluate the trade-off between query performance gains and potential slowdowns in DML operations.

Reorganize and Rebuid Index

Whenever an INSERT, UPDATE and DELETE operation occurs against the underlying data, SQL Server automatically maintains the indexes. These operations cause index fragmentation and cause performance issues. Fragmentation exists when indexes have pages in the logical ordering based on the key values and it does not match the physical ordering inside the data files. To tackle this issue we reorganize or rebuild the index. (Index in MS SQL Server)


When we rebuild an index the SQL Server drops and re-creates the index and removes fragmentation, reclaims disk space by compacting reorders the index rows in contiguous pages.

Reorganize Index:

It will physically reorder the leaf level. The system function is used to check index fragmentation:

If avg_fragmentation_in_percent value is:
avg_fragmentation_in_percent > 5% or < 40% ==> Reorganize index
avg_fragmentation_in_percent > 40% ==> Rebuild index
  • Rebuild an index can be online or offline.
  • Reorganizing an index is always online.

Rebuild Index Online:

In online mode, the new index is built while the old index is accessible to read and write. When the process is completed the table is locked for a brief period and the new index replaces the old index.

Rebuild index offline:

In offline mode, the table is locked upfront for any read and write operation and then the new index gets built from the old index while holding locks on the table no read and write operation is permitted on the table. (Index in MS SQL Server)

Implementation and Best Practices: Optimizing Your Indexes

Once you’ve identified the optimal columns for indexing, it’s time to put theory into practice. Here are some best practices to follow:

  • Start with the most frequently used columns: Prioritize indexing columns involved in filtering, sorting, and joining operations in your most critical queries.
  • Consider covering indexes: Covering indexes include additional columns in the index structure beyond the primary indexed column(s). This can further enhance query performance by avoiding additional table scans to retrieve non-indexed columns used in the query’s WHERE clause.
  • Monitor and maintain your indexes: Regularly analyze your indexes to assess their effectiveness and identify opportunities for optimization. Over time, data distribution and query patterns might change.


In conclusion, mastering the art of indexing in MS SQL Server is pivotal for achieving optimal database performance. By understanding the types of indexes, implementing best practices, and recognizing their impact on performance, you can empower your database to deliver efficient and responsive operations.

Remember, indexing is not a one-size-fits-all solution; it requires thoughtful consideration and ongoing maintenance. So, leverage the power of indexing wisely, and watch as your SQL Server database achieves new heights of speed and efficiency. (Index in MS SQL Server)

What are SQL Server indexes used for?

SQL Server indexes are special data structures that act like organized catalogs for your tables. They accelerate data retrieval by enabling faster searching and filtering of specific rows. Imagine an index like an alphabetized phonebook; searching for a name becomes much quicker compared to checking every entry sequentially.

Where are indexes stored in SQL Server?

By default, indexes reside in the same filegroup as the base table they are associated with. This means they are physically located on the same storage unit, ensuring efficient access during query execution. However, you can also customize their placement across different filegroups for specific performance or administrative needs.

What are SQL Server indexes, and how do they work?

Think of SQL Server indexes as optimized subsets of your table data. They are essentially ordered and filtered copies created to improve query performance. These optimized structures use specific operators like seeks, scans, and lookups to navigate the index and locate the desired rows efficiently.
Similar to how a map helps you find a specific location quickly, an index helps SQL Server locate relevant data faster within your tables.

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