More On Adaptive Joins

Erik Darling has a couple more posts on adaptive joins in SQL Server 2017.  First, he wonders what happens when you add scalar functions to the mix:

See, this isn’t SARGable either (and no, SCHEMABINDING doesn’t change this). When a predicate isn’t SARGable, you take away an index seek as an access choice. You don’t see too many Nested Loops with an index scan on the other end, do you?

No.

So there you go. It’s not the function itself that bops our Adaptive Join on the head, but the lack of SARGability.

Erik also looks at using APPLY and EXISTS to make sure you can use adaptive joins with that dynamic duo:

I do have to point out that Cross Apply used to only be implemented as a Nested Loops Join. I learned that many years ago from one of the best articles written about Cross Apply by Paul White. That changed recently — it’s possible to see it implemented with a Hash Join in at least 2016. I’ve seen it crop up in Cross Apply queries without a TOP operator.

The latter results are a bit surprising.

Related Posts

OS Threads DMV

Ewald Cress moves up the internals stack a little further and looks at a DMV: Broadly speaking, a DMV presents just another iterator that can be plugged into a query plan. The execution engine calls GetRow() repeatedly until it reaches the end, and the iterator emits rows. The only unusual thing is that the ultimate source of […]

Read More

Parameter Sniffing Explained

Bert Wagner looks at how parameter sniffing works, why it’s (mostly) a good thing, and how to gently explain to the server when it’s a bad thing: All subsequent executions of that same query will go to the query cache to reuse that same initial query plan — this saves SQL Server time from having to regenerate […]

Read More

Categories