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Index architectures are classified as clustered or non-clustered. Clustered indexes are indexes that are built based on the same key by which the data is ordered on disk. The leaf node of the clustered index corresponds to the actual data, not simply a pointer to data that resides elsewhere, as is the case with a non-clustered index. Due to the fact that the clustered index corresponds (at the leaf level) to the actual data, the data in the table is sorted as per the index, and therefore, only one clustered index can exist in a given table whereas many non-clustered indexes can exist. Unclustered indexes are indexes that are built on any column. Each relation can have a single clustered index and many unclustered indexes. Clustered indexes usually store the actual records within the data structure and as a result can be much faster than unclustered indexes. Unclustered indexes are forced to store only record IDs in the data structure and require at least one additional I/O operation to retrieve the actual record.
Bluesky
