Tag: SQL Server

There’s Still Time To Catch My PASS Precon: The Professional Performance Tuning Blueprint

Posted on by Erik Darling

Workin’

If the Gods of post scheduling cooperate, this should publish in October. That means you’ve got a little more than a month to buy tickets and book travel to PASS.

And of course, if you’re going to go, I’d love it you came to my precon: The Professional Performance Tuning Blueprint.

Searching the internet for every problem isn’t cutting it.

You need to be more proactive and efficient when it comes to finding and solving database performance fires. I work with consulting customers around the world to put out SQL Server performance fires.

In this day of learning, I will teach you how to find and fix your worst SQL Server problems using the same modern tools and techniques which I use every week.

You’ll learn tons of new and effective approaches to common performance problems, how to figure out what’s going on in your query plans, and how indexes really work to make your queries faster.

Together, we’ll tackle query rewrites, batch mode, how to design indexes, and how to gather all the information you need to analyze performance.

This day of learning will teach you cutting edge techniques which you can’t find in training by folks who don’t spend time in the real world tuning performance.

Performance tuning mysteries can easily leave you stumbling through your work week, unsure if you’re focusing on the right things.

You’ll walk out of this class confident in your abilities to fix performance issues once and for all.

If you want to put out SQL Server performance fires, this is the precon you need to attend. Anyone can have a plan, it takes a professional to have a blueprint.

All attendees will get free access to my performance tuning training library, which also covers the material covered in the precon.

In-person attendees will also get a cool t-shirt. Arguably the coolest t-shirt ever made for a SQL Server conference.

SQL Server MAXDOP shirt
big eight

Dates And Times

The PASS Data Community Summit is taking place in Seattle November 15-18, 2022 and online.

You can register here, to attend online or in-person. I’ll be there in all my fleshy goodness, and I hope to see you there too!

Thanks for reading!

Going Further

If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% 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 database performance problems quickly. You can also get a quick, low cost health check with no phone time required.

SQL Server For Beginners: Searching For Data With Queries

Posted on by Erik Darling

Searching For Data With Queries

Going Further

If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% 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 database performance problems quickly. You can also get a quick, low cost health check with no phone time required.

SQL Server For Beginners: SQL Server Query Plans

Posted on by Erik Darling

SQL Server Query Plans

Going Further

If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% 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 database performance problems quickly. You can also get a quick, low cost health check with no phone time required.

SQL Server For Beginners: SQL Server Indexes

Posted on by Erik Darling

SQL Server Indexes

Going Further

If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% 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 database performance problems quickly. You can also get a quick, low cost health check with no phone time required.

SQL Server For Beginners: Query Execution

Posted on by Erik Darling

Query Execution

Going Further

If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% 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 database performance problems quickly. You can also get a quick, low cost health check with no phone time required.

There’s Still Time To Catch My PASS Precon: The Professional Performance Tuning Blueprint

Posted on by Erik Darling

Workin’

If the Gods of post scheduling cooperate, this should publish in October. That means you’ve got a little more than a month to buy tickets and book travel to PASS.

And of course, if you’re going to go, I’d love it you came to my precon: The Professional Performance Tuning Blueprint.

Searching the internet for every problem isn’t cutting it.

You need to be more proactive and efficient when it comes to finding and solving database performance fires. I work with consulting customers around the world to put out SQL Server performance fires.

In this day of learning, I will teach you how to find and fix your worst SQL Server problems using the same modern tools and techniques which I use every week.

You’ll learn tons of new and effective approaches to common performance problems, how to figure out what’s going on in your query plans, and how indexes really work to make your queries faster.

Together, we’ll tackle query rewrites, batch mode, how to design indexes, and how to gather all the information you need to analyze performance.

This day of learning will teach you cutting edge techniques which you can’t find in training by folks who don’t spend time in the real world tuning performance.

Performance tuning mysteries can easily leave you stumbling through your work week, unsure if you’re focusing on the right things.

You’ll walk out of this class confident in your abilities to fix performance issues once and for all.

If you want to put out SQL Server performance fires, this is the precon you need to attend. Anyone can have a plan, it takes a professional to have a blueprint.

All attendees will get free access to my performance tuning training library, which also covers the material covered in the precon.

In-person attendees will also get a cool t-shirt. Arguably the coolest t-shirt ever made for a SQL Server conference.

SQL Server MAXDOP shirt
big eight

Dates And Times

The PASS Data Community Summit is taking place in Seattle November 15-18, 2022 and online.

You can register here, to attend online or in-person. I’ll be there in all my fleshy goodness, and I hope to see you there too!

Thanks for reading!

Going Further

If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% 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 database performance problems quickly. You can also get a quick, low cost health check with no phone time required.

Changing From Int to Bigint In SQL Server: Annoyances With Backfilling Tables

Posted on by Erik Darling

Quickly, man!

I’m still looking into this a bit, but I ran into this issue when helping set up a process to migrate data to a table because the original table was running out of integer identity values.

The process looks something like:

  • Create a new table with the right definition
  • Write a loop to backfill existing table data
  • Create a trigger to keep updated data synchronized
  • Eventually swap the tables during a short maintenance window

All well and good! Except… You can end up with a really weird execution plan for the backfill process, and some pretty gnarly memory grants for wide tables.

Demonstrating with the Votes table:

CREATE TABLE 
    dbo.Votes_bigint
(
    Id bigint IDENTITY NOT NULL,
    PostId int NOT NULL,
    UserId int NULL,
    BountyAmount int NULL,
    VoteTypeId int NOT NULL,
    CreationDate datetime NOT NULL,
    CONSTRAINT PK_Votes_bigint_Id 
        PRIMARY KEY CLUSTERED (Id ASC)
) 
GO

INSERT
    dbo.Votes_bigint
(
    Id,
    PostId,
    UserId,
    BountyAmount,
    VoteTypeId,
    CreationDate
)
SELECT TOP (100000)
    v.Id,
    v.PostId,
    v.UserId,
    v.BountyAmount,
    v.VoteTypeId,
    v.CreationDate
FROM dbo.Votes AS v
ORDER BY v.Id;

The query plan has a Sort in it, which is weird because… Both tables are primary key/clustered index on the same column. Why re-sort that data?

I’m showing both serial and parallel plans:

SQL Server Query Plan
sickness bag

The Sort is always on this expression:

SQL Server Query Plan
Expr1003

Which in the XML looks like this:

[StackOverflow2013].[dbo].[Votes_bigint].[Id] = [Expr1003]

Thanks for reading!

Going Further

If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% 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 database performance problems quickly. You can also get a quick, low cost health check with no phone time required.

SQL Server Extended Event Duration Filtering Can Make Troubleshooting Frustrating

Posted on by Erik Darling

Shortness, Shortness

I use sp_HumanEvents to troubleshoot all sorts of SQL Server issues.

Most frequently, I used it to troubleshoot stored procedure performance like this:

EXEC sp_HumanEvents
    @event_type = 'query',                   
    @query_duration_ms = 5000,               
    @session_id = N'60',                    
    @keep_alive = 1,
    @debug = 1;

The session it creates will have this definition:

CREATE EVENT SESSION keeper_HumanEvents_query
    ON SERVER  
      ADD EVENT sqlserver.module_end 
        (SET collect_statement = 1
         ACTION (sqlserver.database_name, sqlserver.sql_text, sqlserver.plan_handle, sqlserver.query_hash_signed, sqlserver.query_plan_hash_signed)
         WHERE ( 
            sqlserver.is_system = 0 
     AND duration >= 5000000
     AND sqlserver.session_id = 60
 )),
      ADD EVENT sqlserver.rpc_completed 
        (SET collect_statement = 1
         ACTION(sqlserver.database_name, sqlserver.sql_text, sqlserver.plan_handle, sqlserver.query_hash_signed, sqlserver.query_plan_hash_signed)
         WHERE ( 
            sqlserver.is_system = 0 
     AND duration >= 5000000
     AND sqlserver.session_id = 60
 )),
      ADD EVENT sqlserver.sp_statement_completed 
        (SET collect_object_name = 1, collect_statement = 1
         ACTION(sqlserver.database_name, sqlserver.sql_text, sqlserver.plan_handle, sqlserver.query_hash_signed, sqlserver.query_plan_hash_signed) 
         WHERE ( 
            sqlserver.is_system = 0 
     AND duration >= 5000000
     AND sqlserver.session_id = 60
 )),
      ADD EVENT sqlserver.sql_statement_completed 
        (SET collect_statement = 1
         ACTION(sqlserver.database_name, sqlserver.sql_text, sqlserver.plan_handle, sqlserver.query_hash_signed, sqlserver.query_plan_hash_signed)
         WHERE ( 
            sqlserver.is_system = 0 
     AND duration >= 5000000
     AND sqlserver.session_id = 60
 )),
      ADD EVENT sqlserver.query_post_execution_showplan
        (
         ACTION(sqlserver.database_name, sqlserver.sql_text, sqlserver.plan_handle, sqlserver.query_hash_signed, sqlserver.query_plan_hash_signed)
         WHERE ( 
            sqlserver.is_system = 0 
     AND duration >= 5000000
     AND sqlserver.session_id = 60
 ))

I collect:

  • module_end
  • rpc_completed
  • sp_statement_completed
  • sql_statement_completed
  • query_post_execution_showplan

Not all of them are relevant to stored procedure calls, but in larger contexts where I have no idea where long running queries might be coming from, it’s useful to get all these.

Weirdness, Weirdness

Where I find things getting somewhat annoying is when things start showing up in there that meet the duration filter, but don’t really give me anything further to go on.

To simulate what I mean, I’m going to use this stored procedure:

CREATE OR ALTER PROCEDURE
    dbo.Eventually
AS
BEGIN

    SET NOCOUNT, XACT_ABORT ON;
    
    WAITFOR DELAY '00:00:01.000';
    SELECT TOP (1) * FROM dbo.Badges AS b;
    WAITFOR DELAY '00:00:01.000';
    SELECT TOP (1) * FROM dbo.Comments AS c;
    WAITFOR DELAY '00:00:01.000';
    SELECT TOP (1) * FROM dbo.Posts AS p;
    WAITFOR DELAY '00:00:01.000';
    SELECT TOP (1) * FROM dbo.Users AS u;
    WAITFOR DELAY '00:00:01.000';
    SELECT TOP (1) * FROM dbo.Votes AS v;
    WAITFOR DELAY '00:00:01.000';

END;
GO 

EXEC dbo.Eventually;

There are six waitfor commands that each pause for 1 second. In between them are queries that finish in milliseconds.

If I watch the event output, eventually, I’ll see this:

SQL Server Extended Events
lied to me, cried to me

Okay, so the stored procedure took more than 5 seconds, but… no individual query took more than 5 seconds.

To troubleshoot further, I have to set the duration bar even lower, and then figure out what I can meaningfully tune.

  • Do I have one query that takes four seconds
  • Do I have ten queries that take 500 milliseconds
  • Do I have 500 queries that take ten milliseconds

I see this behavior quite a bit with queries that loop/cursor over tons of data. They might take a whole bunch of rows and do a lot of really fast queries over them, but the time adds up.

There’s not really a good solution for this, either. The closer you look, by reducing the duration filter to lower and lower values, the more you return, the more overhead you cause, and the longer you have to wait for things to finish.

Sure, you can wring someone’s neck about not “thinking in sets”, but that doesn’t help when the processing takes place by passing looped values to stored procedures, unless you have the freedom to make significant app changes by using table valued parameters or something instead.

Thanks for reading!

Going Further

If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% 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 database performance problems quickly. You can also get a quick, low cost health check with no phone time required.

Tricky Situations When Rewriting Functions In SQL Server

Posted on by Erik Darling

Walk In Line

Far and away, some of the trickiest situations I run into when helping clients is rewriting scalar functions that have WHILE loops in them.

This sort of procedural code is often difficult, but not impossible, to replace with set-based logic.

Sure, lots of IF/THEN/ELSE stuff can be tough too, though that’s often easier to  manage with CASE expressions in stacked CTEs or derived tables.

I ran across a really interesting function recently that I had to rewrite that had a couple WHILE loops in it, and I’ve simplified the example here to show my approach to fixing it.

Table Face

The original intent of the function was to do some string manipulation and return a cleaned version of it.

There were several loops that looked for “illegal” characters, add in formatting characters (like dashes), etc.

The problem the function caused wasn’t it running for a long time (we’ll talk more about that tomorrow), it was that the function was called in really critical code paths that Function Repercussions© were messing with:

  • Row by row execution
  • Inhibited parallelism

These are not the kinds of functions that are Froid Friendly© either. If they were, I could largely leave them alone. Maybe.

Depends on bugs.

Bad Way

The bad way of doing this is like so. If you write functions like this, feel bad. Let it burn a little.

Ten years ago, I’d understand. These days, there’s a billion blog posts about why this is bad.

CREATE OR ALTER FUNCTION
    dbo.CountLetters_Bad
(
    @String varchar(20)
)
RETURNS bigint
AS 
BEGIN

DECLARE
    @CountLetters bigint = 0,
    @Counter int = 0;
    
    WHILE 
        LEN(@String) >= @Counter
    BEGIN
        IF PATINDEX
           (
               '%[^0-9]%', 
               SUBSTRING
               (
                   @String, 
                   LEN(@String) - @Counter, 
                   1
               )
           ) > 0
        BEGIN
            SET @CountLetters += 1;
            SET @Counter += 1;
        END;        
        ELSE        
        BEGIN
            SET @Counter += 1;
        END;
    END;

    RETURN @CountLetters;

END;
GO 


SELECT 
    CountLetters = 
        dbo.CountLetters_Bad('1A1A1A1A1A');

Better Way

This is a better way to write this specific function. It doesn’t come with all the baggage that the other function has.

But the thing is, if you just test them with the example calls at the end, you wouldn’t nearly be able to tell the difference.

CREATE OR ALTER FUNCTION 
   dbo.CountLetters
(   
    @String AS varchar(20)  
)  
RETURNS table
AS
RETURN

WITH
    t AS 
(
    SELECT TOP(LEN(@String))
        *,
        s = 
            SUBSTRING
            (
                @String, 
                n.Number +1, 
                1
            )
    FROM dbo.Numbers AS n
)
    SELECT
        NumLetters = 
            COUNT_BIG(*)
    FROM t
    WHERE PATINDEX('%[^0-9]%', t.s) > 0;
GO

SELECT
    cl.*
FROM dbo.CountLetters('1A1A1A1A1A') AS cl;

Pop Quiz Tomorrow

This is a problem I run into a lot: developers don’t really test SQL code in ways that are realistic to how it’ll be used.

  • Look, this scalar UDF runs fine for a single value
  • Look, this view runs fine on its own
  • Look, this table variable is great when I pass a test value to it

But this is hardly the methodology you should be using, because:

  • You’re gonna stick UDFs all over huge queries
  • You’re gonna join that view to 75,000 other views
  • You’re gonna let users pass real values to table variables that match lots of data

In tomorrow’s post, I’m gonna show you an example of how to better test code that calls functions, and what to look for.

Thanks for reading!

Going Further

If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% 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 database performance problems quickly. You can also get a quick, low cost health check with no phone time required.

Decoding Client Options In SQL Server Deadlocks

Posted on by Erik Darling

Has Been

Recently while working with a client to fix some deadlock issues, they asked about the client options stored in the XML.

I could remember looking at them at one point to see if there was a decoder ring for them, but never found anything definitive.

There’s a similar type of bittish thing in dm_exec_plan_attributes. It gets decoded in sp_BlitzCache like so:

SetOptions = SUBSTRING(
             CASE WHEN (CAST(pa.value AS INT) & 1 = 1) THEN ', ANSI_PADDING' ELSE '' END +
             CASE WHEN (CAST(pa.value AS INT) & 8 = 8) THEN ', CONCAT_NULL_YIELDS_NULL' ELSE '' END +
             CASE WHEN (CAST(pa.value AS INT) & 16 = 16) THEN ', ANSI_WARNINGS' ELSE '' END +
             CASE WHEN (CAST(pa.value AS INT) & 32 = 32) THEN ', ANSI_NULLS' ELSE '' END +
             CASE WHEN (CAST(pa.value AS INT) & 64 = 64) THEN ', QUOTED_IDENTIFIER' ELSE '' END +
             CASE WHEN (CAST(pa.value AS INT) & 4096 = 4096) THEN ', ARITH_ABORT' ELSE '' END +
             CASE WHEN (CAST(pa.value AS INT) & 8192 = 8191) THEN ', NUMERIC_ROUNDABORT' ELSE '' END 
             , 2, 200000)

Based on that, I was hopeful that I could use a combination of SSMS and session settings to figure out where bits flip in client options.

I don’t think I got all of them, but this simple demo shows off the ones that were accessible. Some caveats here:

  • No one at Microsoft has validated these
  • They might change in the future
  • There are probably ones I missed

Ring Toss

Here’s what I came up with, using my own experimenting, and also some test data from deadlock XML files I had sitting around.

CREATE TABLE 
    #temptable 
( 
    clientoption1 bigint, 
    clientoption2 bigint 
);

INSERT INTO 
    #temptable 
(
    clientoption1, 
    clientoption2
)
VALUES
(536870944, 128056), 
(671088672, 128056), 
(671088672, 128058), 
(673185824, 128056), 
(673316896, 128056);


SELECT
    q.clientoption1,
    q.clientoption2,
    clientoption1 = 
        SUBSTRING
        (    
            CASE WHEN q.clientoption1 & 0x20 = 0x20 THEN ', QUOTED IDENTIFIER ON' ELSE '' END +
            CASE WHEN q.clientoption1 & 0x40 = 0x40 THEN ', ARITHABORT' ELSE '' END +
            CASE WHEN q.clientoption1 & 0x800 = 0x800 THEN ', USER SET ARITHABORT' ELSE '' END +
            CASE WHEN q.clientoption1 & 0x8000 = 0x8000 THEN ', NUMERIC ROUNDABORT ON' ELSE '' END +
            CASE WHEN q.clientoption1 & 0x10000 = 0x10000 THEN ', USER SET NUMERIC ROUNDABORT ON' ELSE '' END +
            CASE WHEN q.clientoption1 & 0x20000 = 0x20000 THEN ', SET XACT ABORT ON' ELSE '' END +
            CASE WHEN q.clientoption1 & 0x80000 = 0x80000 THEN ', NOCOUNT OFF' ELSE '' END +
            CASE WHEN q.clientoption1 & 0x200000 = 0x200000 THEN ', NOCOUNT ON' ELSE '' END +
            CASE WHEN q.clientoption1 & 0x8000000 = 8000000 THEN ', USER SET QUOTED IDENTIFIER' ELSE '' END +
            CASE WHEN q.clientoption1 & 0x20000000 = 0x20000000 THEN ', ANSI NULL DEFAULT ON' ELSE '' END +
            CASE WHEN q.clientoption1 & 0x40000000 = 0x40000000 THEN ', ANSI NULL DEFAULT OFF' ELSE '' END,
            3,
            8000
        ),
    clientoption2 = 
        SUBSTRING
        (
            CASE WHEN q.clientoption2 & 2 = 2 THEN ', IMPLICIT TRANSACTION' ELSE '' END +
            CASE WHEN q.clientoption2 & 8 = 8 THEN ', ANSI WARNINGS' ELSE '' END +
            CASE WHEN q.clientoption2 & 0x10 = 0x10 THEN ', ANSI PADDING' ELSE '' END +
            CASE WHEN q.clientoption2 & 0x20 = 0x20 THEN ', ANSI NULLS' ELSE '' END +
            CASE WHEN q.clientoption2 & 0x1000 = 0x1000 THEN ', USER CONCAT NULL YIELDS NULL' ELSE '' END +
            CASE WHEN q.clientoption2 & 0x2000 = 0x2000 THEN ', CONCAT NULL YIELDS NULL' ELSE '' END +
            CASE WHEN q.clientoption2 & 0x4000 = 0x4000 THEN ', USER ANSI NULLS' ELSE '' END +
            CASE WHEN q.clientoption2 & 0x8000 = 0x8000 THEN ', USER ANSI WARNINGS' ELSE '' END,
            3,
            8000
        )
FROM #temptable AS q;

I may work on getting these into some deadlock analysis tooling after I have a little more validation that the results are correct.

Thanks for reading!

Going Further

If this is the kind of SQL Server stuff you love learning about, you’ll love my training. I’m offering a 75% 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 database performance problems quickly. You can also get a quick, low cost health check with no phone time required.