I’m Brent Ozar, and I like databases. Not yours though.

Why is my query sometimes fast,
and sometimes terribly slow?

Even weirder, it just gets slow out of nowhere – even when you swear you haven’t changed anything.

Why Is This Query Sometimes Slow? – I show you how parameter sniffing happens, and why reboots/restarts seem to fix it, but it keeps coming back.

How to Start Troubleshooting Parameter Sniffing – a brief introduction of what parameter sniffing is, how to fix it during an emergency, and how to gather the data you’ll need to fix it right for the long term.

Video: Fixing Parameter Sniffing Part 1 – a demo of how parameter sniffing happens, and how to detect it in the plan cache.

Video: Fixing Parameter Sniffing Part 2 – I show why you can’t just comment out CREATE PROC in order to test parameter sniffing, show you how to really test your code, and why OPTION RECOMPILE can produce the wrong query results.

Optimize for Unknown (Mediocre) – this query hint gives you a predictable plan, but it’s just usually predictably BAD, not predictably good.

Slow in the App, Fast in SSMS – Erland Sommarskog’s epic novel (dozens of pages long) about the causes and fixes for parameter sniffing and different execution plans.

Video: Identifying and Fixing Parameter Sniffing

From my session at SQLDay Poland:

Here is the latest version of the demo scripts I use in the demo:

/*
Why is the Same Query Sometimes Slow?

v1.7 - 2019-05-22

I'm going to teach you 4 things:

1. What parameter sniffing is
2. How to react to parameter sniffing emergencies
3. How to test a query with sniffing problems
4. Options to fix the query long term

Learn more later at:
https://www.BrentOzar.com/go/sniff


This demo requires:

* Any supported version of SQL Server or Azure SQL DB
* Any Stack Overflow database: https://www.BrentOzar.com/go/querystack
* DropIndexes proc: https://www.BrentOzar.com/go/dropindexes
* sp_BlitzCache: https://www.BrentOzar.com/blitzcache
*/
USE StackOverflow2013;
GO
ALTER DATABASE CURRENT SET COMPATIBILITY_LEVEL = 140;
GO
EXEC sys.sp_configure N'cost threshold for parallelism', N'50'
GO
EXEC sys.sp_configure N'max degree of parallelism', N'4'
GO
RECONFIGURE WITH OVERRIDE
GO
DropIndexes;
GO
/* Turn on actual execution plans and: */
SET STATISTICS IO, TIME ON;
GO

CREATE INDEX IX_Reputation ON dbo.Users(Reputation)
GO
SELECT TOP 10000 *
FROM dbo.Users
WHERE Reputation=2
ORDER BY DisplayName;
GO

SELECT TOP 10000 * 
FROM dbo.Users
WHERE Reputation=1
ORDER BY DisplayName;
GO



CREATE OR ALTER PROCEDURE dbo.usp_UsersByReputation
  @Reputation int
AS
SELECT TOP 10000 *
FROM dbo.Users
WHERE Reputation=@Reputation
ORDER BY DisplayName;
GO

EXEC dbo.usp_UsersByReputation @Reputation =1;
GO
EXEC dbo.usp_UsersByReputation @Reputation =2;
GO

DBCC FREEPROCCACHE
GO
EXEC dbo.usp_UsersByReputation @Reputation =2;
GO
EXEC dbo.usp_UsersByReputation @Reputation =1;
GO
sp_BlitzCache




DECLARE @Reputation INT = 2

--CREATE PROCEDURE dbo.usp_UsersByReputation
--  @Reputation int
--AS
SELECT TOP 10000 *
FROM dbo.Users
WHERE Reputation=@Reputation
ORDER BY DisplayName;
GO

DBCC SHOW_STATISTICS('dbo.Users', 'IX_Reputation')
GO



CREATE PROCEDURE #usp_UsersByReputation @Reputation int AS
SELECT * FROM dbo.Users
WHERE Reputation= @Reputation
GO

EXEC #usp_UsersByReputation 2
GO




/* "Normal" dynamic SQL: */
CREATE OR ALTER PROCEDURE dbo.usp_UsersByReputation
  @Reputation int
AS
BEGIN
DECLARE @StringToExecute NVARCHAR(4000);
SET @StringToExecute = N'SELECT TOP 10000 * FROM dbo.Users WHERE Reputation=@Reputation ORDER BY DisplayName;';

EXEC sp_executesql @StringToExecute, N'@Reputation INT', @Reputation;
END
GO

DBCC FREEPROCCACHE;
GO
EXEC usp_UsersByReputation @Reputation = 1;
GO
EXEC usp_UsersByReputation @Reputation = 2;
GO
sp_BlitzCache;
GO


/* "Bad" dynamic SQL building unparameterized strings: */
CREATE OR ALTER PROCEDURE dbo.usp_UsersByReputation
  @Reputation int
AS
BEGIN
DECLARE @StringToExecute NVARCHAR(4000);
SET @StringToExecute = N'SELECT TOP 10000 * FROM dbo.Users WHERE Reputation= '
	+ CAST(@Reputation AS NVARCHAR(10))
	+ N' ORDER BY DisplayName;';

EXEC sp_executesql @StringToExecute;
END
GO

DBCC FREEPROCCACHE;
GO
EXEC usp_UsersByReputation @Reputation = 1;
GO
EXEC usp_UsersByReputation @Reputation = 2;
GO
sp_BlitzCache;
GO



/* Parameterized, but with comment injection: */
CREATE OR ALTER PROCEDURE dbo.usp_UsersByReputation
  @Reputation int
AS
BEGIN
DECLARE @StringToExecute NVARCHAR(4000);
SET @StringToExecute = N'SELECT TOP 10000 * FROM dbo.Users WHERE Reputation=@Reputation ORDER BY DisplayName;';

IF @Reputation = 1
	SET @StringToExecute = @StringToExecute + N' /* Big data */';

EXEC sp_executesql @StringToExecute, N'@Reputation INT', @Reputation;
END
GO

DBCC FREEPROCCACHE;
GO
EXEC usp_UsersByReputation @Reputation = 1;
GO
EXEC usp_UsersByReputation @Reputation = 2;
GO
sp_BlitzCache;
GO
/*
Learn more later at:
https://www.BrentOzar.com/go/sniff


License: Creative Commons Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0)
More info: https://creativecommons.org/licenses/by-sa/3.0/

You are free to:
* Share - copy and redistribute the material in any medium or format
* Adapt - remix, transform, and build upon the material for any purpose, even 
  commercially

Under the following terms:
* Attribution - You must give appropriate credit, provide a link to the license,
  and indicate if changes were made.
* ShareAlike - If you remix, transform, or build upon the material, you must
  distribute your contributions under the same license as the original.
*/

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Why is the Same Query Sometimes Slow?

v1.7 - 2019-05-22

I'm going to teach you 4 things:

1. What parameter sniffing is

2. How to react to parameter sniffing emergencies

3. How to test a query with sniffing problems

4. Options to fix the query long term

Learn more later at:

https://www.BrentOzar.com/go/sniff

This demo requires:

* Any supported version of SQL Server or Azure SQL DB

* Any Stack Overflow database: https://www.BrentOzar.com/go/querystack

* DropIndexes proc: https://www.BrentOzar.com/go/dropindexes

* sp_BlitzCache: https://www.BrentOzar.com/blitzcache

USE StackOverflow2013;

ALTER DATABASE CURRENT SET COMPATIBILITY_LEVEL = 140;

EXEC sys.sp_configure N'cost threshold for parallelism', N'50'

EXEC sys.sp_configure N'max degree of parallelism', N'4'

RECONFIGURE WITH OVERRIDE

DropIndexes;

/* Turn on actual execution plans and: */

SET STATISTICS IO, TIME ON;

CREATE INDEX IX_Reputation ON dbo.Users(Reputation)

SELECT TOP 10000 *

FROM dbo.Users

WHERE Reputation=2

ORDER BY DisplayName;

SELECT TOP 10000 *

FROM dbo.Users

WHERE Reputation=1

ORDER BY DisplayName;

CREATE OR ALTER PROCEDURE dbo.usp_UsersByReputation

@Reputation int

SELECT TOP 10000 *

FROM dbo.Users

WHERE Reputation=@Reputation

ORDER BY DisplayName;

EXEC dbo.usp_UsersByReputation @Reputation =1;

EXEC dbo.usp_UsersByReputation @Reputation =2;

DBCC FREEPROCCACHE

EXEC dbo.usp_UsersByReputation @Reputation =2;

EXEC dbo.usp_UsersByReputation @Reputation =1;

sp_BlitzCache

DECLARE @Reputation INT = 2

--CREATE PROCEDURE dbo.usp_UsersByReputation

-- @Reputation int

--AS

SELECT TOP 10000 *

FROM dbo.Users

WHERE Reputation=@Reputation

ORDER BY DisplayName;

DBCC SHOW_STATISTICS('dbo.Users', 'IX_Reputation')

CREATE PROCEDURE #usp_UsersByReputation @Reputation int AS

SELECT * FROM dbo.Users

WHERE Reputation= @Reputation

EXEC #usp_UsersByReputation 2

/* "Normal" dynamic SQL: */

CREATE OR ALTER PROCEDURE dbo.usp_UsersByReputation

@Reputation int

BEGIN

DECLARE @StringToExecute NVARCHAR(4000);

SET @StringToExecute = N'SELECT TOP 10000 * FROM dbo.Users WHERE Reputation=@Reputation ORDER BY DisplayName;';

EXEC sp_executesql @StringToExecute, N'@Reputation INT', @Reputation;

END

DBCC FREEPROCCACHE;

EXEC usp_UsersByReputation @Reputation = 1;

EXEC usp_UsersByReputation @Reputation = 2;

sp_BlitzCache;

/* "Bad" dynamic SQL building unparameterized strings: */

CREATE OR ALTER PROCEDURE dbo.usp_UsersByReputation

@Reputation int

BEGIN

DECLARE @StringToExecute NVARCHAR(4000);

SET @StringToExecute = N'SELECT TOP 10000 * FROM dbo.Users WHERE Reputation= '

+ CAST(@Reputation AS NVARCHAR(10))

+ N' ORDER BY DisplayName;';

EXEC sp_executesql @StringToExecute;

END

DBCC FREEPROCCACHE;

EXEC usp_UsersByReputation @Reputation = 1;

EXEC usp_UsersByReputation @Reputation = 2;

sp_BlitzCache;

/* Parameterized, but with comment injection: */

CREATE OR ALTER PROCEDURE dbo.usp_UsersByReputation

@Reputation int

BEGIN

DECLARE @StringToExecute NVARCHAR(4000);

SET @StringToExecute = N'SELECT TOP 10000 * FROM dbo.Users WHERE Reputation=@Reputation ORDER BY DisplayName;';

IF @Reputation = 1

SET @StringToExecute = @StringToExecute + N' /* Big data */';

EXEC sp_executesql @StringToExecute, N'@Reputation INT', @Reputation;

END

DBCC FREEPROCCACHE;

EXEC usp_UsersByReputation @Reputation = 1;

EXEC usp_UsersByReputation @Reputation = 2;

sp_BlitzCache;

Learn more later at:

https://www.BrentOzar.com/go/sniff

License: Creative Commons Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0)

More info: https://creativecommons.org/licenses/by-sa/3.0/

You are free to:

* Share - copy and redistribute the material in any medium or format

* Adapt - remix, transform, and build upon the material for any purpose, even

commercially

Under the following terms:

* Attribution - You must give appropriate credit, provide a link to the license,

and indicate if changes were made.

* ShareAlike - If you remix, transform, or build upon the material, you must

distribute your contributions under the same license as the original.

Parameter Sniffing

Why is my query sometimes fast, and sometimes terribly slow?

Video: Identifying and Fixing Parameter Sniffing

Why is my query sometimes fast,
and sometimes terribly slow?