Difference Between Clustered and Non-clustered Index

In SQL Server, indexes are utilized to accelerate the query process, resulting in an excellent performance. They are comparable to indexes in textbooks. If you need to locate a specific chapter in a textbook, you consult the index, locate the chapter's page number, and navigate straight to that page. Without indexes, locating the necessary chapter would have been a lengthy procedure.

The same holds true for database indexes. Without indexes, a DBMS must search through every entry in a database to extract the needed results. This procedure is known as table scanning and is exceedingly time-consuming. In contrast, if you establish indexes, the database will access the index and obtain the table entries directly.

SQL Server provides two kinds of Indexes:

• Clustered Index
• Non-Clustered Index

Before discussing their differences, let's first examine their definitions.

What is Clustered Index?

A clustered index is only formed if both of the following requirements are met:

• The data or file transferred to secondary memory should be sequential or sorted.
• There must be a key value, meaning that it cannot contain duplicate data.

Every time clustered indexing is applied to a table, just that table will be sorted. Similar to the primary key, only one clustered index may be created in a table. A clustered index is identical to a dictionary in which the entries are organized alphabetically.

Features:

• Only stores crucial values.
• Pointers to rows in a Heap or Clustered Index.
• Permits access to Secondary data.
• Index Scan and Index Seek operation.

Advantages:

• A clustered index is advantageous for range queries as the data is logically ordered by key.
• By replicating the clustered index on a separate filegroup, you may relocate a table to a different filegroup.

Disadvantages:

• The so-called Last Page Insert Latch Contention may be caused by data entered exclusively at the end of the Clustered Index, since there is a single hotspot at the end of the Clustered Index where numerous queries compete against one another while traversing the B-tree structure.

What is Non-clustered Index?

Non-Clustered Index is comparable to a book's index. The index of a book contains the chapter name and page number; if you want to read a certain subject or chapter, you may use the index to get straight to that page. There is no need to read every page of a book.

The data is stored in one location, while the index is kept in another. Since the data and non-clustered index are kept independently, a table may have many non-clustered indexes.

Features:

• The data in a table is stored as key-value pairs.
• It is possible to construct indexes for tables and views.
• It holds references to entries in the clustered index.
• Provides Index Scanning and Searching Capabilities.

Advantages:

• Additionally, a non-clustered index is employed to accelerate search operations. A non-clustered index, unlike a clustered index, does not dictate the actual order in which data are entered into a table. A non-clustered index is really kept in a different place than the data table.

Disadvantages:

• On the non-clustered index, the lookup operation is quite costly. Every time the clustering key is updated, a matching change must be stored on the non-clustered index.

Clustered Index vs. Non Clustered Index

Conclusion

Finally, we have come to the end of this detailed comparison between clustered index vs. non-clustered index. We hope you like this tutorial. We have started with a brief introduction to clustered index and non-clustered index. We also explored the advantages, disadvantages, and features of clustered index vs. non-clustered index. Finally, we have compared clustered index vs. non-clustered index.

Please let us know in the comment box if you have difficulty following along. Happy learning!

Related Questions

1. How many clustered and nonclustered indexes are there?

Each table may have just one clustered index. However, numerous non-clustered indexes may be created on a same table.

2. Is primary key a clustered index?

When you construct a PRIMARY KEY constraint, if a clustered index on the table does not already exist and you do not provide a unique nonclustered index, a unique clustered index is automatically built on the column or columns. The key column cannot contain NULL values.

3. Is B-tree a non-clustered index?

B-Tree index is another name for Non-Clustered Index. In a non-clustered index, the data is arranged logically. In a non-clustered index, rows may be stored physically in a different order than columns.

4. Can a SQL query use multiple indexes?

MySQL may use multiple-column indexes for queries that test all the columns in the index, as well as queries that test just the first column, the first two columns, the first three columns, etc.