Any new value inserted into that index will follow the index key ordering sequence among the existing rows. The clustered index can be beneficial for the queries that read large result sets of ordered sequential data.
In this case, the SQL Server Engine will locate the row with the first requested value using the clustered index, and continue sequentially to retrieve the rest of rows that are physically adjacent within the index pages with the correct order, without consuming the SQL Server Engine time and resources in sorting the data that is already sorted in the clustered index, affecting the overall query performance positively.
For example, if the query returns all rows with ID value large than , the SQL Server Engine will use the clustered index to locate the row with ID value equal to and continue retrieving the rest of rows sequentially. The characteristics of the best clustering keys can be summarized in few points that are followed by most of the designers:. When designing a clustered index, you should consider that some data types are generally better than other data types to be used as clustering keys.
In addition, the IDENTITY integer values are narrow, due to its small size, unique, if you enforce the column uniqueness with a constraint, and static, as they are generated automatically by the system and not visible to the users. Although the GUIDs values, that are stored in the uniqueidentifier columns, are commonly used as clustered index key, there are some challenges that accompany that design.
The main challenge that affects the clustered index key sorting performance is the nature of the GUID value that is larger than the integer data types, with 16 bytes size, and that it is generated in random manner, different from the IDENTITY integer values that are increasing continuously.
The large size and randomness generation of the GUID values will always lead to the page splitting and index fragmentation problems, which negatively affect the clustered index usage performance. The Character data types can be also used, but not recommended, as clustered index keys.
This is due to the limited sorting performance of the character data types, the large size, non-increasing values, non-static values that often tend to change in the business applications and not compared as binary values during the sorting process, as the characters comparison mechanism depends on the used collation.
Even though the Date data types are not unique, it has a small size and provides good sorting performance, especially for the queries that search for data ranges. SQL Server allows you to specify the type of the index that will be created automatically when you create a UNIQUE constraint to be clustered index, if there is no clustered index created on that table, due to the fact that, only one clustered index can be created per each table.
Having no clustered index defined in the previous table, we can create a clustered index using the SQL Server Management Studio by browsing the table on which we need to create the clustered index on, then right-click on the Indexes node under that table and from the New Index option choose the clustered index type, as shown below:. From the displayed New Index dialog box, the name of the table on which the index will be created, and the type of the index will be filled automatically.
You need to provide the name of the index, following your company naming convention, the uniqueness of that index key values and from the Add button you can choose the column or list of columns that will participate in that index key, as shown below:. You can also perform the same task from the Table Designer, by right-clicking on the table on which the index will be created and choose Design option as below:.
From the appeared dialog box, click on the Add bottom to add a new clustered index, by setting Create As Clustered to Yes, Specify the index name, the clustered index key uniqueness and the list of columns, with the proper sorting order, that will be included in the clustered index key, as shown below:. ManJan 3, 3 3 gold badges 18 18 silver badges 22 22 bronze badges. Thanks for responding. I am already aware of that.
My question is would there be a situation where creating a non-clustered index would benefit more than creating a clustered index — armulator. Yes, you should avoid a clustered index when; the column has low cardinality, no particular order, frequently updated, non-sequential, it is a composite of many columns Thanks for your answer. Its helpful — armulator.
Perhaps this question would help you: stackoverflow. Possible duplicate of What column should the clustered index be put on? Show 1 more comment. Active Oldest Votes. You should use extreme care when picking a clustering key - it should be: narrow 4 bytes ideal unique it's the "row pointer" after all. Ever-increasing clustering key - the Clustered Index Debate Disk space is cheap - that's not the point!
Improve this answer. Zain Rizvi But technically speaking, what does non clustered index do for us. That makes a HUGE difference!
Add a comment. By default a clustered index is created on a primary key column. Execute the following script:. The above script creates a dummy database Hospital. The database has 4 columns: id, name, gender, age. The id column is the primary key column. When the above script is executed, a clustered index is automatically created on the id column.
You can see the index name, description and the column on which the index is created. If you add a new record to the Patients table, it will be stored in ascending order of the value in the id column. If the first record you insert in the table has an id of three, the record will be stored in the third row instead of the first row since clustered index maintains physical order. You can create your own clustered indexes. However, before you can do that you have to create the existing clustered index.
We have one clustered index due to primary key column. If we remove the primary key constraint, the default cluster will be removed. The following script removes the primary key constraint. Owing to this index, all the records in the Patients table will be stored in ascending order of the age. In the above script, we add 5 dummy records. Notice the values for the age column. They have random values and are not in any logical order.
However, since we have created a clustered index, the records will be actually inserted in the ascending order of the value in the age column. You can verify this by selecting all the records from the Patients table.
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