Comments on: Joins without JOIN

By: Chirag

Chirag — Fri, 10 Jun 2016 20:27:09 +0000

Hi Rob,
Nice work , just loved your article. Your post give so much clarity to the topic/concepts.
thanks for posting.

By: Kawahee

Kawahee — Sun, 09 Oct 2011 10:06:12 +0000

Just an update, I looked at the query plans myself and they seem to be equivalent in performance – both had the same number of (re)binds. I added a few more predicates to the WHERE and ON clauses to try and get the query optimizer to revert to rebinding for each record but both plans had the same number of rebinds.
The LEFT JOIN was actually slower, by a small margin, once I’d added the extra predicates.
I also changed the last query from SELECT * to SELECT s.*, since it’s only fair we ask for the same data each time.
I’m going to keep looking at this.

By: Kawahee

Kawahee — Sun, 09 Oct 2011 07:54:09 +0000

What is the number of rebinds on the operators at the bottom of the NOT EXISTS plan? If there is rebinding then it will be slower than the LEFT JOIN, which should bind to the table once.
Correlated subqueries are bad for performance because of rebinds, no?

By: Rob Farley

Rob Farley — Wed, 05 Oct 2011 08:23:24 +0000

Hi Todd,
EXISTS isn’t actually an Inner Join, it’s a Semi Join. Slight difference… an Inner Join will project rows onto each match. A Semi Join simply filters based on existence.
Hope this helps…
Rob

By: Brad Schulz

Brad Schulz — Wed, 05 Oct 2011 04:10:10 +0000

I love that we both touched on the OUTER + IS NULL inefficiency. :) http://bradsruminations.blogspot.com/2011/10/t-sql-tuesday-023-flip-side-of-join.html --Brad

By: Todd Everett

Todd Everett — Tue, 04 Oct 2011 15:26:52 +0000

Thanks for this post Rob. I’ve always wondered what those anti joins were. Coming from a background of always using correlated EXISTS and NOT EXISTS I had only recently realized EXISTS is just INNER JOIN. I never made the leap that NOT EXISTS is of course outer join with a WHERE filter so thank you for showing that. Where I work folks like EXISTS and NOT EXISTS because it makes it clear what you are doing logically where writing as a join makes that a little more ambiguous. I was just recently tending to use the inner join instead of EXISTS as I thought it performed better. Now I see that may not be the case. Thanks for rounding out my understanding of this topic!

By: Rob Farley

Rob Farley — Tue, 04 Oct 2011 14:24:19 +0000

Indeed it can. I figured it wasn’t necessary to show it though, as I discussed APPLY plenty in a previous post.

By: vijay kashyap

vijay kashyap — Tue, 04 Oct 2011 13:53:54 +0000

outer apply can be used for this below code
SELECT *
, (SELECT COUNT(*) FROM Production.Product AS p
WHERE p.ProductSubcategoryID = s.ProductSubcategoryID) AS ProdCount
FROM Production.ProductSubcategory AS s;