DISTINCT, GROUP BY, And Transaction Isolation Levels

Rob Farley has an interesting post where two similar-looking queries can provide different outputs given certain transaction isolation levels:

Now, it’s been pointed out, including by Adam Machanic (@adammachanic) in a tweet referencing Aaron’s post about GROUP BY v DISTINCT that the two queries are essentially different, that one is actually asking for the set of distinct combinations on the results of the sub-query, rather than running the sub-query across the distinct values that are passed in. It’s what we see in the plan, and is the reason why the performance is so different.

The thing is that we would all assume that the results are going to be identical.

But that’s an assumption, and isn’t a good one.

Rob starts out with READ UNCOMMITTED but then gets into the “normal” READ COMMITTED transaction isolation level that most places use.

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