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Clustered Index Scan

The Clustered Index Scan operator is non-blocking and represents a scan operation on a clustered index. Remember that a clustered index contains the data pages of the table at the leaf level, so this is effectively a table scan. Because the clustered index is organized into a tree structure, the data is logically ordered by the keys of the index. This doesn't necessarily mean the data will be returned in order; if no ORDER BY clause is specified in the query, the data may be returned in random order. If there is an ORDER BY in the query that matches the clustered index key or there is some other benefit to outputting the data in order, SQL Server may choose to do an ordered scan of the clustered index. This is helpful ...