Category: tempdb

Screech

Links:

• Table Variables
• Table Variables In SQL Server 2019
• What’s Really Different About In Memory Table Variables?
• A Trick For Dealing With Table Variable Modification Performance
• Starting SQL: I Don’t Care How Many Rows Are In Your Table Variable

Video Summary

In this video, I delve into the often misunderstood world of table variables in SQL Server, exploring their uses and benefits, particularly when dealing with high concurrency and frequent execution. Using a series of stored procedures to test both temp tables and table variables, I demonstrate how these objects perform under stress using RML Utilities’ O Stress tool. The results show that while temp tables can lead to significant contention and slower performance due to their serialization issues, table variables shine in scenarios requiring quick, parallel query execution. This video aims to provide clarity on when it’s appropriate to use table variables over other temporary storage options, highlighting the importance of choosing the right tool for the job based on your specific workload requirements.

Full Transcript

Erik Darling here with Erik Darling Data, named one of the top 1001 sexiest SQL Server consulting companies in the world by BeerGut Magazine. And on this lovely Sunday, and it is Sunday, and so am I, I am enjoying a GinFizz of sorts. Mmm. Mmm. What a delicacy. Today we’re going to talk about when you should use table variables. Table variables get sort of an understandably bad rap for a lot of reasons. You know, you get the forced serialization when you modify them, you get the crappy estimates unless you recompile or throw some hokey trace flags at them. You get the the, um, what do you call it? What was the other one? There was another good one I had. Oh yeah. SQL Server has no idea what’s in them. Not only does it have no idea how much is in them, but it has no idea what the contents of that much is. And that’s not good. Often not good when we care about things like queries that have good cardinality estimates and perform well and plan shape makes a, makes a huge difference. But, uh, they’re there for a reason. They, they do not exist just to make consultants money finger wagging at you. Poor people who are there for a reason. They do not exist just to make consultants money finger wagging at you. Poor people who are there for a reason. They do not exist just to make consultants money finger wagging at you.

Who, who, who keep using them for no particular reason. Uh, they do exist for a good reason. And, uh, I’ve, I’ve talked a little bit about how they help when you, uh, have very high frequency execution for queries. Uh, but what I didn’t explore then was, uh, what happens when you also have very high concurrency along with the high frequency execution. And that’s what we’re going to look at today. Now I am going to use a, uh, a couple store procedures. that I used, uh, in the past to look at how these two different types of objects perform. Uh, one of them is a temp table test. And, um, you know, I know it’s not maximally, maximally realistic, but that’s fine. Cause all it has to do is show you behavior. Not everything is going to be a hundred percent true to life, real world. This is what happened demo this, but it just shows you what happens in the real world when you use different objects for different purposes. And this is the whole point. Thanks for watching. I’m just going to leave now, go enjoy my, my gin fizz. So this, uh, this one here is a temp table tester and we create a temp table. I don’t know how I missed a semi colon. There’s a sloppy of me. It’s been gone. It’s going to kill me next time I see him and I get assassinated. Uh, actually no, it’s not even my fault. Cause I, I use SQL prompt and I hit, uh, insert semi colons. And I don’t know what, what was going on with that. I don’t know. Maybe SQL prompt is dumb, but all this does is create a temp table. Uh, insert into the temp table and then we’re done with it. Uh, this one down here, look, see this, the semi colons here. I don’t know what, what the H E double, double doglegs happened.

No, but this one declares a table variable and then does the exact same thing. Now, of course, we have to pass something in to these store procedures and, uh, I don’t want to have to do that in a weird way. So what I did is I wrote a wrapper store procedure, uh, for, for these things. And, uh, it’s called the temp test wrapper. You know, I’ll have all this code somewhere. I mean, probably, probably in the blog post. I’m not going to stick all the code in the YouTube description. That’s annoying.

Uh, but it’ll be, it’ll, it’ll accompany the blog post along with some links to other things that I’ve written about table variables in the past. Uh, including the miraculous video where I, I test running these store procedures in a loop. Won’t that be fun for you? Uh, so yeah, so this thing, uh, declares a, uh, declares a variable and, uh, we set that variable to some random number between, uh, uh, one and the max ID in the post database, which is first exact overflow 2013 is currently 2119501.

In case you had trouble reading that number. And then, uh, you know, with the test procedure is, oh, you know, uh, temp, temp table test, then we’ll run temp table test. And if the test proc that we pass in up here is that, then we’ll run that. And I guess I should, I mean, I don’t really need to make it unicode. I guess I, I guess I should for completeness, right?

And make sure I don’t go against any of my, any of my own best practices mismatching data types. Not that this is going to matter, but whatever. I don’t want to get hoisted by my own petard or whatever that’s called unitard. Um, but you know, if we run these things, we get query plans turned on over here.

If we, uh, if we test these things out and run them, uh, we will get an execution plan that, uh, accurately portrays what we wanted to do. We have an insert into our pound sign temp table and then insert into our at sign table variable. We are all good. So what I’m going to do next is run use O stress from the lovely and talented RML utilities.

And I’m going to use that to, uh, run a bunch of concurrent threads 1000 times. So 300 sessions for 1000 iterations. And I just want to make a lot of busy work for the server. There’s no other point to it. Just plain busy work.

And let’s go over here and let’s, uh, plug that into. Oh, stress and get off that off and running. And let’s come over to here to SP who is active.

And we’re going to use this get task info parameter to get, uh, additional weight stat information. We see a whole bunch of page latch EX weights. And if we run this a whole bunch of times, uh, that’s what we’re just going to see piling up, right? Maybe they’re not terribly long.

Maybe this is no, not the worst, but you know, we can obviously see the, the tempDB contention occurring. We can see SQL Server trying to figure out where to put stuff, where things go, where do you belong? Um, I don’t know.

I don’t know where to go anymore. So if we keep running this, we’re just going to keep seeing it, right? It’s obvious signs of contention. Now, if, uh, we have code where it matters that, uh, that like, you know, what, what we’re putting into our, our temp table, we might just have to deal with some of that contention.

We might just have to, you know, live with it because we might need some of the, uh, the benefits of temp tables that table variables don’t have. You can put data into them in a, in a parallel query. You can, uh, you can create indexes on them that actually make a big difference because, you know, you get statistical information, you get all accurate.

Well, I mean, you know, usually accurate row counts. So there are some, you know, kind of iffies around, um, uh, temp tables getting cached and stored procedures. But as I always say, if that is your biggest problem, you should just go on vacation and find a new job where you will have interesting problems to solve again. Uh, but if you look, that whole thing took about 40 seconds to complete.

39.368, which is close enough to 40 for me. And I’m going to order a 40 later in remembrance of this fine demo. Uh, but next let’s go and look at table variables.

All right. Let’s, for good luck, we’re going to clear that out. And now we’re going to run the table variable version. Come back over here and let’s start running who is active.

And we’ll start seeing that wait info column. It doesn’t have anything in it. We’re not really waiting on anything. And, uh-oh, we’re done in eight seconds. So, if we were measuring this workload, right?

And I know it’s not the most realistic thing in the world, but, you know, let’s just say you have a lot of user sessions that come in, do something quick, and get out. So, well, if, you know, the idea is to make those user sessions as quick as possible because you don’t want them hanging around doing things, well, then we need to maybe consider the type of temporary object that we’re using to push those queries through. So, table variables do have very good uses in this case.

In this case, you know, we had an 8.3 second workload versus, what was that, almost 40 seconds if I’m remembering correctly. And I should remember correctly because now I have to order a 40 later and remembrance of that fine query. So, I’ll let you know how that smells in another video.

So, anyway, you know, table variables do get a lot of flack, and rightly so. They have probably messed up performance for people more times than they have solved a big performance problem. But this is a case where I would absolutely use one.

It’s a little bit of a niche case, but there are definitely industries, and there are definitely workloads where this kind of thing is, you know, the norma. The norma. The normal.

The norma. This is the norma gene. But, yeah, if you have that type of workload where you have very, very high frequency execution, and you need to push a bunch of tiny little queries through very, very quickly, table variables just might be the solution for you.

Now, of course, like, you know, one step further is, you know, the in-memory table variable, which I blogged about recently. I suppose I should put a link to that somewhere, too. I don’t know where it’s going to end up, though.

I don’t like to make promises about putting things in the YouTube description because quite often I forget. And I’m bad about remembering to go back and do it until someone leaves a comment and says, Hey, you said you were going to put this in the thing, and then you didn’t.

I’m like, yeah, sorry. Happens to me all the time. I’m forgetful. That’s why I blog a lot and record videos so I can remember. I don’t have to remember.

Anyway, I’m going to get back to my gin fizz and my Sunday because I am Sunday. And I will see you in another video another time. Thank you for watching.

Going Further

If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 25% discount to my blog readers if you click from here. I’m also available for consulting if you just don’t have time for that and need to solve performance problems quickly.

USE StackOverflow2013;
EXEC dbo.DropIndexes;
SET NOCOUNT ON;
DBCC FREEPROCCACHE;
GO 

CREATE INDEX whatever 
ON dbo.Posts
    (OwnerUserId) 
INCLUDE
    (Score);
GO 

CREATE OR ALTER PROCEDURE dbo.TempTableTest (@Id INT)
AS
BEGIN
SET NOCOUNT, XACT_ABORT ON;
    
    CREATE TABLE #t(i INT NOT NULL);

    INSERT 
        #t ( i )
    SELECT 
        p.Score
    FROM dbo.Posts AS p
    WHERE p.OwnerUserId = @Id;

END;
GO 

CREATE OR ALTER PROCEDURE dbo.TableVariableTest (@Id INT)
AS
BEGIN
SET NOCOUNT, XACT_ABORT ON;
    
    DECLARE @t TABLE (i INT NOT NULL);

    INSERT 
        @t ( i )
    SELECT 
        p.Score
    FROM dbo.Posts AS p
    WHERE p.OwnerUserId = @Id;

END;
GO 


CREATE OR ALTER PROCEDURE dbo.TempTestWrapper (@TestProc sysname)
AS
BEGIN
SET NOCOUNT, XACT_ABORT ON;

DECLARE @rando int = 
    ((ABS(CHECKSUM(NEWID())) % 21195018) + 1); /*this is the max id in posts for SO2013*/

IF @TestProc = N'TempTableTest'
    BEGIN
        EXEC dbo.TempTableTest @rando;
    END;

IF @TestProc = N'TableVariableTest'
    BEGIN
        EXEC dbo.TableVariableTest @rando;
    END;

END;
GO 

/*Testing*/
EXEC dbo.TempTestWrapper @TestProc = 'TempTableTest';
EXEC dbo.TempTestWrapper @TestProc = 'TableVariableTest';

/*

ostress -SSQL2019S -d"StackOverflow2013" -Q"EXEC dbo.TempTestWrapper @TestProc = 'TempTableTest';" -U"ostress" -P"ostress" -q -n300 -r1000 -o"C:\temp\crap"

ostress -SSQL2019S -d"StackOverflow2013" -Q"EXEC dbo.TempTestWrapper @TestProc = 'TableVariableTest';" -U"ostress" -P"ostress" -q -n300 -r1000 -o"C:\temp\crap"

*/