Collapsable Subqueries

Dmitry Pilugin notes a new query simplification rule in SQL Server vNext:

You may see that in the first plan, there are two clustered index scans of the table SalesOrderDetail, however the subquery is exactly the same “exists (select * from Sales.SalesOrderDetail f where f.SalesOrderID = d.SalesOrderID)” but referenced twice.

In the second case, compiled under next compatibility level, the double reference of the subquery is collapsed and we see only one reference to the SalesOrderDetails table and more efficient plan, despite the query still has two subqueries with SalesOrderDetails.

In the third case, also compiled under vNext level, we see the second branch with the SalesOrderDetail again, but that is because we turned off the rule CollapseIdenticalScalarSubquery with an undocumented hint queryruleoff (which I originally described in my blog post).

I think Dmitry has the expected use case nailed:  ORMs.  But I can see people writing (well, copy-pasting) similar queries, so maybe it’ll be useful in more contexts as well.

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