Learn T-SQL Querying

Clustered indexes

As we discussed in the previous chapter, if a table does not have a clustered index, it is stored as a heap. While heap data structures allow for fast inserts, they are generally less efficient for data access, so it is recommended that most tables in a SQL Server database have a clustered index.

The clustered index defines the structure of the table itself. Once we create a clustered index, the data rows are stored in the index itself, ordered by the index keys. For this reason, there can be only one clustered index on a table. When choosing which column or columns should be used for the clustered index key, consider the following guidelines:

• The key should be unique if possible: While uniqueness is not required, because ...