1. Read Dirty to Me: SQL Server Isolation Levels Wendy Pastrick Arrow IT Consulting

2. About me SQL Server DBA since 2000 PASS Women In Technology PASS Board of Directors SQL Server MVP @wendy_dance Wendyverse.blogspot.com

3. Agenda Transaction Isolation Dirty Reads and Other Phenomena Isolation Levels Management Views Lock Manager Transaction Control The Bottom Line

4. Transaction Isolation The level at which a transaction is prepared to accept inconsistent data Concurrency Consistency ConcurrencyConsistency

5. Who Cares? Just “Set It” and “Forget It” Locking & Blocking

6. Dirty Reads and… Dirty Read ◦ Cheating on a Test

7. …Other Phenomena Non-Repeatable Read ◦ It was there a second ago!

8. …Other Phenomena Phantom Read ◦ Now, where did THAT come from?

9. Isolation Levels Transaction Based Defines the degree to which one transaction must be isolated from resource or data modifications made by other transactions Area of Impact ◦ Transaction (Query) ◦ Database

10. Half Full or Half Empty?

11. Isolation Levels Read UnCommitted ◦ The Dirty Read ◦ Okay sometimes  Data that does not change often Concurrency Consistency

12. Isolation Levels Read Committed ◦ Pessimistic ◦ Default SQL Server Isolation Level ◦ Rows only locked when being accessed ◦ Demo Time! Concurrency Consistency

13. Isolation Levels Repeatable Read ◦ Pessimistic ◦ Rows locked as they are read ◦ Rows remain locked until transaction completes ◦ Phantom Rows possible ◦ Demo Time! Concurrency Consistency

14. Isolation Levels Serializable ◦ Key Range Locks ◦ Pessimistic ◦ All data locked until Transaction completes  Locking & Blocking ◦ Only one transaction at a time ◦ Marbles in a bag Concurrency Consistency

15. Serializable Tran #1: Change all White to RED Tran #2: Change all RED to White Who Wins?

16. Isolation Levels Snapshot Isolation (2005+) ◦ Row Versioning ◦ Uses Tempdb ◦ Optimistic ◦ Possible to have two transactions update the EXACT SAME DATA  But wait, there’s Conflict Detection! ◦ Marbles in a bag again Concurrency Consistency

17. Snapshot Who Wins? Tran #1: Change all RED to White Tran #2: Change all White to RED

18. Snapshot Merge Results: Tran #2: Change all White to RED Tran #1: Change all RED to White

19. Isolation Levels Read Committed Snapshot Isolation (2005+) ◦ Row Versioning ◦ Optimistic Concurrency Consistency

20. Management Views sys.dm_trans_locks ◦ View locks set by isolation sys.dm_tran_version_store ◦ Shows versions in the ‘store’ sys.dm_os_waiting_tasks ◦ Join with sys.dm_trans_locks to see blocking

21. Lock Manager ResourceDescription RIDA row identifier used to lock a single row within a heap. KEYA row lock within an index used to protect key ranges in serializable transactions. PAGEAn 8-kilobyte (KB) page in a database, such as data or index pages. EXTENTA contiguous group of eight pages, such as data or index pages. HoBT A heap or B-tree. A lock protecting a B-tree (index) or the heap data pages in a table that does not have a clustered index. TABLEThe entire table, including all data and indexes. FILEA database file. APPLICATIONAn application-specified resource. METADATAMetadata locks. ALLOCATION_UNITAn allocation unit. DATABASEThe entire database.

22. Transaction Control SET TRANSACTION ISOLATION LEVEL Table Hints override ◦ With NO LOCK – not as good as it sounds

23. The Bottom Line Transaction Processing ◦ CLOSE Transactions Choose Wisely ◦ Concurrency vs. Consistency Hidden Impact ◦ Query Plans ◦ Indexing Choices

24. Many MANY Thanks MSDN Kendra Little Craig Freedman Michael J Swart Grant Fritchey Itzik Ben-Gan http://wendyverse.blogspot.com/p/presentat ions-links.html

25. Cool Links (NOLOCK) for YESFUN by Kendra Little Itzik Ben-Gan Michael J Swart …and MORE! Wendyverse.blogspot.com

26. Contact me wendy.pastrick@gmail.com @wendy_dance on Twitter wendyverse.blogspot.com