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Kimberly L. Tripp President, SYS olutions, Inc. – SQLSkills Email: Kimberly@SQLSkills

Designing for Performance General Database Optimizations Presentation Highlight: The Anatomy of a Data Modification Wednesday, 12 November 2003 – 16:30-18:00 S207. Kimberly L. Tripp President, SYS olutions, Inc. – SQLSkills.com Email: Kimberly@SQLSkills.com

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Kimberly L. Tripp President, SYS olutions, Inc. – SQLSkills Email: Kimberly@SQLSkills

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  1. Designing for PerformanceGeneral Database OptimizationsPresentation Highlight: The Anatomy of a Data ModificationWednesday, 12 November 2003 – 16:30-18:00S207 Kimberly L. Tripp President,SYSolutions, Inc. – SQLSkills.comEmail:Kimberly@SQLSkills.com Principal Mentor, Solid Quality Learning – SolidQualityLearning.comEmail:Kimberly@SolidQualityLearning.com

  2. Speaker – Kimberly L. Tripp • Independent Consultant/Trainer/Speaker/Writer • President, SYSolutions, Inc.www.SQLSkills.com • Principal Mentor, Solid Quality Learning * In-depth, high quality training around the world! www.SolidQualityLearning.com • SQL Server MVP (http://mvp.support.microsoft.com/) • Microsoft Regional Director (http://www.microsoftregionaldirectors.com/Public/) • Writer/Editor for TSQL Solutions/SQL Magazine www.tsqlsolutions.com and www.sqlmag.com • Coauthor for MSPress title: SQL Server 2000 High Availability • Presenter/Technical Manager for SQL Server 2000 High Availability Overview DVD (MS Part# 098-96661)

  3. The Anatomy of a Data Modification • User sends UPDATE • Update is highly selective (only 5 rows) • Indexes exist to aid in finding these rows efficiently • The update is a SINGLE statement batch NOT enclosed in BEGIN TRAN…COMMIT TRAN block therefore this is IMPLICIT transaction • Server receives the request and locates the data in cache OR reads the data from disk into cache • Since this is highly selective only the necessary pages are read into cache (maybe a few extra but that’s not important here) • Let’s use an example where the 5 rows being modified are located on 3 different data pages

  4. What it looks like - Data Data Log UPDATE… Server… Cache

  5. The Anatomy of a Data Modification • SQL Server proceeds to lock the necessary data • Locks are necessary to give us a consistent point FOR ALL rows from which to start • If any other transaction(s) have ANY of these rows locked we will wait until ALL locks have been acquired before we can proceed. • In the case of this update (because it’s highly selective and because indexes exist to make this possible) SQL Server will use row level locking. • The rows are locked but there are also “intent” locks at higher levels to make sure other larger locks (like page or table level locks) are not attempted (and fail) • There are a few locks that have already occurred – within indexes, etc. to read the data – but they are not significant here This sounds complex but it’s not too bad…

  6. What it looks like - Locks Update Lock Row Update Lock Row Page Update Lock Row Page Update Lock Cache Row Update Lock Row Page

  7. The Anatomy of a Data Modification • SQL Server can now begin to make the modifications – for EVERY row the process will include: • Change the lock to a stricter lock (eXclusive lock) • An update lock helps to allow better concurrency by being compatible with other shared locks (readers). Readers can read the pre-modified data as it is transactionally consistent • The eXclusive lock is required to make the change because once modified no other reads should be able to see this un-committed change • Make the modification to the data row (yes, in cache) • Log the modification to the transaction log pages (also in cache)

  8. What it looks like - Modifications x Exclusive Lock Update Lock x Row x x Exclusive Lock Update Lock Row Page L x Exclusive Lock Update Lock x Row Page x Exclusive Lock Update Lock x Cache x Row x Exclusive Lock Update Lock Row Page

  9. The Anatomy of a Data Modification • Finally, the transaction is complete – this is the MOST critical step • All rows have been modified • There are no other statements in this transaction – i.e. Implicit transaction • Steps are: • Write all log pages to transaction log ON DISK • Release the locks • Send a message to the user: (5 Rows Affected)

  10. What it looks like Write-Ahead Logging Data L Log 5 Rows Affected Server… Log Cache ~~~~~~~~~~~~~~~~~~~~~~~~~~~ Sequential writes Change Change Change Change … After the log entries are made and the locks are released…

  11. So now what? • The transaction log ON DISK – is up to date • The data in CACHE – is up to date • But when does the data get written from cache to disk? CHECKPOINT It’s important to realize that the sole purpose of checkpoint is NOT just to write committed pages… Instead a checkpoint writes ALL pages which have changed since they were brought into cache – regardless of the state of the transaction which changed them!

  12. Transaction Recovery and Checkpoints Transactions… Action Requiredif restart recovery L D 1 None L/D L 2 Roll forward L/D 3 Roll back L 4 Roll forward 5 Roll back Checkpoint System Failure Time

  13. Resources • Check out www.sqlskills.com for information about upcoming events, useful downloads and excellent scripts! There are quite a few resources and/or links to use. • MSPress title: SQL Server 2000 High AvailabilityAuthors: Allan Hirt with Cathan Cook, Kimberly L. Tripp, Frank McBathISBN: 0-7356-1920-4 • Check out the main page of www.sqlskills.com for asample chapter to download!

  14. Resources • From Books Online “Home Page” select White Papers to get to msdn • For Tech Net articles use: http://www.microsoft.com/technet/prodtechnol/sql/default.asp?frame=true • See www.microsoft.com/sql for all sorts of useful links, resources and whitepapers, etc. • Support Resources listed: http://www.microsoft.com/sql/support/default.asp

  15. Resources • Whitepaper: Using Partitions in a Microsoft SQL Server 2000 Data Warehouse,http://msdn.microsoft.com/library/techart/partitionsindw.htm • Whitepaper: Index Tuning Wizard for Microsoft SQL Server 2000, http://msdn.microsoft.com/library/en-us/dnsql2k/html/itwforsql.asp?frame=true • Support WebCast: SQL Server 2000 Profiler: What's New and How to Effectively Use It http://support.microsoft.com/default.aspx?scid=%2Fservicedesks%2Fwebcasts%2Fwc111400%2Fwcblurb111400%2Easp

  16. Resources • Improving Performance with SQL Server 2000 Indexed Views http://msdn.microsoft.com/library/en-us/dnsql2k/html/indexedviews1.asp?frame=true • Microsoft SQL Server 2000 Index Defragmentation Best Practices http://www.microsoft.com/technet/prodtechnol/sql/maintain/Optimize/SS2KIDBP.asp?frame=true • Support Resources listed: http://www.microsoft.com/sql/support/default.asp

  17. Thank you! Kimberly L. Tripp President, SYSolutions, Inc.Website: www.SQLSkills.com Email: Kimberly@SQLSkills.com Principal Mentor, Solid Quality LearningWebsite: www.SolidQualityLearning.comEmail: Kimberly@SolidQualityLearning.com Please fill out your evaluation!

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