Clustered indexes sort and store the data rows for a table, based on the columns defined in the index. For example, if you were to create a clustered index on the
FirstName columns in a table, the data rows for that table would be organized or sorted according to these two columns. This has some obvious advantages for data retrieval. Queries that search for data based on the clustered index keys have a sequential path to the underlying data, which helps reduce I/O.
A clustered index is analogous to a filing cabinet where each drawer contains a set of file folders stored in alphabetical order, and each file folder stores the files in alphabetical order. Each file drawer contains a label that indicates which folders ...