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Category: Query Tuning

Statistics TIME and IO Analysis and Statistics Reporter

Published 2024-07-16 by Kevin Feasel

Rod Edwards does a bit of analysis:

Again, this is another one of those little bits of functionality that has been present since year dot. Unfortunately, it seems to be forgotten by many in the great game of query investigation.

Being able to see the IO patterns of a query that we’re running to help us see where the heavy lifting is occurring is really useful in allowing us to get the bottom of problems quickly, or see the benefits of our tuning efforts (or otherwise!).

One neat thing I did not know about before was a product called Statistics Reporter, an extension for SQL Server Management Studio that parses the results from time and I/O statistics results. I like Richie Rump’s Statistics Parser website, but the idea of having something built-in is pretty nice.

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Keyset Pagination and Descending Order

Published 2024-07-16 by Kevin Feasel

Laurenz Albe digs into keyset pagination:

Keyset pagination is the most performant way to retrieve a large result set page by page. However, the neat trick with composite type comparison doesn’t always work. This article explains why and how you can work around that shortcoming.

All of the examples are for Postgres, though in my read-through of the post, I’d say it applies almost as well to SQL Server.

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Merge Join vs Hash Join in Postgres

Published 2024-07-08 by Kevin Feasel

Andrei Lepikhov compares two physical join operators:

Today’s post is sparked by a puzzling observation: users, especially those who use an abstraction layer like REST or ORM library to interact with databases, frequently disable the MergeJoin option across the entire database instance. They justify this action by citing numerous instances of performance degradation.

Considering how many interesting execution paths MergeJoin adds to the system elaborating IncrementalSort or sort orderings derived from underlying IndexScan, it looks strange: one more bug of skewed cost balance inside the PostgreSQL cost model?

This is an interesting peek into how complex the query optimizers in database engines are, as well as how small amounts of information (via statistics or indexes) can matter to a query.

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The Core of a Window Function’s Execution Plan

Published 2024-06-26 by Kevin Feasel

Grant Fritchey lays it out:

I showed the missing index suggestion (and let’s remember, they’re just suggestions) just for completion and to show I’m not hiding anything. Potentially, adding an index could speed up the query. However, that doesn’t affect what we’re going to talk about within this plan.

Logically, what’s happening here?

Click through for this overview, as well as a very helpful comment from Hugo Kornelis on what the Segment operator does.

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Execution Plans for Keyset Cursors

Published 2024-06-25 by Kevin Feasel

Hugo Kornelis talks about a cursor I’d never heard of before:

Welcome to plansplaining, part 32, where we once more look at cursors. We already discussed the basics, and looked at static cursors and dynamic cursors. It is now time to cast our gaze upon the keyset cursor. The keyset cursor is sort of in between the static cursor (which presents a snapshot of the data is at was when the cursor was opened and disregards future changes) and the dynamic cursor (that always shows the current data). To be precise, a keyset cursor sees changes made to already existing rows, but does not see new rows that were added after the cursor was opened.

Read on to learn more about it.

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Enumerations and Ordering in Postgres

Published 2024-06-24 by Kevin Feasel

Christoph Schiessl sorts things out:

Custom ENUM types in PostgreSQL are an excellent tool for enforcing certain database constraints, but you must be careful if you use SELECT queries and want to ORDER BY these columns. Recently, I had to fix a bug whose root cause was a misunderstanding of this behavior. It’s just a contrived example, but imagine a table of people with their marital status, which is implemented as a custom ENUM type.

Read on to learn more about the misunderstanding and some of the unexpected trickiness involved in getting a good query plan.

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Using Query Store to Fix a Cardinality Estimation Problem

Published 2024-06-07 by Kevin Feasel

Michael Bourgon solves an issue:

This morning I had a performance issue on a piece of code that worked long ago on a different server, and they were trying to put it in place today.  It was SLOW. Like, 10 minutes slow. With the added bonus that it’s done through a web app, so it never finishes, it just always times out. After dealing with various approaches, I finally tried using the old Cardinality Estimator, and it went from 10 minutes to 3 seconds. But the query is inside the application, it doesn’t call a stored procedure. Which means the devs changing it is Non-Trivial. So I went to an updated version of an old trick – query store hints (which used to be Plan Guides)

Click through for a list of actions Michael took.

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Dynamic Cursors in SQL Server

Published 2024-06-04 by Kevin Feasel

Hugo Kornelis continues a series on cursors:

We’re already at part 31 of the plansplaining series. And this is also the third part in my discussion of execution plans for cursors. After explaining the basics, and after diving into static cursors, it is now time to investigate dynamic cursors. As a quick reminder, recall that a static cursor presents data as it was when the cursor was opened (and does so by simply saving a snapshot of that data in tempdb), whereas a dynamic cursor is supposed to see all changes that are committed while the cursor is open. Let’s see how this change in semantics affects the execution plan.

Read on as Hugo gives it the college try and also admits he might be missing something in the explanation.

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Impossible Execution Plan Timings

Published 2024-06-03 by Kevin Feasel

Paul White puts up an article:

I showed a hidden option to make all operators report only their individual times in

More Consistent Execution Plan Timings in SQL Server 2022

. That feature isn’t complete yet, so the results aren’t perfect, and it’s not documented or supported.

I mention all that in case you are interested in the background. None of the foregoing explains what we see in this mixed mode plan. The row mode Gather Streams elapsed time ought to include its children. The batch mode Sort should just be reporting its own elapsed time.

With that understanding in mind, there’s no way the Sort could run for longer than the Gather Streams. What’s going on here?

Read on for a Paul White-level discussion of the topic, including a demo from Erik Darling.

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Finding Duplicate Post Titles and Tuning the Query

Published 2024-05-31 by Kevin Feasel

Erik Darling makes a friend:

I’m going to be totally open and honest with you, dear reader: I’ve been experimenting with… AI.

See, I’m just a lonely independent consultant, and sometimes it’s just nice to have someone to talk to. It’s also kind of fun to take a query idea you have, and ask “someone” else to write it to see what they’d come up with.

ChatGPT (for reference, 4 and 4o) does a rather okay job sometimes. In fact, when I ask it to write a query, it usually comes up with a query that looks a lot like the ones that I have to fix when I’m working with clients.

Considering that the clients probably stole the query idea from Stack Overflow as well, that makes sense. But there was a clever trick that the query returned, so check it out.

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