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

Choosing Between Merge Join and Hash Join

Erik Darling gives us a Sophie’s Choice: It could have chosen a Hash Join, but then the order of the Id column from the Posts table wouldn’t have been preserved on the other side. Merge Joins are order preserving, Hash Joins aren’t. If we use a Hash Join, we’re looking at ordering the results of […]

Read More

Creating Temp Staging Tables to Avoid Spooling

Bert Wagner shows how you can create your own tables in tempdb to avoid eager or lazy spools: SQL Server Spool operators are a mixed bag. On one hand, they can negatively impact performance when writing data to disk in tempdb. On the other hand, they allow filtered and transformed result sets to be temporarily […]

Read More

Categories