May 2019
Intermediate to advanced
484 pages
11h 23m
English
Content preview from Learn T-SQL Querying
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Sorts and aggregation
Sort and aggregation operators are present in an execution plan when a query contains an ORDER BY and/or a GROUP BY clause. In some cases, SQL Server will introduce a sort in order to optimize the execution of a query, such as to enable a merge join or to improve the performance of a Nested Loops join. We may also see a Sort operator in an execution plan that contains SELECT DISTINCT. DISTINCT is effectively an aggregation as it requires grouping the rows and only returning one row per distinct set of values. A sort operation is a simple way to perform this type of aggregation. As discussed in the Query Optimization Essentials section of Chapter 2, Understanding Query Processing, SQL Server may also add a sort operator ...
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