Parameter Sniffing - Need more info
Mardy
it. What I don't understand is why it happens. Is it a bug? Why does it
consistently result in less efficient execution plans? Is it only apparent on
multiple processor db's? Are there any other configurations that are more
susceptible to it.
I've been using SQL Sever for a long time and I've never experienced this
before. The envorinment I'm in now is a little different than at prevoius
clients so I'm wondering if the db config or other changes may exacerbate the
situation.
Thanks
Mardy
Adam Machanic
informed decisions. Why do you think that it makes for less efficient
plans?
--
Adam Machanic
SQL Server MVP
http://www.sqljunkies.com/weblog/amachanic
--
"Mardy" <Ma...@discussions.microsoft.com> wrote in message
news:8C8078AB-7FAE-4CEE...@microsoft.com...
oj
The reason for the performance difference stems from a feature called
"parameter sniffing". Consider a stored proc defined as follows:
CREATE PROC proc1 @p1 int AS
SELECT * FROM table1 WHERE c1 = @p1
GO
Keep in mind that the server has to compile a complete execution plan for
the proc before the proc begins to execute. In 6.5, at compile time SQL
didn't know what the value of @p1 was, so it had to make a lot of guesses
when compiling a plan. Suppose all of the actual parameter values for
"@p1 int" that a user ever passed into this stored proc were unique
integers that were greater than 0, but suppose 40% of the [c1] values in
[table1] were, in fact, 0. SQL would use the average density of the
column to estimate the number of rows that this predicate would return;
this would be an overestimate, and SQL would might choose a table scan
over an index seek based on the rowcount estimates. A table scan would
be the best plan if the parameter value was 0, but unfortunately it
happens that users will never or rarely pass @p1=0, so performance of the
stored proc for more typical parameters suffers.
In SQL 7.0 or 2000, suppose you executed this proc for the first time
(when the sp plan is not in cache) with the command "EXEC proc1 @p1 =
10". Parameter sniffing allows SQL to insert the known value of
parameter @p1 into the query at compile time before a plan for the query
is generated. Because SQL knows that the value of @p1 is not 0, it can
compile a plan that is tailored to the class of parameters that is
actually passed into the proc, so for example it might select an index
seek instead of a table scan based on the smaller estimated rowcount --
this is a good thing if most of the time 0 is not the value passed as
@p1. Generally speaking, this feature allows more efficient stored proc
execution plans, but a key requirement for everything to work as expected
is that the parameter values used for compilation be "typical".
In your case, the problem is that you have default NULL values for your
parameters ("@Today DATETIME = NULL, ...") that are not typical because
the parameter values are changed inside the stored proc before they are
used -- as a result NULL will never actually be used to search the
column. If the first execution of this stored proc doesn't pass in an
explicit value for the @Today parameter, SQL believes that its value will
be NULL. When SQL compiles the plan for this sp it substitutes NULL for
each occurrence of @Today that is embedded within a query.
Unfortunately, after execution begins the first thing the stored proc
does is change @Today to a non-NULL value if it is found to be NULL, but
unfortunately SQL doesn't know about this at compile time. Because NULL
is a very atypical parameter value, the plan that SQL generates may not
be a good one for the new value of the parameter that is assigned at
execution time.
So, the bottom line is that if you assign defaults to your sp parameters
and later use those same parameters in a query, the defaults should be
"typical" because they will be used during plan generation. If you must
use defaults and business logic dictates that they be atypical (as may be
the case here if app modifications are not an option), there are two
possible solutions if you determine that the substitution of atypical
parameter values is causing bad plans:
1. "Disable" parameter sniffing by using local DECLARE'd variables that
you SET equal to the parameters inside the stored proc, and use the local
variables instead of the offending parameters in the queries. This is the
solution that you found yourself. SQL can't use parameter sniffing in
this case so it must make some guesses, but in this case the guess based
on average column density is better than the plan based on a specific but
"wrong" parameter value (NULL).
2. Nest the affected queries somehow so that they run within a different
context that will require a distinct execution plan. There are several
possibilities here. for example:
a. Put the affected queries in a different "child" stored proc. If
you execute that stored proc within this one *after* the parameter @Today
has been changed to its final value, parameter sniffing will suddenly
become your friend because the value SQL uses to compile the queries
inside the child stored proc is the actual value that will be used in the
query.
b. Use sp_executesql to execute the affected queries. The plan won't
be generated until the sp_executesql stmt actually runs, which is of
course after the parameter values have been changed.
c. Use dynamic SQL ("EXEC (@sql)") to execute the affected queries.
An equivalent approach would be to put the query in a child stored proc
just like 2.a, but execute it within the parent proc with EXEC WITH
RECOMPILE.
Option #1 seems to have worked well for you in this case, although
sometimes one of the options in #2 is a preferable choice. Here are some
guidelines, although when you're dealing with something as complicated as
the query optimizer experimentation is often the best approach <g>:
- If you have only one "class" (defined as values that have similar
density in the table) of actual parameter value that is used within a
query (even if there are other classes of data in the base table that are
never or rarely searched on), 2.a. or 2.b is probably the best option.
This is because these options permit the actual parameter values to be
used during compilation which should result in the most efficient query
plan for that class of parameter.
- If you have multiple "classes" of parameter value (for example, for
the column being searched, half the table data is NULL, the other half
are unique integers, and you may do searches on either class), 2.c can be
effective. The downside is that a new plan for the query must be
compiled on each execution, but the upside is that the plan will always
be tailored to the parameter value being used for that particular
execution. This is best when there is no single execution plan that
provides acceptable execution time for all classes of parameters.
HTH -
Bart
------------
Bart Duncan
Microsoft SQL Server Support
Please reply to the newsgroup only - thanks.
This posting is provided "AS IS" with no warranties, and confers no
rights.
Also, we had a good discusion a while back:
http://tinyurl.com/4vhy4
--
-oj
"Mardy" <Ma...@discussions.microsoft.com> wrote in message
news:8C8078AB-7FAE-4CEE...@microsoft.com...
Bob Barrows [MVP]
> It is a feature, not a bug -- it helps the query optimizer make more
> informed decisions. Why do you think that it makes for less efficient
> plans?
>
take several minutes when used in a procedure. :-)
Bob Barrows
--
Microsoft MVP -- ASP/ASP.NET
Please reply to the newsgroup. The email account listed in my From
header is my spam trap, so I don't check it very often. You will get a
quicker response by posting to the newsgroup.
Adam Machanic
news:eWp3WVy$EHA....@TK2MSFTNGP12.phx.gbl...
>
> Tell that to somebody who has seen a query run in QA in less than a second
> take several minutes when used in a procedure. :-)
I'll tell them that they ran the wrong query! :D
Mardy
Thanks oj. That really helps.
Mardy
Gert-Jan Strik
>
> I recently learned about parameter sniffing and I understand how to deal with
> it. What I don't understand is why it happens.
See Bart Duncan's full explanation that was posted by oj.
> Is it a bug?
No, it is a feature. But if your table data is not evenly distributed
and you have bad luck, then later calls to the same stored procedure
might use a suboptimal query plan for the supplied parameter(s). Note
that that behavior is an exception, not the rule.
> Why does it consistently result in less efficient execution plans?
It doesn't. It usually increases performance a little or a lot. But
nobody is ever complaining about good performing queries in this forum
:-)
> Is it only apparent on multiple processor db's?
AFAIK there is no relation between parameter sniffing and number of
CPU's.
> Are there any other configurations that are more susceptible to it.
No, because it is not linked to configuration. Only to data distribution
and statistics on this distribution. Bad statistics can result in bad
query plans (with or without parameter sniffing). By default, statistics
are automatically created and updated, which is generally a good thing.
> I've been using SQL Sever for a long time and I've never experienced this
> before. The envorinment I'm in now is a little different than at prevoius
> clients so I'm wondering if the db config or other changes may exacerbate the
> situation.
If you do not have skewed data and use auto update statistics, then you
would never encounter this problem. If you do have skewed data, then you
should read the posted article so you can assess what "risk" there is
for your situation.
HTH,
Gert-Jan
Adam Machanic
news:uN9aWFy$EHA....@TK2MSFTNGP15.phx.gbl...
>
> 2. Nest the affected queries somehow so that they run within a different
> context that will require a distinct execution plan. There are several
> possibilities here. for example:
> a. Put the affected queries in a different "child" stored proc. If
> you execute that stored proc within this one *after* the parameter @Today
> has been changed to its final value, parameter sniffing will suddenly
> become your friend because the value SQL uses to compile the queries
> inside the child stored proc is the actual value that will be used in the
> query.
> b. Use sp_executesql to execute the affected queries. The plan won't
> be generated until the sp_executesql stmt actually runs, which is of
> course after the parameter values have been changed.
> c. Use dynamic SQL ("EXEC (@sql)") to execute the affected queries.
> An equivalent approach would be to put the query in a child stored proc
> just like 2.a, but execute it within the parent proc with EXEC WITH
> RECOMPILE.
Here's an article I've written that shows a technique combining 2b and
2c:
http://sqljunkies.com/WebLog/amachanic/articles/StoredProcedureCaching.aspx
Mardy
Gert-Jan Strik
See below. Your original text is now prefixed with "> > > ", my comments
with "> > ". Maybe I should have added the message "see inline".
Gert-Jan