Execution Plans

What Is Skewed Parallelism? When queries go parallel, some assumptions get made: There’s enough work to keep multiple threads busy Each thread will get an equal amount of work to do The ‘equal amount of work’ part is particularly important, because in a parallel plan, each thread gets an equal share of memory up front.…

Way Back When I blogged about the new USE HINT that logs query plans that use it to a new XE session. The use hint and session both share the same name — query_plan_profile. There are currently some rather unfortunate limitations to the event in the GUI. Right now, there’s no query plan tab so you…

The preview of the next version of SQL Server Management Studio is out, and it has a radical improvement to query plans that will shock and amaze you. It’s best to just show you: Estimated vs actual rows You can see the estimated and actual number of rows right there on the query plan just…

Recent Updates To SQL Server 2016 and 2017 introduced a new USE HINT that lets you direct an actual execution plan to a new Extended Event, called query_plan_profile. The hint by itself doesn’t do anything, and the XE by itself doesn’t do anything. You need to have both. Alright then, let’s do that. Sessions Assuming you’re on…

Say your database has a view, and everybody’s queries use it. Let’s take the Stack Overflow database and create this view: Transact-SQL CREATE OR ALTER VIEW dbo.vwQuestionsAndAnswers AS SELECT q.Title, q.Id AS QuestionId, a.Id AS AnswerId, a.Body AS Answer, uQuestioned.DisplayName AS Questioner_DisplayName, uAnswered.DisplayName AS Answerer_DisplayName, COUNT(DISTINCT vQ.Id) AS VotesOnQuestion, COUNT(DISTINCT uVotedQuestion.Id) AS UsersWhoVotedOnQuestion FROM dbo.Posts…

For years, when you right-clicked on a database and click Properties, the “Compatibility Level” dropdown was like that light switch in the hallway: You would flip it back and forth, and you didn’t really understand what it was doing. Lights didn’t go on and off. So after flipping it back and forth a few times,…

You And Ert This is a quick post because it came up with a client. I like having stuff to point people to — that’s sort of like automation, right? Anyway! Lots of plans have Assert operators in them. But they’re usually performing modifications. Assert operators are used to check Foreign Key and Check Constraint…

Innocent Enough I was writing another query, and became enamored with the fact that HAVING will accept IS NULL or IS NOT NULL as a predicate. What I ended up writing as an example was this query: Transact-SQL SELECT v.PostId, SUM(v.UserId) AS whatever FROM dbo.Votes AS v WHERE v.UserId IS NULL GROUP BY v.PostId HAVING…

In which I show why you can’t trust “actual” execution plans for branching stored procedures, and you’re better off using sp_BlitzCache to figure out what branches you need to tune: Demo code: Transact-SQL CREATE TABLE dbo.TableA (ID INT); CREATE TABLE dbo.TableB (ID INT); CREATE TABLE dbo.TableC (ID INT); CREATE TABLE dbo.TableD (ID INT); CREATE TABLE…

Bailing, Takes Me Away Both of these queries will return zero rows, in the 2010 version of the Stack Overflow database. Transact-SQL SELECT COUNT(*) AS records FROM dbo.Votes AS v JOIN dbo.Posts AS p ON v.UserId = p.OwnerUserId WHERE v.CreationDate >= '2011-01-01'; SELECT COUNT(*) AS records FROM dbo.Votes AS v JOIN dbo.Posts AS p ON…