With every SQL Server query execution, the primary goal is to fetch the requested data from the database using as few system resources as possible. This is where indexes play a major role in data retrieval. In this blog, we will learn about covering index in SQL Server with examples, along with clustered index and non clustered index in SQL Server as well, and how they are efficient for fetching data. 

What is a Covering Index in SQL Server? Overview

SQL Server database is known to store and manage a large amount of data. However, when the database grows from storing thousands of records to millions, it becomes quite challenging to fetch the data from the database. Every time a query is executed, the SQL Server engine looks for the requested record, and in case it is not available in an index, the engine looks for the required data in the complete table. With these additional lookups for the record, the consumption of CPU resources increases, further resulting in slowing down the query execution. 

This is where a covering index in SQL Server comes in handy. These indexes contain all the required columns for a successful query execution. As the name suggests, this index covers every column specified in the query, further eliminating the additional lookups by the SQL Server engine. To understand the covering index better, we need to learn about SQL Server indexes first. 

Clustered Index and Non Clustered Index in SQL Server

These are the two primary types of SQL Server indexes. The covering index in SQL Server can also be specified as a specially designed non-clustered index. But what is a non clustered index? Let’s take a look.

  • Clustered indexes in SQL Server is an index that determines the physical order in which the record rows are stored. As the data is organized in the index, SQL Server can easily locate and fetch the requested data. The clustered index is ideal for sorting and frequently queried columns. A table can have only a single clustered index, as the data can only be stored in one physical order. 
  • The non clustered index in SQL Server is a data structure that often stores the indexed column values along with a row locator for the actual data rows in the table. This index doesn’t change the storage order of the records like clustered indexes do. Nonclustered indexes also support included columns, making it optimal for creating covering indexes. 

Now that we know what a clustered index and non clustered index in SQL Server are, let’s take a look at how covering indexes work and the syntax to create one. 

Covering Index in SQL with Example – Working and Syntax Explained

This index stores the requested columns as well as the additional columns that satisfy the executed query. These columns allow the database engine to retrieve the requested record directly from the index, further eliminating the need to lookup complete table and reducing CPU usage as well. Let’s now take a look at the syntax of the covering index:

CREATE NONCLUSTERED INDEX ‘Index_Name’
ON Table_Name (key_column1, key_column2, ...)
INCLUDE (included_column1, included_column2, ...);

As we can see, the covering index is basically a non clustered index created using key columns for searching, and the INCLUDE clause is used for additional columns required by the query. Let’s take a look at a similar example now:

Let’s assume there is a table named Students, having the following columns:

  • StudentID
  • Name
  • Age
  • Address
  • Contact_Number

Now, let’s execute a normal query to retrieve records from this table:

SELECT Name, Address
FROM Students
WHERE Age = ‘18’

Here, SQL Server might use the index on the Age column to locate the required rows, but it would still require accessing and looking up the base table. To prevent the complete database lookup, a covering index can be created. The syntax to create a covering index is as follows:

CREATE NONCLUSTERED INDEX IX_Students_Age
ON Students (Age)
INCLUDE (Name, Address);

In this index, Age is the key column that will help filter the rows. Additionally, Name and Address are the included columns stored in the index to fulfil the SELECT statement. Now, these indexes help with improving SQL Server performance and enhancing query optimization; however, there are many scenarios where corruption impacts query execution and also brings the complete database at risk of inaccessibility. Let’s learn more about this index corruption and how to resolve it.

How to Repair Covering Index Corruption in SQL Server? Complete Solution

When we work with covering index in SQL Server, the most common question users have is ‘can covering indexes become corrupted?’ and the answer is yes. Even though the indexes help improve performance in the database, many reasons and causes can corrupt these indexes as well. We will now take a look at some common reasons for corruption in covering indexes and an optimal solution for the issue as well.

  • Sudden power outage
  • MDF file corruption
  • Storage failure
  • File system errors
  • SQL Server page corruption

These are a few common causes that lead to corruption in covering indexes. In such scenarios, dedicated tools like SysTools SQL Recovery Tool come in handy.

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This tool is capable of repairing index corruption in a quick and hassle-free way. The tool also helps repair and recover data from other database objects as well. Let’s take a quick look at the steps on how to use the utility:

  1. Install and run the suggested software. Click on Open to browse .mdf file.browse mdf files
  2. Next, choose a scan mode to detect corruption in the covering index in SQL Server.scan files
  3. After the scan, preview the recovered database objects along with the indexes.preview recovered data
  4. Click on the Export Tab to proceed with the repair of index corruption. recover covering index in sql server
  5. Then, select a desired destination to save the recovered indexes, and click on the Export button.export recovered data

With the help of these steps, users can efficiently recover data from indexes and save them to a healthy database. 

Conclusion

With the help of this technical guide, we have learned about covering index in SQL Server and how they work. To understand the concept better, we have also discussed clustered index and non clustered index in SQL Server. In case of covering index corruption, we have recommended an optimal solution that can help resolve the issue as well as restore the data to a healthy database.