# Hash Aggregate: Efficient Processing without Ordering > [!Note] > `The **Hash Aggregate** operator excels at summarizing large, unordered datasets by building an **in-memory hash table**, updating or creating entries per grouping key, and emitting a final result table without the need for prior sorting.` Hash Aggregate begins by computing a **hash function** on each grouping key to assign rows into buckets within its hash table. As rows are read, it **updates aggregate values** for existing keys or **creates new entries** when a key is encountered for the first time. Once all input is processed, it **produces a result set** containing every grouping key alongside its computed aggregates. In execution plans, this operator appears as **Hash Match (Aggregate)** and is chosen by the optimizer when data is not sorted and volumes are large. It leverages the same **Hash Match** mechanism used by **Hash Join**, enabling complex queries to run efficiently without imposing an explicit sort order. --- ## ## References - Korotkevitch, D. (2022). _SQL Server advanced troubleshooting and performance tuning: Best practices and techniques_. O’Reilly Media. - Nevarez, B. (2022). _SQL Server query tuning and optimization: Optimize Microsoft SQL Server 2022 queries and applications_. Packt Publishing.