1. SQLintersection Locks, Blocks, and Deadlocks Oh My! Randy Knight randy@sqlsolutionsgroup.com Wednesday, 2:15-3:15

2. © SQLintersection. All rights reserved. http://www.SQLintersection.com Randy Knight  Microsoft Certified Master in SQL Server  20+Years of database experience, focusing on SQL Server since 1997.  Worked in a variety of settings, including six years as a Database Architect for match.com  Founder, SQL Solutions Group

3. © SQLintersection. All rights reserved. http://www.SQLintersection.com Overview  Review  ACID Properties  Transaction Isolation Levels  Defined  Locking  Blocking  Deadlocks  Detection and Prevention  Excessive Blocking  Deadlocks

4. © SQLintersection. All rights reserved. http://www.SQLintersection.com ACID  Atomic  Consistent  Isolated  Durable

5. © SQLintersection. All rights reserved. http://www.SQLintersection.com Isolation Levels  ANSI Standard Levels  Level 0: Read Uncommitted  Level 1: Read Committed  Level 2: Repeatable Read  Level 3: Serializable  Snapshot / Versioning

6. © SQLintersection. All rights reserved. http://www.SQLintersection.com Locking Defined  This is how SQL Server handles Isolation  Read Locks  Shared  Concurrent  Write Locks  Exclusive  Isolated  Lock Levels  Table, Page, Row

7. © SQLintersection. All rights reserved. http://www.SQLintersection.com Blocking Defined  Definition  User A has a lock on Resource X  User B needs a lock on Resource X  Locks are incompatible  This is a Block  This is normal behavior!  What we care about is EXCESSIVE blocking  Blocking Chains  User A block B, B blocks C, and on and on

8. Demo Locks and Blocks

9. © SQLintersection. All rights reserved. http://www.SQLintersection.com Preventing Excessive Blocking  Short transactions  Appropriate Indexes  SNAPSHOT isolation level

10. © SQLintersection. All rights reserved. http://www.SQLintersection.com Deadlocks Defined  User A and User B both have locks that prevent the other’s transaction from completing  Will never clear itself  SQL server handles automatically  Never normal

11. © SQLintersection. All rights reserved. http://www.SQLintersection.com Deadlock Detection  Lock Monitor Thread  Every 5 seconds (normally)  Looks at blocks  Can the block clear itself  Cyclic blocks would last forever  Chooses a victim if needed  How is the victim chosen  DEADLOCK_PRIORITY  If they are the same priority, the least expensive transaction to rollback

12. © SQLintersection. All rights reserved. http://www.SQLintersection.com Viewing Deadlock Information  SQL Server Error Log  Trace Flags  1204  1222  SQL Server Profiler  Extended Events  Already on in the default session  XML is different so doesn’t show up as a deadlock graph

13. Demo Deadlocks

14. © SQLintersection. All rights reserved. http://www.SQLintersection.com Common Deadlock Causes  Table Order  Key Lookup  Serializable Isolation Level  Parallelism

15. © SQLintersection. All rights reserved. http://www.SQLintersection.com Recommendations  Minimize blocking  Short transactions  SNAPSHOT when appropriate  Locking is normal  Blocking is normal  Excessive Blocking is not  Deadlocks are not normal behavior

16. © SQLintersection. All rights reserved. http://www.SQLintersection.com Review  Review  ACID Properties  Transaction Isolation Levels  Defined  Locking  Blocking  Deadlocks  Detection and Prevention  Excessive Blocking  Deadlocks

17. © SQLintersection. All rights reserved. http://www.SQLintersection.com References  Forums  MSDN  SQLServerCentral  pluralsight.com  SQL Server: Deadlock Analysis and Prevention  Twitter  #sqlhelp, #sqlblog  @randy_knight  @SQLGroup

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