Optimize stored Procedures

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DBA 322Optimizing Stored Procedure Performance:

DBA 322 Optimizing Stored Procedure Performance Kimberly L. Tripp Solid Quality Learning – SolidQualityLearning.com Email: Kimberly@SolidQualityLearning.com SYSolutions, Inc. – SQLSkills.com Email: Kimberly@SQLSkills.com

Introduction:

Introduction Kimberly L. Tripp, SQL Server MVP Principal Mentor, Solid Quality Learning * In-depth, high quality training around the world! www.SolidQualityLearning.com Content Manager for www.SQLSkills.com Writer/Editor for TSQL Solutions/SQL Mag www.tsqlsolutions.com and www.sqlmag.com Consultant/Trainer/Speaker Coauthor for MSPress title: SQL Server 2000 High Availability Presenter/Technical Manager for SQL Server 2000 High Availability Overview DVD Very approachable. Please ask me questions!

Overview:

Overview Initial Processing - Review Resolution Compilation/Optimization Execution/Recompilation Recompilation Issues When do you want to Recompile? Options for Recompilation? What to Recompile? Stored Procedure Best Practices Naming Conventions Writing Solid Code Excessive Recompilations – How? Detecting?

Processing of Stored Procedures:

Processing of Stored Procedures

Resolution:

Resolution When a stored procedure is created all objects referenced are resolved (checked to see whether or not they exist). The create will succeed even if the objects do not exist Procedures called that do not exist generate error Cannot add rows to sysdepends for the current stored procedure because it depends on the missing object 'missingobjectname'. The stored procedure will still be created. Benefit: Recursion is allowed! Tables, Views, Functions called that do not exist - do NOT generate error (unless in 6.5 compatibility mode) Verify dependencies with sp_depends before dropping an object

Compilation/Optimization:

Compilation/Optimization Based on parameters supplied Future executions will reuse the plan Complete optimization of all code passed (more on this coming up…modular code!) Poor coding practices can cause excessive locking/blocking Excessive recompilations can cause poor performance

Execution/Recompilation:

Execution/Recompilation Upon Execution if a plan is not already in cache then a new plan is compiled and placed into cache What can cause a plan to become invalidated and/or fall out of cache: Server restart Plan is aged out due to low use DBCC FREEPROCCACHE (sometime desired to force it) Base Data within the tables - changes: Same algorithm as AutoStats, see Q195565 INF: How SQL Server 7.0 and SQL Server 2000 Autostats Work

Recompilation Issues:

Recompilation Issues RECOMPILATION = OPTIMIZATION OPTIMIZATION = RECOMPILATION When do you want to recompile? What options do you have Recompilation? How do you know you need to recompile? Do you want to recompile the entire procedure or only part of it? Can you test it?

When to recompile?:

When to recompile? When the plan for a given statement within a procedure is not consistent in execution plan – due to parameter and/or data changes Cost of recompilation might be significantly less than the execution cost of a bad plan! Why? Faster Execution with a better plan Saving plans for reuse is NOT always beneficial Some plans should NEVER be saved

Options for Recompilation:

Options for Recompilation CREATE … WITH RECOMPILE When procedure returns widely varying results When the plan is not consistent EXECUTE … WITH RECOMPILE For testing and to determine if CREATE WITH RECOMPILE is necessary sp_recompile objname Forces all plans with regard to that object to be invalidated (note: this does not force recompilation on views even though a view name is supported) Statement Recompilation Dynamic String Execution or Modularized Code

How do you know?:

How do you know? You Test! Test optimization plans consistency using EXECUTE WITH RECOMPILE Choose what needs to be recompiled Whole Procedure Portions of the procedure Test final performance using the chosen strategy Procedure Recompilation (CREATE with RECOMPILE) Statement Recompilation (Dynamic String Execution) Modularized Code (Sub procedures created with or without WITH RECOMPILE)

EXECUTE WITH RECOMPILE:

EXECUTE WITH RECOMPILE Excellent for Testing Verify plans for a variety of test cases EXEC dbo.GetMemberInfo ‘Tripp’ WITH RECOMPILE EXEC dbo.GetMemberInfo ‘T%’ WITH RECOMPILE EXEC dbo.GetMemberInfo ‘%T%’ WITH RECOMPILE Do the execution plans match? Are they consistent? Yes  then create the procedure normally No  Determine what should be recompiled

What Should be Recompiled?:

What Should be Recompiled? Whole Procedure CREATE with RECOMPILE Procedure is recompiled for each execution EXECUTE with RECOMPILE Procedure is recompiled for that execution NOTE: Consider forcing recompilation through another technique – you should not expect users will know when/why to use EXECUTE … WITH RECOMPILE once in production! Statement(s) Recompilation Limited number of statements cause excessive recompilation Dynamic String Execution Modular Code

CREATE … WITH RECOMPILE:

CREATE … WITH RECOMPILE Use when the procedure returns drastically different results based on input parameters. May not be the only – or even the best option… How do you know? CREATE PROCEDURE GetMemberInfo ( @LastName varchar(30) ) AS SELECT * FROM Member WHERE LastName LIKE @LastName go EXEC GetMemberInfo 'Tripp' -- index+bookmark EXEC GetMemberInfo 'T%' -- plan already exists (s/b a table scan) EXEC GetMemberInfo '%T%' -- definitely should use a table scan

Statement Recompilation:

Statement Recompilation What if only a small number of statements need to be recompiled? The SQL Statement is not likely safe (i.e. it will not be saved and parameterized) Dynamic String Execution! Amazingly Flexible Permission Requirements Potentially Dangerous Advanced Examples Complex large strings Changing database context Output parameters

Modular Code – The Better Solution!:

Modular Code – The Better Solution! IF (expression operator expression) SQL Statement Block1 ELSE SQL Statement Block2 Scenario 1 – upon first execution… Parameters are passed such that the ELSE condition executes – BOTH Block1 and Block2 are optimized with the input parameters Scenario 2 – upon first execution… Parameters are passed such that the IF condition executes – ONLY Block1 is optimized. Block2 will be optimized when a parameter which forces the ELSE condition is passed. See ModularProcedures.sql Solution? Do not use a lot of conditional SQL Statement Blocks Call separate stored procedures instead!

sp_recompile:

sp_recompile Can be used to periodically and directly force recompilation of a procedure (or trigger) Can be used on tables and views to indirectly force the recompilation of all procedures and triggers that reference the specified table or view Does not actually recompile the procedures  Instead it invalidates plans for next execution SQL Server invalidates plans as data changes Never really negative – especially if you run it at night as part of batch processing after index rebuilds or statistics updates with FULLSCAN

Stored Procedure Best Practices:

Stored Procedure Best Practices Naming Conventions Owner Qualify Do not use sp_ Modifying Procedures Write Solid Code Writing Better Queries/Better Search Arguments Changing Session Settings Interleaving DML/DDL Temp Table Usage Modular Code Detecting Excessive Recompilations

Naming Conventions:

Naming Conventions Owner Qualify to Eliminate Ambiguity On execution EXEC dbo.procname On creation CREATE PROC dbo.procname AS SELECT columnlist FROM dbo.tablename EXEC dbo.procname Minimize Blocking – initial cache lookup by owner will fail. It will not cause a recompile but excessive lookups can cause significant blocking and cache misses. Do not use sp_ in stored procedure names – causes cache misses on lookup as well because SQL Server looks in master first! See KB Article Q263889

Modifying Procedures:

Modifying Procedures DROP and RECREATE Loses the dependency chain stored in sysdepends Loses the permissions already granted Invalidates all plans ALTER PROC Loses the dependency chain stored in sysdepends Retains the permissions Invalidates all plans To retain the dependency chain you must also ALTER all procedures that depend on the procedure being altered.

Changing SESSION Settings:

Changing SESSION Settings Certain Session Settings can be set within a stored procedure – some can be desired: SET NOCOUNT ON SET QUOTED_IDENTIFIER OFF (not recommended except for backward compatibility and upgrades) Some Session Settings will cause EVERY execution to force a recompile: ANSI_DEFAULTS ANSI_NULLS (tip: do not use WHERE col = null, use col IS NULL) ANSI_PADDING ANSI_WARNINGS CONCAT_NULL_YIELDS_NULL (tip: use the ISNULL function to concatenate strings) Recommendation: DO NOT Change these session settings in the client or the server! See “SET Options that Affect Results” in the BOL

Interleaving DML/DDL Statements:

Interleaving DML/DDL Statements Objects that don’t exist at procedure first execution cannot be optimized until statement execution Upon execution of a DDL statement the procedure gets recompiled to recompile the plans for the DML But wait – not all of the objects are created…so later executions of DDL force recompilation AGAIN… Don’t interleave DDL and DML separate it… All DDL at the beginning of the proc, all DML later!

Data Manipulation:

Data Manipulation Derived Tables Nested Subquery in FROM clause May optimize better than temp tables/variables Views Another option – rewrite existing temp table code to use views instead (simple rewrite) May optimize better than temp tables/variables Temp Tables Should be considered Table Variables Limitations might not affect you Might be the most optimal

Temp Table Usage:

Temp Table Usage Temp Table can create excessive recompilations for procedures. Consider creating permanent tables (with indexes) and manipulating data there. Consider dropping and re-creating or rebuilding indexes as part of the procedure instead! Try not to create tables conditionally (IF create… ELSE create…) Use Profiler to see if there are significant recompiles Use KEEP PLAN on SELECT statements if data changes more than 6 times but the plan should not change

Table Variable Usage:

Table Variable Usage Scope is limited to the local procedure\transaction Does not cause excessive recompiles due to local only access No re-resolution on CREATE/ALTER Temp Tables need re-resolution for nested procedures Only Key Indexes can be created Definition of Table allows PRIMARY KEY/UNIQUE constraint indexes Use TEMP TABLES if large volumes of data will be manipulated – create the right indexes for access Population Does not support INSERT EXEC Does not support SELECT INTO

Temp Table vs. Table Variables:

Temp Table vs. Table Variables Temp Table PROs Can create useful nonclustered non-unique indexes to improve join performance Can access from other nested procedures Can populate with INSERT EXEC or SELECT INTO CONs Potential for excessive recompiles due to resolution Table Variable Table PROs Local only – no excessive recompiles CONs Cannot create additional nonclustered indexes Not flexible on population

Detecting SP Recompilation :

Detecting SP Recompilation Event = SP:Recompile & Column = EventSubClass

Profiling SP Performance:

Profiling SP Performance Create New Trace (SQLProfilerTSQL_sps) Replace SP:StmtStarting w/SP:StmtCompletion Better if you want to see a duration (starting events don’t have a duration) Add Duration as a Column Value If short term profiling for performance: Add columns: Reads, Writes, Execution Plan Always use Filters Database Name (only the db you want) Exclude system IDs (checkbox on filter dialog)

Review:

Review Initial Processing - Review Resolution Compilation/Optimization Execution/Recompilation Recompilation Issues When do you want to Recompile? Options for Recompilation? What to Recompile? Stored Procedure Best Practices Naming Conventions Writing Solid Code Excessive Recompilations – How? Detecting?

Slide30:

DAT 335 – SQL Server Tips and Tricks for DBAs and Developers Tuesday, 1 July 2003, 15:15-16:30 DBA 324 – Designing for Performance: Structures, Partitioning, Views and Constraints Wednesday, 2 July 2003, 08:30-09:45 DBA 328 – Designing for Performance: Optimization with Indexes Wednesday, 2 July 2003, 16:45-18:00 DBA 322 – Optimizing Stored Procedure Performance in SQL Server 2000 Thursday, 3 July 2003, 08:30-09:45 Other Sessions…

Slide31:

Articles in TSQLSolutions at www.tsqlsolutions.com (FREE, just register) All About Raiserror, InstantDoc ID#22980 Saving Production Data from Production DBAs, InstantDoc ID#22073 Articles in SQL Server Magazine, Sept 2002: Before Disaster Strikes, InstantDoc ID#25915 Log Backups Paused for Good Reason, InstantDoc ID#26032 Restoring After Isolated Disk Failure, InstantDoc #26067 Filegroup Usage for VLDBs, InstantDoc ID#26031 Search www.sqlmag.com and www.tsqlsolutions.com for additional articles Articles…

Slide32:

Whitepaper: Query Recompilation in SQL Server 2000 http://msdn.microsoft.com/library/default.asp?url=/nhp/Default.asp?contentid=28000409 Resources…

Community Resources:

Community Resources Community Resources http://www.microsoft.com/communities/default.mspx Most Valuable Professional (MVP) http://www.mvp.support.microsoft.com/ Newsgroups Converse online with Microsoft Newsgroups, including Worldwide http://www.microsoft.com/communities/newsgroups/default.mspx User Groups Meet and learn with your peers http://www.microsoft.com/communities/usergroups/default.mspx

Ask The ExpertsGet Your Questions Answered:

Ask The Experts Get Your Questions Answered I will be available in the ATE area after most of my sessions!

Slide35:

Kimberly L. Tripp Principal Mentor, Solid Quality Learning Website: www.SolidQualityLearning.com Email: Kimberly@SolidQualityLearning.com President, SYSolutions, Inc. Website: www.SQLSkills.com Email: Kimberly@SQLSkills.com Thank You!

Slide37:

evaluations