Clustered Index vs. Non-Clustered Index

Attribute	Clustered Index	Non-Clustered Index
Physical order of data	Data is physically stored in the order of the index key	Data is not physically stored in the order of the index key
Number of indexes per table	Only one clustered index per table	Multiple non-clustered indexes per table
Impact on INSERT, UPDATE, DELETE operations	Slower for INSERT, UPDATE, DELETE operations as data needs to be physically reorganized	Faster for INSERT, UPDATE, DELETE operations as data does not need to be physically reorganized
Storage space	Requires less storage space compared to non-clustered index	Requires more storage space compared to clustered index

Introduction

When it comes to organizing and accessing data in a database, indexes play a crucial role. Two common types of indexes used in database management systems are Clustered Index and Non-Clustered Index. Both types have their own set of attributes and advantages, which make them suitable for different scenarios.

Clustered Index

A Clustered Index is a type of index in which the physical order of the rows in the table is the same as the order of the key values. This means that the data rows in a table are stored in the same order as the index key, which allows for faster retrieval of data based on the key values. When a Clustered Index is created on a table, the table is physically reordered based on the index key.

One of the key advantages of a Clustered Index is that it can improve the performance of queries that involve range searches or sorting operations. Since the data rows are stored in the same order as the index key, the database engine can quickly locate and retrieve the required data without having to perform additional sorting operations.

Another advantage of a Clustered Index is that it can reduce the amount of disk space required to store the index. Since the data rows are physically ordered based on the index key, there is no need to store additional pointers to the data rows, which can result in a more compact index structure.

However, one limitation of a Clustered Index is that there can be only one Clustered Index per table. This means that if a table already has a Clustered Index, you cannot create another Clustered Index on the same table. Additionally, when a Clustered Index is created on a table, the data rows are physically reordered, which can lead to fragmentation and decreased performance if the table undergoes frequent insertions and deletions.

In summary, a Clustered Index offers fast retrieval of data based on key values, improved performance for range searches and sorting operations, and reduced disk space requirements. However, it is limited to one per table and can lead to fragmentation with frequent data modifications.

Non-Clustered Index

A Non-Clustered Index is a type of index in which the physical order of the rows in the table is different from the order of the index key values. In a Non-Clustered Index, the index key values are stored separately from the actual data rows, and the index contains pointers to the data rows. This allows for faster retrieval of data based on the index key values.

One of the key advantages of a Non-Clustered Index is that it can improve the performance of queries that involve searching for specific values or joining multiple tables. Since the index key values are stored separately from the data rows, the database engine can quickly locate the required data rows using the pointers in the index.

Another advantage of a Non-Clustered Index is that it allows for the creation of multiple indexes on a single table. This means that you can create multiple Non-Clustered Indexes on different columns of a table to improve the performance of various types of queries.

However, one limitation of a Non-Clustered Index is that it can result in higher disk space requirements compared to a Clustered Index. Since the index key values are stored separately from the data rows, additional disk space is required to store the index and the pointers to the data rows.

In summary, a Non-Clustered Index offers fast retrieval of data based on key values, improved performance for specific value searches and joins, and the ability to create multiple indexes on a single table. However, it can result in higher disk space requirements compared to a Clustered Index.

Comparison

When comparing Clustered Index and Non-Clustered Index, it is important to consider the specific use cases and requirements of the database. A Clustered Index is ideal for tables that are frequently queried for range searches or sorting operations, as it offers fast retrieval of data based on key values and reduced disk space requirements.

On the other hand, a Non-Clustered Index is more suitable for tables that are frequently queried for specific value searches or joins, as it allows for the creation of multiple indexes on a single table and offers fast retrieval of data based on key values.

Ultimately, the choice between a Clustered Index and Non-Clustered Index depends on the specific performance requirements and data access patterns of the database. By understanding the attributes and advantages of each type of index, database administrators can make informed decisions to optimize the performance of their databases.