Grouping dates without blocking operators

It’s not entirely uncommon to want to group by a computed expression in an aggregation query. The trouble is, whenever you group by a computed expression, SQL Server considers the ordering of the data to be lost, and this will turn your buttery-smooth Stream Aggregate operation into a Hash Match (aggregate) or create a corrective Sort operation, both of which are blocking.

Is there anything we can do about this? Yes, sometimes, like when those computed expressions are YEAR() and MONTH(), there is. But you should probably get your nerd on for this one.

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Windowed DISTINCT aggregates

You may have discovered that the use of DISTINCT is not supported in windowed functions. A query that uses a distinct aggregate in a windowed function,

SELECT COUNT(DISTINCT something) OVER (PARTITION BY other)
FROM somewhere;

will generate the following error message:

Msg 10759, Level 15, State 1, Line 1
Use of DISTINCT is not allowed with the OVER clause.

There are, however, a few relatively simple workarounds that are suprisingly efficient.

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Indexed views

Indexed views are views where the computed view data is already calculated and stored in the database, which make them super-fast for querying. And if you’re using the expensive Enterprise Edition of SQL Server, you’ll enjoy performance increases in other queries that can make use of the indexed view as well.

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