What’s a Clustered Index and How Does it Work?

What's a Clustered Index and How Does it Work?

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A clustered index is a valuable tool in the SQL Server. If you are interested in the field of scripting and creation of web pages, you need to know what clustered indexes are. In any event, you will probably meet it in an interview for an SQL-related job.

What is a Clustered Index?

It is an on-disk structure that determines the order on which records (rows) should arrange themselves?physically. Such an index holds its data in its leaf nodes. It is important in speeding up the retrieval and viewing of data rows from the table. An index has keys developed from one or multiple columns within the table.

You can find the keys in?structures called B-trees. These allow the SQL server to locate the single or multiple rows associated with key values efficiently.

Clustered vs Nonclustered Index

Clustered indexes organize and store data rows in the table depending on their key values. A table is only limited to a single index that is clustered in nature since data rows can only be arranged in one order.

Data rows can only be stored in a sorted order if the table itself contains an index that is clustered. Therefore, that kind of a table is referred to as a clustered table while the data structure without it is known as a heap. This is because data rows aren?t arranged in a particular order.

The nonclustered indexes, on the other hand, have structures that are different from the data rows. They have a nonclustered index key with each key value entry containing a pointer to the data row with the key content. In a nonclustered index, the pointer from an index to a record is referred to as a row locator. The nature of the row locator is dependent on the nature of arrangement of the data pages, either in a heap or ordered rows.

In a clustered table, the clustered index key usually acts as a locator of the row.

Advantages of the Clustered Index

? The data is arranged physically by the clustered key in your storage subsystem.

These indexes scale very well because they use a B-tree structure. The SQL Servers can, therefore, utilize the structure effectively to perform index search operations on your table. It is important that you know the distinction between clustered vs nonclustered index. We hope that you find this information on how to answer the question useful as you proceed to the interview room. Make sure to share your thoughts on these answers with us.

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