Make It Count
When queries go parallel, you want them to be fast. Sometimes they are, and it’s great.
Other times they’re slow, and you end up staring helplessly at a repartition streams operator.
Sometimes you can reduce the problem with higher DOP hints, or better indexing, but overall it’s a crappy situation.
Let’s admire a couple familiar looking queries, because that’s been working really well for us so far.
WITH Comments AS ( SELECT ROW_NUMBER() OVER(PARTITION BY c.UserId ORDER BY c.CreationDate) AS n FROM dbo.Comments AS c ) SELECT * FROM Comments AS c WHERE c.n = 0 OPTION(USE HINT('QUERY_OPTIMIZER_COMPATIBILITY_LEVEL_140')); WITH Comments AS ( SELECT ROW_NUMBER() OVER(PARTITION BY c.UserId ORDER BY c.CreationDate) AS n FROM dbo.Comments AS c ) SELECT * FROM Comments AS c WHERE c.n = 0 OPTION(USE HINT('QUERY_OPTIMIZER_COMPATIBILITY_LEVEL_150'));
One is going to run in compatibility level 140, the other in 150, as foretold by ancient alien prophecy.
The two query plans will have a bit in common, but…
The second query, which runs in batch mode, runs about 15 seconds faster. One big reason why is that we skip that most unfortunate repartition streams operator.
It’s a cold sore. An actual factual cold sore.
The only ways I’ve found to fix it completely are:
- Induce batch mode
- Use the parallel apply technique
But the parallel apply technique doesn’t help much here, because of local factors.
In this case, me generating the largest possible result set and then filtering it down to nothing at the end.
Thanks for reading!
A Word From Our Sponsors
First, a huge thank you to everyone who has bought my training so far. You all are incredible, and I owe all of you a drink.
Your support means a lot to me, and allows me to do nice stuff for other people, like give training away for free.
So far, I’ve donated $45k (!!!) worth of training to folks in need, no questions asked.
Next year, I’d like to keep doing the same thing. I’d also like to produce a whole lot more training to add value to the money you spend. In order to do that, I need to take time off from consulting, which isn’t easy to do. I’m not crying poor, but saying no to work for chunks of time isn’t easy for a one-person party.
I’m hoping that I can make enough in training bucks to make that possible.
Because this sale is extra sale-y, I’ve decided to name it after the blackest black known to man.
From today until December 31st, you can get all 25 hours of my recorded training content for just $100.00. If you click the link below to add everything to your cart, and use the discount code AllFor100 to apply a discount to your cart.
Some fine print: It only works if you add EVERYTHING. It’s a fixed amount discount code that you need to spend a certain amount to have kick in.
Thank for reading, and for your support.