I will be able to not care about this sort of thing. But for now, here we are, having to write multiple blogs in a day to cover a potpourri of grievances.
Let’s get right to it!
First, without a where clause, the optimizer doesn’t think that an index could improve one single, solitary metric about this query. We humans know better, though.
WITH Votes AS ( SELECT v.Id, ROW_NUMBER() OVER(PARTITION BY v.PostId ORDER BY v.CreationDate) AS n FROM dbo.Votes AS v ) SELECT * FROM Votes AS v WHERE v.n = 0;
The tough part of this plan will be putting data in order to suit the Partition By, and then the Order By, in the windowing function.
Without any other clauses against columns in the Votes table, there are no additional considerations.
What often happens is that someone wants to add an index to help the windowing function along, so they follow some basic guidelines they found on the internet.
What they end up with is an index on the Partition By, Order By, and then Covering any additional columns. In this case there’s no additional Covering Considerations, so we can just do this:
CREATE INDEX v2 ON dbo.Votes(PostId, CreationDate);
If you’ve been following my blog, you’ll know that indexes put data in order, and that with this index you can avoid needing to physically sort data.
The trouble here is that, even though we have Cost Threshold For Parallelism (CTFP) set to 50, and the plan costs around 195 Query Bucks, it doesn’t go parallel.
Creating the index shaves about 10 seconds off the ordeal, but now we’re stuck with this serial calamity, and… forcing it parallel doesn’t help.
Our old nemesis, repartition streams, is back.
Even at DOP 8, we only end up about 2 seconds faster. That’s not a great use of parallelism, and the whole problem sits in the repartition streams.
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.