Session Code: DAT454
Avoiding Stored Procedure
Recompiles
Dr Greg Low
Managing Director
Solid Q Australia
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Who am I?
Managing Director Solid Q Australia
Host of SQL Down Under Podcast
Microsoft Regional Director
Microsoft MVP for SQL Server
Organizer for SDU Code Camp
Co-organizer for CodeCampOz
PASS Board Member
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Acknowledgements/Sources
Material for this session has been distilled
from:
Batch Compilation, Recompilation and Plan Caching Issues in
SQL Server 2005 (Arun Marathe – MSDN Library)
Query Recompilation in SQL Server 2000 (Thomas Davidson
– MSDN Library)
Statistical maintenance functionality (autostats) in SQL
Server (KB 195565)
Troubleshooting stored procedure recompilation (KB
243586)
How to identify the cause of recompilation in an
SP:Recompile event (KB 308737)
Tibor Karaszi’s article on SET options and Recompilation
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What we will cover
Query compilation and plan caching in SQL
Server
Using tools to view caching and recompiles
Recompilation triggers in SQL Server 2000 and
2005/8
Best practices
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SQL Server Code Execution
SQL Server selects an execution plan for a
stored procedure (and other query batches)
and then compiles it, prior to execution.
Concept of a batch
After compilation, plans are stored in the
procedure cache (Arun calls it a plan cache)
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SQL Server Code Execution
Cached plan reuse is highly desirable
Recompilation vs compilation
SQL Server may choose not to use a stored plan
- (validity & optimality)
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SQL Server 2005/2008
SQL Server 2000 always recompiled entire
batches
SQL Server 2005/8 may use statement-level
recompilation
Faster
Less memory
Fewer compile locks
Avoids the need to break up large stored
procedures
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Pop – Quiz
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Cached vs Non-Cached
Ad-hoc queries (exact match required – case
and space sensitivity)
Auto-parameterized queries (unless the value
of a constant literal can influence the plan – if
so, treated as ad-hoc)
sp_executesql
Stored Procedures
Batches (exact match)
EXEC () (checks resultant string)
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Showplan – Auto-parameterization
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Query Plans
Query plans are re-entrant
At most, one sequential plan and one parallel
plan (no matter how many processors)
In SQL Server 2000, serialized action, in 2005
and 2008, multiples may temporarily exist but
only one cached
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Execution Contexts
Execution contexts are derived from Query
Plans
Not re-entrant – one per user executing – data
and parameters – multiples per Query Plan
possible
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Execution Contexts (Continued)
Not all execution contexts for a single Query
Plan are identical (eg branching)
Errors of severity 11 or above cause the
Execution Context to be destroyed
Execution contexts from parallel plans are not
cached (but parallel query plans are)
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Caching In SQL Server 2000
A cost is held for each query plan and execution
context
SQL Server 2000 – measure of server resources to
optimize the batch (note not from executions). Based
on:
CPU time generating the plan
Read page count
Write page count
Memory pages occupied
Lazy writer divides costs by 4 each pass
0 entries (+ ad-hoc queries) are deletion targets
Cost gets reset to original value on reuse
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Caching In SQL Server 2005/2008
Ad-hoc still zero
Cost is calculated in ticks (max 31)
Two I/O’s cost 1 tick (max 19 ticks)
Two context switches cost 1 tick (max 8 ticks)
Sixteen memory pages cost 1 tick (max 4 ticks)
When procedure cache 50% of buffer pool size, next
cache access decrements all ticks by one.
If over 75%, a separate resource monitor takes over
decrementing.
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New Option In SQL Server 2008
sp_configure option to:
optimize for adhoc workloads
Cache only parameterized stubs for adhoc
queries
Should help for applications with large numbers
of non-parameterized batches
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What we will cover
Query compilation and plan caching in SQL
Server
Using tools to view caching and recompiles
Recompilation triggers in SQL Server 2000 and
2005/2008
Best practices
18
sys.syscacheobjects / Pop Quiz Revisited
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SQL Profiler
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What we will cover
Query compilation and plan caching in SQL
Server
Using tools to view caching and recompiles
Recompilation triggers in SQL Server 2000 and
2005/2008
Best practices
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Recompilation Reasons
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Recompilation Triggers
Correctness
Optimality
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SET Options (Correctness)
Various SET options trigger recompilations due
to constant folding
Tibor mentions ANSI_DEFAULTS, ANSI_NULLS,
ANSI_PADDING, ANSI_WARNINGS, ARITHABORT,
CONCAT_NULL_YIELDS_NULL, DATEFORMAT,
FORCEPLAN, LANGUAGE, NUMERIC_ROUNDABOUT
Arun also adds DATEFIRST, NO_BROWSETABLE,
QUOTED_IDENTIFIER
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Name Resolution Issues
(Correctness)
Objects with single-part names cannot be
resolved until execution time.
SQL Server 2000 (owners)
Products -> Mary.Products vs Tom.Products
SQL Server 2005 and 2008 (schemas)
Products -> Production.Products vs
Advertising.Products
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Name Resolution Issues
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Database Context (Correctness)
EXEC dbo.Proc1
EXEC anotherdb.dbo.Proc1
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Miscellaneous
Batch inside vs outside CLR
CREATE PROCEDURE … WITH RECOMPILE
EXEC … WITH RECOMPILE (but preferred to the
CREATE PROC version)
Mixing DDL and DML
sp_recompile
DBCC FREEPROCCACHE/DBCC FLUSHPROCINDB
RECONFIGURE
Autoclose
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Mixing DDL and DML
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Schema Changes
Add/drop columns
Add/drop indexes (where used in the plan)
Manual statistics changes
ATTACH/DETACH
ONLINE/OFFLINE
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No Recompile
Trivial plans
KEEPFIXED_PLAN
All read-only tables
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Optimality
Each table has a Recompilation Threshold (RT)
Based on number of rows in table
rowmodctr (2000), colmodctr (2005/2008)
Permanent tables ( <500 = 500, >500 = 500 +
0.2 x n)
Temporary tables ( < 6 = 6, else same)
Table variables (no RT)
rowmodctr, colmodctr not transactional
rowmodctr value present but not useful in
2005/2008, use colmodctr
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Optimality (Continued)
INSERT/DELETE count as one
UPDATE counts as 2 in 2000
UPDATE counts as 1 in 2005/2008 (non-key) or
2 (key)
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Working with Statistics
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Parameter Sniffing
Problem with atypical parameters
SQL Server 2005/2008 hint for default
parameter values
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Parameter Sniffing and 2005/2008
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What we will cover
Query compilation and plan caching in SQL
Server
Using tools to view caching and recompiles
Recompilation triggers in SQL Server 2000 and
2005/2008
Best practices
37
Best Practices
In SQL Server 2000, it may be advantageous to
break up large procedures.
Avoid SET options from the list of those that
cause recompilations. In general, try to work
with connection default settings.
Keep server, database & connection settings as
stable as possible.
Avoid name resolution issues by two-part
object names.
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Best Practices (Continued)
Avoid WITH RECOMPILE option on CREATE
PROCEDURE and EXEC
Consider using new SQL Server 2005 hints to
minimize the need for these
Or execute the query using sp_executesql
Consider table variables rather than temporary
tables where recompiles are an issue but
consider the impact of the lack of statistics
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Best Practices (Continued)
Consider KEEPFIXED_PLAN
Avoid manually controlling statistics updates.
Avoid mixing DDL and DML
Consider cost of recompilations, not just the
number (2005/2008 may appear to have more)
Avoid literals longer than 8K (never cached).
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Additional Resources
Batch Compilation, Recompilation and Plan
Caching Issues in SQL Server 2005 (Arun
Marathe – MSDN Library)
http://www.microsoft.com/technet/prodtechnol/sql/2005/recomp.mspx
Query Recompilation in SQL Server 2000
(Thomas Davidson – MSDN Library)
http://msdn.microsoft.com/library/default.asp?url=/library/enus/dnsql2k/html/sql_queryrecompilation.asp
Statistical maintenance functionality (autostats)
in SQL Server (KB 195565)
http://support.microsoft.com/default.aspx?scid=kb;EN-US;195565
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Additional Resources (Continued)
Troubleshooting stored procedure recompilation (KB
243586)
http://support.microsoft.com/default.aspx?scid=kb;en-us;243586
How to identify the cause of recompilation in an
SP:Recompile event (KB 308737)
http://support.microsoft.com/?id=308737
Tibor Karaszi’s article on SET options and
Recompilation
http://www.karaszi.com/SQLServer/info_sp_recompile_set.asp
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Thank You!
[email protected]
http://sqlblog.com/blogs/greg_low
http://www.sqldownunder.com
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