SQL Server hash match aggregate operator – Part1

Hi Friends,

SQL Server hash match aggregate is selected by query optimizer for the tables with large data and when they are not sorted. Cardinality estimates few groups only and there is no need to sort it.

As an example, TerritoryID column on SalesOrderHeader table has no index and following query will use a Hash Match Aggregate operator.

USE [AdventureWorks2012]

SELECT SalesOrderHeader.TerritoryID, COUNT(*)
FROM Sales.SalesOrderHeader
GROUP BY SalesOrderHeader.TerritoryID


As discussed yesterday, hash operation builds a hash table in memory and hash key used for this table is displayed in properties window which is TerritoryID.



Since this column is not sorted by required column TerritoryID, every row is scanned. Hash aggregate algorithm is almost similar to Stream Aggregate with only exception that, input data is not sorted here.

A hash table is created in memory and a hash value is calculated for each row processed and for each hash value calculated, the algorithm is checked if the corresponding group exist or not, if not it creates a new entry. Values for each record are aggregated in this entry within hash table and you have only one row each group.

That’s all for today, we will continue with hash join aggregate operator tomorrow, till then.

Happy learning.



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About Kanchan Bhattacharyya

Kanchan is an astute IT professional, a seasoned SQL Database Administrator with 13+ years of industry experience. A calculated risk taker with deep technical knowledge and has gained proficiency in database consulting and solutions across different environments. Kanchan holds MBA degree in IT and an International Executive MBA in Project Management. He is very passionate about SQL Server and holds MCSE (Data Platform), MCSA – SQL 2012, MCITP – SQL 2008 certifications. Currently he is focusing on cloud and latest releases of SQL Server. When not working, Kanchan likes to spend his time reading on new technical developments specifically on SQL Server and other related technologies.

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