SQL Indexes

Updated March 16, 2023

Introduction

SQL Indexes are primarily used for data retrieval purposes. When retrieving data from a database, SQL Indexes significantly carry out the process at a faster rate. If you want fast data retrieval from a database, SQL Indexes are an option. The concept of data indexing promises better performance for associated applications and queries.

Every book comprises an index, probably in the last section. You see that there are lists or column-wise words or phrases given with page numbers mentioned beside those. This page helps in identifying certain words or phrases from the book within the shortest time. The same concept works for SQL Indexes. The data present in the table of SQL can be easily sorted out using the indexes chart.

Key Highlights

• SQL indexes are special tables created to make the data searching process easier in a database
• SQL Indexes make the data retrieval process faster.
• Using SQL Indexes, users can quickly search for records and data from a vast database.
• The basic commands of SQL Indexes are CREATE INDEX, CREATE UNIQUE INDEX, and DROP INDEX.
• There are 6 types of SQL indexes: clustered, non-clustered, unique, filtered, column store, and hash.

What are SQL Indexes?

SQL indexes are reference lists for users to make their search process easier and faster. Using the SQL Indexes table, it becomes a lot easier to find records and data that users are searching for from the database.

How do SQL Indexes work?

To keep it simple, SQL indexes help maintain a segregated, ordered table to improve the efficiency level of the search query process.

If you look at an unindexed table, sorting data from it will be a cumbersome procedure. It will be time-consuming and extremely tough. You, as the user, will have to look through every row and column of the table to search for the desired data. Therefore, the concept of indexing proves to be an efficient option for data retrieval and searching procedures.

Examples

Have a look at an unindexed table EMP_Details:

Emp name	Emp id	Dept
Jack	851	FN
Ravi	259	DB
Megha	455	HR
Hazel	369	DB
Joseph	345	DB
Sandy	785	FN
Mohan	547	DB

Now, let’s search a specific data by inserting the following code:

Code:

SELECT
	Emp name,
	Emp id,
	Dept 
FROM
	EMP_Details
WHERE
	Emp id = 785

What will this do?

The process will search through every row and column from the table before coming up with the search query result. As you can understand, this is a lengthy process to derive the required data from the table.

Looking at the above table, you may not get an idea of the time-consuming stature of this process. Tables inside a database are longer than what you may expect. Say, for example, there are one million rows in a table. Can you imagine the time it will take to look for specific data from such a humongous list? This is where indexing proves effective.

Have a look at the above table, but the Dept section indexed:

Emp name	Emp id	Dept
Jack	851	FN
Sandy	785	FN
Ravi	259	DB
Hazel	369	DB
Joseph	345	DB
Mohan	547	DB
Megha	455	HR

Since the above table shown has the Dept section indexed, searching for Dept FN would be easier and less time-consuming. When the search process commences, the database will search for all the Dept with FN values. Since indexed, it will stop searching once the FN value list ends.

In actuality, the table within the database cannot auto-reorganize itself each time the query criteria change to maximize the query performance because that would be impossible.

The database creates a data structure as a result of the index.

Probably a B-Tree is the type of data structure. Though the B-Tree has several benefits, its ability to be sorted is the most important one for your purpose. Because of the clear benefits we mentioned earlier, an ordered data structure improves the effectiveness of your search.

How to Create SQL Indexes Database Statements?

When creating Indexes in SQL Database, you need to use the CREATE INDEX statement.

As mentioned in the above sections, indexes help retrieve data from a database much faster than regular procedures. Using indexes, searching for data becomes faster and a lot easier.

Wait, you need to know another thing. Indexes are best suited only for those columns that are expected to have frequent searching. When updated with indexes, it is because a table will take a lot more time compared to an unindexed regular form table.

The CREATE INDEX syntax is used for creating SQL indexes database statements.

CREATE INDEX index_name
ON table_name (column1, column2, ...);

Example:

Consider the table EMP_Details we used in the above section.

CREATE INDEX Dept
ON EMP_Details (Dept, Emp id);

Types of SQL Indexes

Different types of Indexes in SQL server are given below:

1. Clustered Indexes

• In Clustered Indexes, the fundamental values of the rows of data are used for storing the data in a tabular format or for easy viewing.
• This type of index is required primarily when there is a requirement for modification of a huge amount of data within a database.

2. Non-Clustered Indexes

• In Non-Clustered Indexes, a structure is not common to the data rows. This structure is separate and comprises non-clustered key values.
• Non-Clustered Indexes make it easier for users to include non-key columns at the lead level.

3. Unique Indexes

• In Unique Indexes, you will find unique values for every row in the index key.
• There are no duplicate items present. This suggests that every row in the table is unique.

4. Filtered Indexes

• In Filtered Indexes, you will notice a very minimal amount of relevant values in a column for querying purposes.

5. Column Store Indexes

• In Column Store Indexes, you will notice a large amount of data stored in columns.
• This index helps to improve the overall query performance when dealing with high-quantity data.

6. Hash Indexes

• In Hash Indexes, you will notice unique values like an email address or primary key on the column upon which it is created.
• This type of index is popular due to its fast performance.

SQL Indexes Command

Here are the following commands for SQL Indexes:

1. Create Index command

Used for creating index values on a table. Here, duplicate values can be used

Example (please refer to the EMP_Details table)

CREATE INDEX IDX_EMP
ON EMP_Details (Dept);

2. Create Unique Index command

Used for creating index values on a table. Here, no duplicate values can be used

CREATE UNIQUE INDEX index_name
ON table_name (column1, column2, ...);

Example (please refer to the EMP_Details table)

CREATE UNIQUE INDEX UIDX_EMP 
ON EMP_Details (Dept);

3. Drop Index

Used for deleting an index value from the table.

DROP INDEX table_name.index_name;

Example (please refer to the EMP_Details table)

DROP INDEX EMP_Details, IDX_EMP;

SQL Indexes rename and remove

When you rename an index, it replaces the existing index name with the new name you just used for renaming the same. However, you must ensure that the new name is unique within the view or table.

The command used for renaming the index is sp_rename.

EXEC sp_rename 
    index_name, 
    new_index_name, 
    N'INDEX';

To delete or remove indexes from a table, you need to use the DROP INDEX command.

DROP INDEX table_name.index_name;

Conclusion

SQL Indexes are portable and designed to make it easier for users to search for records and data compared to looking into those large, complicated tables. These indexes work best for sizable structured SQL databases.

FAQs

Q1. What is the importance of indexes in SQL?

Answer: Queries utilize indexes to find information in tables quickly. Views and tables both have indexes built in. Indexes make it easier to search for specific data from a table containing hundreds of columns and rows. Proper indexing makes the search process smoother and faster.

Q2. How can you differentiate between clustered and non-clustered indexes in SQL?

Answer: In SQL, you can have a single clustered index for a table. However, there can be multiple numbers of non-clustered indexes for the table. As far as the speed is concerned, clustered indexes are faster compared to the non-clustered variants. Clustered indexes never require extra disk space but the same is not true for non-clustered indexes.

Q3. When is the best time to create indexes?

Answer: The best time to create indexes in SQL is when the database consists of vast data and values (not null values). Creating indexes helps in the fast retrieval of data.

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