Aggregations

Aggregations are operations used in databases to summarize information about some set of data. The result of aggregate operations can be a single value, such as the average salary for a company, or it can be a per-group value, such as the average salary by department. SQL Server has two operators to implement aggregations, Stream Aggregate and Hash Aggregate, and they can be used to solve queries with aggregation functions (such as SUM, AVG, and MAX), or when using the GROUP BY clause and the DISTINCT keyword.

Sorting and hashing

Before introducing the remaining operators of this chapter, let’s discuss sorting and hashing, both of which play a very important role in some of the operators and algorithms of the execution engine. For example, two of the operators covered in this chapter, Stream Aggregate and Merge Join, require data to be already sorted. To provide sorted data, the query optimizer might employ an existing index, or it might explicitly introduce...