DATEDIFF performs implicit conversions

As I was performance tuning a query, I found that a number of date calculation functions in SQL Server appear to be forcing a conversion of their date parameters to a specific datatype, adding computational work to a query that uses them. In programming terms, it seems that these functions do not have “overloads”, i.e. different code paths depending on the incoming datatype.

So let’s take a closer look at how this manifests itself.

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Aggregating date and time values

Date and time values are not entirely intuitive to aggregate into averages in T-SQL, although the business case does arguably exist. Suppose, for instance, that you have a production log with a “duration” column (in the “time” datatype), and you want to find the totalt or average duration for a certain group of items.

It’s possible, but I would still call it a workaround.

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