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Grouping algorithms compute a summary of their input. They are applicable only if the query contains a GROUP BY clause, or if the query contains aggregate functions (such as SELECT COUNT(*) FROM T).

SELECT COUNT(*) FROM T

For more information, see Hash Group By algorithm, Ordered Group By algorithm, and Single Group By algorithm.

Clustered Hash Group By algorithm Hash Group By algorithm Hash Group By Sets algorithm Ordered Group By algorithm Ordered Group By Sets algorithm Single Group By algorithm Single XML Group By algorithm Sorted Group By Sets algorithm