Download Powerpoint - Data Realized

Plan Guides
Jeremy Lowell
Data Realized
[email protected]
Agenda
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Who is it for?
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Why do they exist?
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What is it?
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Query processing
• Query Plans
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Plan Guides
• Demo
Overview
• Plan Guides
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Why?
• Increased complexity in database environments today
– Linq to SQL
– C.O.T.S. implementations
» Commercial-off-the-shelf
– Proactive vs. Reactive
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Who?
• Database Administrators
• Database & BI Developers
Plan Guides
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What are they?
• Plan Guides can be used to optimize the performance of
queries when you cannot or do not want to change the text of
the query directly.
• Plan guides can be useful when a small subset of queries in a
database application deployed from a third-party vendor are
not performing as expected.
• Plan guides influence optimization of queries by attaching
query hints or a fixed query plan to them. -- Microsoft
Query Plans
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Query Processing
Query Plans
• Creation
• One note of importance::
– SQL Server uses the optimal access plan, but query optimization is limited by what is
possible.
» Meaning… Even a great optimizer can’t always make poor code run efficiently.
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Optimizer
• Evaluates
– Keys
– Indexes
» Clustered
» Non-clustered
– Cardinality (selectivity)
– Quantity of data
– Statistics
– Etc…
• Generates Query Plan
– Execution context (parameter variables)
• Stores Query Plan in Cache
– This is where it gets FUN!
– Known as Procedure Cache, Statement Cache, Plan Cache etc…
Query Plans
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Cache
• Hard to “see” the cache in memory on a server…
– dbcc proccache
num proc buffs
-------------------510917
•
•
•
•
•
•
num proc buffs used num proc buffs active proc cache size
----------------------------------------------------------230
230
18844
proc cache used
-------------------28
proc cache active
-------------------28
num proc buffs
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Total number of pages used by all entries in the procedure cache.
num proc buffs used
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Total number of pages used by all entries that are currently being used.
num proc buffs active
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For backward compatibility only. Total number of pages used by all entries that are currently being used.
proc cache size
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Total number of entries in the procedure cache.
proc cache used
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Total number of entries that are currently being used.
proc cache active
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For backward compatibility only. Total number of entries that are currently being used.
Query Plans
• Cache
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DMV’s
– SELECT cacheobjtype,objtype, sum(refcounts) as TotalRefcounts, sum(usecounts) as
TotalUseCounts from sys.dm_Exec_Cached_plans
group by cacheobjtype,objtype
cacheobjtype
objtype
TotalRefcounts
TotalUseCounts
-------------------------------------------------- -------------------- --------------------------Parse Tree
View
242
48049
Compiled Plan
Proc
2044
9140155
Compiled Plan
Trigger
75
149753
Extended Proc
Proc
25
320641
Compiled Plan
Adhoc
31907
294193
Parse Tree
UsrTab
3
5782
Compiled Plan
Prepared
3696
1951629
Parse Tree
Check
13
213
• Refcounts
– Number of cache objects that are referencing this cache object.
• UseCounts
– Number of times this object has been used since it has existed in cache
• Cacheobjtype
– Type of object in cache
• Ojbtype
– Type of object in cache
Query Plans
• Cache
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DMV’s
• Return attributes for a specific plan
– SELECT plan_handle, refcounts, usecounts, size_in_bytes, cacheobjtype, objtype FROM
sys.dm_exec_cached_plans; GO SELECT attribute, value, is_cache_key FROM
sys.dm_exec_plan_attributes(<plan_handle>);
• Set options
– SELECT plan_handle, pvt.set_options, pvt.sql_handle FROM ( SELECT plan_handle,
epa.attribute, epa.value FROM sys.dm_exec_cached_plans OUTER APPLY
sys.dm_exec_plan_attributes(plan_handle) AS epa WHERE cacheobjtype = 'Compiled Plan' ) AS
ecpa PIVOT (MAX(ecpa.value) FOR ecpa.attribute IN ("set_options", "sql_handle")) AS pvt;
• Optimizer info
– select * from sys.dm_exec_query_optimizer_info
• Memory breakdown of all cached complied plans
– SELECT plan_handle, ecp.memory_object_address AS CompiledPlan_MemoryObject,
omo.memory_object_address, pages_allocated_count, type, page_size_in_bytes
FROM sys.dm_exec_cached_plans AS ecp JOIN sys.dm_os_memory_objects AS omo
ON ecp.memory_object_address = omo.memory_object_address
OR ecp.memory_object_address = omo.parent_address
WHERE cacheobjtype = 'Compiled Plan’ order by page_size_in_bytes desc
Query Plans
• Cache
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DMV’s
SELECT st.text, cp.usecounts, cp.size_in_bytes, cp.cacheobjtype,
cp.objtype from sys.dm_exec_cached_plans cp
cross apply
sys.dm_exec_sql_text(cp.plan_handle) st order by cp.usecounts desc
Sql_text
usecounts size_in_bytes cacheobjtype objtype
--------------------------------create procedure [dbo]… 358968
CREATE PROC [dbo]… 308201
CREATE PROC [dbo]… 290058
CREATE PROCEDU [db 280376
CREATE FUNC [dbo]… 280253
(@P1 ntext,@P2 nvarch… 274484
(@p0 int)SELECT [t0].[… 233054
-----------------------450560
245760
114688
565248
155648
40960
98304
• sys.dm_exec_sql_text(cp.plan_handle)
– Imagine the possibilities!!
-----------------------Compiled Plan
Compiled Plan
Compiled Plan
Compiled Plan
Compiled Plan
Compiled Plan
Compiled Plan
-----------Proc
Proc
Proc
Proc
Proc
Prepared
Prepared
Query Plans
• Cache
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Top 10 ….
• Physical Reads
select top 10 substring(st.text, (qs.statement_start_offset/2) + 1, ((case statement_end_offset when -1 then
datalength(st.text) else qs.statement_end_offset end - qs.statement_start_offset)/2) + 1) as statement_text, *
from sys.dm_exec_query_stats qs cross apply
sys.dm_exec_sql_text(qs.sql_handle) st
where execution_count > 1 and dbid = 5 order by total_physical_reads desc
• Max Physical Reads
– order by max_physical_reads desc
• Max Logical Reads
– Order by max_logical_reads desc
• Max Logical Writes
– Order by max_logical_writes desc
• Total Elapsed time
• Max Elapsed time
• Etc….
Query Plans
• Cache
• Things to look for :
– Determine if the adhoc or dynamic statements are generating plans that
are re-used, or in this case, not being re-used.
select Count (*), refcounts, usecounts, objtype, left(text,50) from
sys.dm_Exec_cached_plans cross apply sys.dm_exec_sql_text(plan_handle)
where cacheobjtype = 'Compiled Plan’ and objtype = 'Adhoc' and usecounts = 1
group by refcounts, usecounts, objtype, left(text,50) order by 1 desc
Count
1670
1603
1538
Refcount
2
2
2
Usecount
1
1
1
objtype
Adhoc
Adhoc
Adhoc
sql – first 50 characters….
Select DISTINCT TOP 100 W…………..
SELECT TOP 1000 W……….
select distinct top 100 v.W………
– Means that there are 4811 plans for three nearly identical statements
– Significant memory required for caching plans
– Finite amount of cache for plans
Query Plans
• Cache
• Life of…
– Execution cache is re-cycled based on age, usage and statistics they are
dependent upon
– Engine will not always choose “best” plan; rather it will find a balance
between the best plan and one quick to create and easy to maintain.
– The algorithms to match new SQL statements to existing, unused
execution plans in the cache require that all object references be fully
qualified. For example, the first of these SELECT statements is not
matched with an existing plan, and the second is matched:
» SELECT * FROM Contact
» SELECT * FROM Person.Contact
– Execution plan is marked for de-allocation from cache when the following
is true:
» The memory manager requires memory and all available memory is
currently being used.
» The age field for the object is 0.
» The object is not currently referenced by a connection.
» http://msdn.microsoft.com/en-us/library/ms181055.aspx
Query Plans
• Cache
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Recompile
• Use Profiler to determine reason: BOL has great explanations.
• Changes made to a table or view referenced by the query (ALTER TABLE
and ALTER VIEW).
Changes to any indexes used by the execution plan.
Updates on statistics used by the execution plan, generated either explicitly
from a statement, such as UPDATE STATISTICS, or generated
automatically.
• Dropping an index used by the execution plan.
• An explicit call to sp_recompile.
• Large numbers of changes to keys (generated by INSERT or DELETE
statements from other users that modify a table referenced by the query).
• For tables with triggers, if the number of rows in the inserted or deleted
tables grows significantly.
• Executing a stored procedure using the WITH RECOMPILE option.
Query Plans
• Cache
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Parameterization
• Simple (Default)
– SQL 2k – known as Auto-Parameterization
– Enables the engine to better match queries and their plans
– SQL Server parameterizes a relatively small set of queries
» Example :
– SELECT * FROM AdventureWorks2008.Production.Product WHERE ProductSubcategoryID=1;
» This query will be parameterized by the engine at compile time.
– SELECT * FROM AdventureWorks.Production.Product WHERE ProductSubcategoryID = 4;
» This query will use the same plan.
– More complex statements may not be parameterized.
» Statements which reference more than one table.
» Delete or update with a from clause
» Many more …
• Forced
Query Plans
• Cache
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Parameterization
• Forced Parameterization
– Forces all statements to be parameterized.
– Any literal value will be parameterized with many exceptions.
http://msdn.microsoft.com/en-us/library/ms175037.aspx
» Xquery, Cursors, ANSI_PADDING or ANSI_NULLS set to OFF.
» The TOP, TABLESAMPLE, HAVING, GROUP BY, ORDER BY, OUTPUT...INTO, or
FOR XML clauses of a query; Statements that reference variables, ETC….
– Alter Database <name> SET parameterization FORCED / SIMPLE
– Decreases compilations – increases concurrency
– Quite a few “does this”, “does not do this” set’s of rules.
» Ensure that you test it on your unique situation and reference the
link above.
Plan Guides
• Creation
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Context
• OBJECT Plan Guide
– Stored proc’s, UDF’s, Triggers (dml), CTE’s, etc…
• SQL Plan Guide
– t-sql statements (dynamic, ad-hoc, prepared etc…)
• Template Plan Guide
– Standalone queries that parameterize to a specified form.
» These can OVERRIDE the parameterization setting.
» The PARAMETERIZATION database option is SET to FORCED,
but there are queries you want compiled according to the rules of
simple parameterization.
» The PARAMETERIZATION database option is SET to SIMPLE
(the default setting), but you want forced parameterization to be
attempted on a class of queries.
(http://msdn.microsoft.com/en-us/library/ms190417.aspx)
Plan Guides
• Demo
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Create an object plan guide
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Disable the Plan Guide
Enable the Plan Guide
Drop the Plan Guide
View the Plan Guides
Create a SQL plan guide
Create a Template plan guide
Create a SQL plan guide using XML_SHOWPLAN
Create a Plan guide against an existing query plan.
Plan Guides
• Profiler
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Capture event
• SQL:BatchStarting
– Extract event data to a file ** Open in Notepad
– Must be exact spacing
» Exception is to escape ‘ ‘s
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Demo …
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Validation
• Profiler
– Performance category >> Plan_Guide_Successful & Plan_Guide_Unsuccessful
• Perfmon
– Guided Plan Executions/sec & MisGuided Plan Executions/sec
Questions
Your Feedback is Important
Please fill out a session evaluation form and
either put them in the basket near the exit
or drop them off at the conference
registration desk.
Thank you!
Contact: [email protected]
Updated Slides – Page 1 of the schedule guide