Execution Plans

Execution Plans, Memory Grants, Parameter Sniffing, SQL Server 2017

Where have you been all my life? Whenever I’m teaching people about query tuning, two things come up almost without fail: parameter sniffing and tempdb spills. They’re not mutually exclusive — some queries will seemingly always spill to tempdb, whether parameter sniffing is present of not, and some queries experiencing issues with parameter sniffing will never spill…

SQL Server 2017 CU3 add a new CXCONSUMER wait type

Last Updated August 9, 2020

Execution Plans, SQL Server 2017, Wait Stats

SQL Server 2017’s new CXCONSUMER wait type was originally announced by Microsoft’s Pedro Lopes, and now it’s out. Here’s what it means for performance tuners. According to Pedro, this wait is the “safe” type of parallelism wait, as opposed to the CXPACKET wait type, which means work isn’t evenly balanced across all of our cores. Pedro blogged…

SQL Server 2017 CU3 adds execution statistics for scalar-valued, user-defined functions

Last Updated December 9, 2018

Execution Plans, SQL Server 2017

Update This seems to finally be fixed as of CU12 for SQL Server 2017. If you’re not already patched up, head over to the link to get there. Estimated Plans Need Not Apply This is, of course, only available in actual plans. All together now: If a query uses one or more user-defined scalar functions…

Query Plan Oddities: Two Identical Missing Indexes

Last Updated December 9, 2017

As I’ve been building labs for my Mastering Query Tuning class, I’ve been spelunking through data.stackexchange.com. It’s where anybody can write queries against the Stack Overflow databases, and share them with friends. For example, Daniel Vandersluis wrote a query to see how many edits he has: Transact-SQL -- How many edits do I have? --…

sp_BlitzQueryStore: A Gentle Introduction

Last Updated May 26, 2020

Execution Plans, First Responder Kit, Parameter Sniffing, Query Store (QDS), sp_BlitzQueryStore, SQL Server 2016, SQL Server 2017

Odds and ends During the precon, we asked how many people were on 2016, how many people were aware of query store, how many people were using it, and how many people were using sp_BlitzQueryStore. About 1/3 of the hands went up at first, then for each successive question, fewer and fewer hands stayed up.…

Implied Predicate and Partition Elimination

Last Updated December 4, 2017

>implying Way back when, I posted about turning the Votes table in the Stack Overflow database into a Partitioned View. While working on related demos recently, I came across something kind of cool. It works for both partitioned tables and views, assuming you’ve done some things right. In this example, both versions of the table…

Partitioned Views, Aggregates, and Cool Query Plans

Last Updated December 4, 2017

The Max for the Minimum Paul White (obviously of course as always) has a Great Post, Brent® about Aggregates on partitioned tables Well, I’m not that smart or good looking, so I’ll have to settle for a So-So Post about this. There are actually quite a few similarities between the way a partitioned table and…

Microsoft’s Query Tuning Announcements from #PASSsummit

Last Updated November 2, 2017

Microsoft’s Joe Sack & Pedro Lopes held a forward-looking session for performance tuners at the PASS Summit and dropped some awesome bombshells. Pedro’s Big Deal: there’s a new CXPACKET wait in town: CXCONSUMER. In the past, when queries went parallel, we couldn’t differentiate harmless waits incurred by the consumer thread (coordinator, or teacher from my…

How to Get Live Query Plans with sp_BlitzWho

Last Updated November 18, 2017

Execution Plans, First Responder Kit, sp_BlitzWho, SQL Server 2016

sp_BlitzWho is our open source replacement for sp_who and sp_who2. It has all kinds of really neat-o outputs like how many degrees of parallelism the query is using, how much memory it’s been granted, how long it’s been waiting for memory grants, and much more. If you’re on SQL Server 2016 SP1 or newer, it…

What Is Estimated Subtree Cost? Query Bucks. No, Really.

Last Updated June 3, 2022

When you look at a query plan, SQL Server shows a tooltip with an Estimated Subtree Cost: Estimated Subtree Cost Now I can run all the bad queries I want! A long time ago in a galaxy far, far away, it meant the number of seconds it would take to run on one guy’s Dell…

Adaptive Blog Posts

Last Updated October 2, 2017

Adaptive Joins, Execution Plans

It turns out I can be dumb In a previous blog post about Adaptive Joins, I thought that EXISTS wasn’t supported by the new feature. It turns out that EXISTS is totally 100% supported, as long as your indexes support EXISTS. To show this, I need to show you a query that gets an Adaptive…

SQL Server 2017 Showplan Schema Is Available

Last Updated September 28, 2017

Execution Plans, SQL Server, SQL Server 2017

File this under… Things probably only I care about. But the Showplan Schema for SQL Server 2017 popped up… well, I caught it last night. I probably could have written this sooner, huh? So what’s new? Well, if I compare the 2016 SP1 schema using KDiff, a few things stand out. Some I’ve blogged about,…

Which sp_configure Options Clear the Plan Cache?

Last Updated May 26, 2020

Execution Plans, Parameter Sniffing

When you change MAXDOP and run RECONFIGURE, SQL Server clears the plan cache. (It’s documented, by the way, as is the rest of the behavior I’m about to show you, but sometimes I like to learn the hard way by running experiments.) Here’s the easiest way to see it – but make sure to only…

Live Query Plans and Blocking

Last Updated September 21, 2017

Execution Plans, Indexing, Locking, Blocking, and Isolation Levels, SQL Server

You’d think this would be cool After all, you’ve got a query plan being executed LIVE ON NATIONAL TELEVISION! But it’s not. It’s actually one of those weird reasons why you still need other ways of looking at DMVs for issues. Demo Library Let’s take a closer look! First, we need an index. Indexes are…

Why Columnstore Indexes May Still Do Key Lookups

Last Updated June 25, 2021

Columnstore Indexes, Execution Plans, Indexing, SQL Server

I was a bit surprised that key lookups were a possibility with ColumnStore indexes, since “keys” aren’t really their strong point, but since we’re now able to have both clustered ColumnStore indexes alongside row store nonclustered indexes AND nonclustered ColumnStore indexes on tables with row store clustered indexes, this kind of stuff should get a…

How Much Can One Row Change A Query Plan? Part 2

Last Updated January 8, 2019

Execution Plans, Memory Grants

Earlier this week, I showed you two nearly identical queries and their estimated plans: One of these things is much like the other They look pretty darned identical – so how much worse can one query be? Here’s how much worse: Top 100: 51.0 seconds duration, 141.6 seconds CPU time Top 101: 160.0 seconds duration,…

Optional Parameters and Missing Index Requests

Last Updated September 15, 2017

Execution Plans, Indexing, SQL Server

That’s when it all gets blown away At one point or another in everyone’s SQL-querying career, they end up writing a query that goes something like this: Transact-SQL SELECT something FROM stuff WHERE (@thing1 is NULL or whatever = @thing1) AND ... 1234 SELECT somethingFROM stuffWHERE (@thing1 is NULL or whatever = @thing1)AND ... These…

Indexing for Windowing Functions: WHERE vs. OVER

Last Updated March 12, 2018

Execution Plans, Indexing, SQL Server

Life Is Messy Demo queries have this nasty habit of being clean. Even using a pit of despair like Adventure Works or World Wide Importers, it’s easy to craft demo queries that fit the scenario you need to make yourself look like a genius. Stack Overflow, in all its simplicity, makes this even easier (lucky…

How Much Can One Row Change A Query Plan? Part 1

Last Updated September 16, 2017