Welcome Dev, in this article we are going to dive deep into the concept of Index in SQL Server. If you are a seasoned developer or just starting out, having a good understanding of indexes is crucial to optimize your database performance. So, let’s get started!
What is Index in SQL Server?
Index in SQL Server is an object that helps to improve the performance of your database by allowing fast retrieval of data rows. In simple terms, an index is like a pointer to the data rows in a table. Instead of scanning the entire table, indexes can be used to retrieve data quickly and efficiently.
When you create an index on a table, SQL Server creates a data structure that holds the indexed columns’ values and a pointer to the table’s data rows. This data structure is called B-Tree or Balanced Tree. It is a hierarchical data structure that allows for quick searches and retrievals of data.
Types of Indexes
SQL Server supports various types of indexes. Let’s take a look at some of the most commonly used indexes:
Clustered Index
A clustered index is a type of index in SQL Server that determines the physical order of data in a table. In other words, when you create a clustered index on a table, the data rows in the table are physically ordered based on the index key value. A table can have only one clustered index because it determines the physical order of data in the table.
Let’s consider an example. Suppose we have a table named Employee with columns such as Emp_Id, Emp_Name, Emp_Salary, and Emp_Department. If we create a clustered index on Emp_Id column, the data rows in the table will be physically ordered based on the Emp_Id value.
Non-Clustered Index
A non-clustered index is a type of index in SQL Server that does not affect the physical order of data in a table. It is created on a separate structure known as B-Tree, which holds the indexed column’s values and a pointer to the data row in the table. A table can have multiple non-clustered indexes, and it is recommended to create non-clustered indexes on columns that are frequently used in search conditions.
Let’s consider the same example we discussed above. We can create a non-clustered index on Emp_Name column to perform searches on the Employee table based on the employee name.
Unique Index
A unique index is a type of index in SQL Server that enforces the uniqueness of values in one or more columns. It works similar to non-clustered indexes, but it does not allow duplicate values in the indexed columns. A table can have multiple unique indexes, and a column can be included in only one unique index.
For example, we can create a unique index on Emp_Id column in the Employee table to ensure that each employee has a unique id.
Filtered Index
A filtered index is a type of index in SQL Server that is created on a subset of data rows based on a filter predicate. It helps to improve the performance of queries that work on a specific subset of data rows. A filtered index can be created only as a non-clustered index.
For example, suppose we want to create an index only on the employees with a salary greater than 50000. In that case, we can create a filtered index on Emp_Salary column with the filter predicate ‘Emp_Salary > 50000’.
How Indexes Work in SQL Server
Indexes in SQL Server work by creating a data structure known as B-Tree. It is a hierarchical data structure that allows for quick searches and retrievals of data. When you create an index on a table, SQL Server creates a B-Tree structure that holds the indexed column’s values and a pointer to the data row in the table.
When you execute a query that uses an indexed column, SQL Server uses the B-Tree structure to quickly locate the data rows that match the search criteria. It performs a binary search operation to find the exact match, and once it finds the first matching row, it uses the pointer to access the data row in the table.
For example, suppose we want to retrieve all the employees with a salary greater than 50000. If we have an index on the Emp_Salary column, SQL Server will use the B-Tree structure to quickly locate the data rows that match the search criteria rather than scanning the entire table.
How to Create Indexes in SQL Server
Creating indexes in SQL Server is a straightforward process. You can create indexes on a table using SQL Server Management Studio or Transact-SQL language. Let’s take a look at some of the commonly used commands to create indexes:
Creating Clustered Index
Command |
Description |
|---|---|
CREATE CLUSTERED INDEX index_name ON table_name (column_name) |
Creates a clustered index on a table |
For example, if we want to create a clustered index on the Emp_Id column in the Employee table, we can use the following command:
CREATE CLUSTERED INDEX idx_Emp_Id ON Employee(Emp_Id)
Creating Non-Clustered Index
Command |
Description |
|---|---|
CREATE NONCLUSTERED INDEX index_name ON table_name (column_name) |
Creates a non-clustered index on a table |
For example, if we want to create a non-clustered index on the Emp_Name column in the Employee table, we can use the following command:
CREATE NONCLUSTERED INDEX idx_Emp_Name ON Employee(Emp_Name)
Creating Unique Index
Command |
Description |
|---|---|
CREATE UNIQUE INDEX index_name ON table_name (column_name) |
Creates a unique index on a table |
For example, if we want to create a unique index on the Emp_Id column in the Employee table, we can use the following command:
CREATE UNIQUE INDEX idx_Emp_Id ON Employee(Emp_Id)
Creating Filtered Index
Command |
Description |
|---|---|
CREATE NONCLUSTERED INDEX index_name ON table_name (column_name) WHERE filter_predicate |
Creates a filtered index on a table |
For example, if we want to create a filtered index on the Emp_Salary column in the Employee table with a filter predicate ‘Emp_Salary > 50000’, we can use the following command:
CREATE NONCLUSTERED INDEX idx_Emp_Salary ON Employee(Emp_Salary) WHERE Emp_Salary > 50000
FAQ
Why do we need Indexes in SQL Server?
Indexes are essential in SQL Server to optimize the performance of your database. Without indexes, SQL Server would scan the entire table to retrieve data rows that match the search criteria, resulting in slow response times. With indexes, SQL Server can quickly locate the data rows that match the search criteria, resulting in faster response times.
How many indexes should we create on a table?
There is no fixed number of indexes that you should create on a table. It depends on the size of the table, the query workload, and the frequency of data modification operations. In general, you should create indexes on columns that are frequently used in search conditions and avoid creating too many indexes as it can lead to performance issues during data modification operations.
Can we modify an index after it has been created?
Yes, you can modify an index after it has been created. You can alter an existing index to change its properties or drop the index and create a new one with modified properties.
What is the difference between Clustered and Non-Clustered Index?
The primary difference between clustered and non-clustered index is the order in which data is stored in a table. A clustered index determines the physical order of data in a table, while a non-clustered index does not affect the physical order of data in a table. A table can have only one clustered index, and it determines the physical order of data in the table, while a table can have multiple non-clustered indexes.
What is the performance impact of creating too many indexes on a table?
Creating too many indexes on a table can have a negative impact on the table’s performance, especially during data modification operations such as insert, update, and delete. It can also lead to increased storage requirements and slow down the database backups and restores.
Can we create an index on a view?
Yes, we can create an index on a view. A view is a virtual table that does not store data physically, but we can create an index on a view to improve its performance.
Conclusion
In this article, we have covered the concept of index in SQL Server in detail. We have discussed the types of indexes, how they work, and how to create them. We have also answered some frequently asked questions related to indexes in SQL Server. Having a good understanding of indexes is crucial for optimizing your database performance, and we hope this article has helped you in that regard.