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Window Functions and Tips on Sorting

Itzik Ben-Gan shares some good advice:

A supporting index can potentially help avoid the need for explicit sorting in the query plan when optimizing T-SQL queries involving window functions. By a supporting index, I mean one with the window partitioning and ordering elements as the index key, and the rest of the columns that appear in the query as the index included columns. I often refer to such an indexing pattern as a POC index as an acronym for partitioningordering, and covering. Naturally, if a partitioning or ordering element doesn’t appear in the window function, you omit that part from the index definition.

But what about queries involving multiple window functions with different ordering needs? Similarly, what if other elements in the query besides window functions also require arranging input data as ordered in the plan, such as a presentation ORDER BY clause? These can result in different parts of the plan needing to process the input data in different orders.

Read on for six tips. By the way, if you see broken images on the page (which I saw at the time of posting), click the broken image icon to see the image. It appears that the problem is just with inline images.

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