Tuesday 9 June 2009

Optimizing a query with Forced Parameterization

SQL 2005 Forced Parameterization Overview

SQL 2005 has a feature which will take AdHoc queries containing literal values and remove those values replacing them with parameters. This means that the query plan which gets cached can be re-used for similar queries that have different values which can aid performance as it will reduce compilation time. This is called FORCED PARAMETERIZATION however the default mode is SIMPLE which will only cache and re-use AdHoc plans containing the same literal values. This cool feature is useful for systems where stored procs and prepared statements aren't being used as these forms of query will already benefit from cached plan re-use. You should read BOL about the suitability of this feature as certain constructs and features will prevent it from being utilized. It may also result in unsuitable query plans being re-used in certain cases rather than a new plan being generated for a query.

Using Forced Parameterization

I came across this feature on the wonderful SQLServerCentral.com site on a discussion about dynamic SQL. I read up about it and thought it might come in useful on a site that has intermittent problems on a results page when hit by concurrent crawlers which results in high CPU on the SQL Server. The page in question is a results page that allows filtering and ordering by multiple columns, both directions and also uses paging. However the query is an AdHoc query that is built up on the client using literal values that are appended to a string rather than using parameters and something like ADO's sqlCommand object to pass those parameter values to the database.

I looked into rewriting this query to make use of parameters but the WHERE clause was being cached and re-used by multiple queries on the same page so it wasn't a simple change therefore it seemed like a good candidate for FORCED PARAMETERIZATION.

Testing the theory

Before enabling this feature for the live site I wanted to prove on our development box that
a) Simple mode just wasn't good enough and was causing multiple plans to be compiled and cached for this query.
b) Forced mode would enable one plan to be cached and re-used for multiple parameter variations of this query.

Enabling Forced Parameterization

I had some trouble with this due to a number of other sites/blogs showing the incorrect way to set this but the correct way is:
-- enable forced mode

-- enable simple mode

You can also use the Management Console and the properties option under the relevant database to switch the setting over manually. When switching from one mode to another I made sure to clear the current cache by running the following:
I then used the following query which makes use of SQL 2005's DMV's (data management views) to check the queries that were currently cached and the type of method being engaged.

FROM sys.dm_exec_query_stats qs
CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) AS qt
INNER JOIN sys.dm_exec_cached_plans as cp on qs.plan_handle=cp.plan_handle
WHERE cp.plan_handle=qs.plan_handle
AND (ObjType = 'Adhoc' OR ObjType='Prepared')

Notice that I am filtering on AdHoc and Prepared and ignoring Stored Procs, Triggers and Views. I initially tried filtering by the dbid (database id) column to narrow down the results to my particular database however for AdHoc and Prepared queries this column contains NULL values.

The columns I am particular interested in are the plan_handle which is the ID of the query plan related to the SQL statement and execution_count which tells me how many times the SQL related to the plan has been executed.

A very simple version of the query in question which conveys the point I am trying to make is below.


With SIMPLE mode enabled a test page running my query with different criteria values for my BETWEEN statement caused multiple records to be returned from sys.dm_exec_cached_plans each with a different plan_handle value and the value for ObjType set to AdHoc. This tells me that the plans are not being re-used by similar queries when they could be. Running the same query with the same values does increment the execution_count against the relevant plan record which shows that a very simple form of re-use is being utilized. However as I want the best performance as possible this isn't good enough.

With FORCED mode enabled and the cache cleared the test pages with different criteria for my BETWEEN statement results in a single record to be returned with one plan_handle. The value for ObjType has now changed from AdHoc to Prepared and looking at the start of the text column which contains the SQL I can see the following before the start of my SQL:
(@0 int,@1 int,@2 int,@3 int,@4 varchar(8000),@5 int,@6 int)

Which is the database creating the parameters required to replace the literal values from my query. The values from my BETWEEN clause have been changed from literals to parameters e.g:
--Simple Mode Enabled
--Forced Mode Enabled
WHERE Row BETWEEN @5 and @6
Re-running the test page with different values for these parameters causes the execution_count to increment which informs me that the query plan is being cached and re-used across similar queries which is what I wanted to prove.

Enabling Forced Parameterization for a single query

There maybe situations where you want to enable this feature for one or more queries rather than set it on for the whole database. You can do this by setting up a plan guide for the query in question. An easy way to do this is to run the following SQL making sure to replace the first parameter value with the SQL you want to parametrize.

DECLARE @stmt nvarchar(max);
DECLARE @params nvarchar(max);
EXEC sp_get_query_template
N'SELECT * FROM JOBS WHERE Row between 1 AND 20;',
@stmt OUTPUT,
@params OUTPUT;
EXEC sp_create_plan_guide

You can then set you database to SIMPLE mode but still benefit from having parametrized queries that match the template you have just created in the previous plan. Some useful SQL related to managing query plans is below:

--View all plans in your system
SELECT * FROM sys.plan_guides

--Disable the plan guide.
EXEC sp_control_plan_guide N'DISABLE', N'MyPlanGuide_JobResults';

--Enable the plan guide.
EXEC sp_control_plan_guide N'ENABLE', N'MyPlanGuide_JobResults';

--Drop the plan guide.
EXEC sp_control_plan_guide N'DROP', N'MyPlanGuide_JobResults';

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