Types Of Index In Sql

Types of SQL Server Indexes

In SQL, an Index is like a special table that enhances data retrieval speed from the database tables. It’s similar to an index in a book and significantly improves query performance. However, it can slow down UPDATE and INSERT operations while speeding up SELECT and WHERE queries. 

Indexes are created on table fields and can be added or removed without impacting the database’s actual data. Let us look into this article to gain a deeper understanding of the various types of indexes in SQL Server.

Why SQL Server Index?

An SQL table index accelerates data retrieval through efficiently locating matching rows and columns for queries. Instead of having to search through every row, it enables users to quickly access a portion of the table for exact results. 

To keep this efficiency, a logically ordered duplicate of the database needs to be stored in more storage space. Complete data is stored in this index and is logically arranged in rows and columns. The format can be either column-wise (known as Columnstore) or row-wise (known as Row Store).

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Types of Indexes in SQL Server

The following are six different types of indexes in SQL Server

Clustered Index

Clustered indexes organize and store data from tables or views depending on their primary values. There is usually only one clustered index per table, which enables data to be arranged in a particular sequence. When data is kept in this sorted order, it is referred to as a clustered table. 

For effectively handling significant alterations to data inside a database, clustered indexes are suitable. Data in a database or table that isn’t arranged in either ascending or descending order is referred to as a heap.

Non-Clustered Index

A non-clustered index is a structure that’s separate from data rows in SQL. It stores non-clustered key values, each paired with a pointer to a crucial data row. The pointer connecting an index row to a data row is called a row locator. This row locator helps identify whether data pages are part of a clustered table or a heap. In a clustered table, the row locator is known as the clustered index key. 

Non-clustered indexes also allow the addition of non-key columns at the leaf level, which helps overcome key limitations and enables fully covered indexed queries. These types of indexes in SQL are created to enhance the overall performance of frequently asked queries not covered by clustered indexes.

Column store Index 

A column store index in SQL Server is a sort of index that is designed for efficiently storing and searching massive data warehouse fact tables. It’s designed to speed up queries in situations with large amounts of data. Performance is greatly improved by this index, which enables data to be stored in small locations. 

Using this index can result in up to ten times greater query performance when compared to conventional row-oriented storage. Compared to other SQL indexes, the Columnstore Index performs better because it stores values from the same location with similar values, increasing total data compression rates.

Filtered Index

A filtered index is used when a column includes a small number of relevant values for specific queries. This type of index in SQL can be generated for one or more data types, even in a database containing a variety of data rows. The query optimizer suggests this filtered index even if it does not explicitly cover any query. 

Compared to full-table indexes, this improves query performance while lowering index maintenance and storage expenses. Furthermore, data changes have a lower impact on a filtered index because it is updated only when new entries are added or the indexed data is changed.

Unique Index

To guarantee that every table row is unique in SQL Server, a unique index makes sure that the index key is free of duplicate entries. When preserving the uniqueness of data properties is essential for preserving data integrity, this kind of index is especially useful. It also provides more details about the database table, which is advantageous for query optimization.

Hash Index

A hash index is one of the types of index in SQL Server where each slot or bucket contains a record. These indexes are made using arrays of N buckets or slots with pointers. It is based on the Hash function F(K, N), where N is the number of buckets and K indicates the key. This function associates the key with a certain bucket in the hash index. 

Eight bytes are stored in each bucket of the hash index in order to store the memory address of the linked list of primary entries.


In conclusion, SQL Server provides a variety of index types to improve database efficiency and streamline query processing. SQL Server training in Chennai is essential for mastering these types of indexes in SQL, ensuring database integrity, and achieving excellent performance. Choosing the right index type is essential for ensuring database integrity and achieving excellent performance.

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