Deadlocks Everything you ever wanted to ask but were too shy Denis Reznik Deadlocks Everything you ever wanted to ask but were too shy

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About Me Denis Reznik Kyiv, Ukraine Data Architect at Intapp, Inc. Microsoft Data Platform MVP Co-Founder of Ukrainian Data Community Kyiv PASS Regional Mentor, CEE Co-author of “SQL Server MVP Deep Dives 2”

Agenda Classic Deadlock Common Deadlock When Deadlock Graph doesn’t Make Sense Object Deadlock Range Deadlock Implicit Deadlock NOLOCK Deadlock Hidden Deadlock One Query Deadlock Undetectable Deadlock

Agenda – More Background Locks (S) Shared (X) Exclusive (U) Update (Range*-*) Range (I*) Intent (Sch-S) Schema Stability (Sch-M) Schema Modification (BU) Bulk Update

Agenda – More Background Transaction Isolation Levels Pessimistic READ UNCOMMITTED READ COMMITTED REPEATABLE READ SERIALIZABLE Optimistic READ COMMITTED SNAPSHOT SNAPSHOT

Classic Deadlock DEADLOCK! wait wait ID City 1 Kyiv 2 Sofia 3 BEGIN TRAN UPDATE Users SET CityId = 2 WHERE Id = 4 UPDATE City SET Name = 'Dnipro' WHERE Id = 3 BEGIN TRAN UPDATE City SET Name = 'Dnipro' WHERE Id = 3 UPDATE Users SET CityId = 2 WHERE Id = 4 ID City 1 Kyiv 2 Sofia 3 Dnipropetrovsk X wait wait ID User City_Id 1 Dejan Sarka 2 Mihail Mateev 3 Marcin Sheliga 4 John Smith X

Demo Common Deadlock

Lock Types - Shared X S S

Lock Types - Exclusive S X X

Demo Common Deadlock

Demo When Deadlock Graph does not make sense

Lock Types - Update s U U X S U X X U

Return different results so they are non-repeatable NON-REPEATABLE READS BEGIN TRAN SELECT * FROM Users WHERE City = 'Sofia' ID City 1 Kyiv 2 Sofia 3 4 5 Tokyo 6 New York 7 ID City 1 Kyiv 2 3 Sofia 4 5 Tokyo 6 New York 7 UPDATE Users SET City = 'Kyiv' WHERE Id = 2 X S Return different results so they are non-repeatable S ... S SELECT * FROM Users WHERE City = 'Sofia'

REPEATABLE READ ID City 1 Kyiv 2 Sofia 3 4 5 Tokyo 6 New York 7 ID BEGIN TRAN SELECT * FROM Users WHERE City = 'Sofia' ID City 1 Kyiv 2 Sofia 3 4 5 Tokyo 6 New York 7 ID City 1 Kyiv 2 3 Sofia 4 5 Tokyo 6 New York 7 UPDATE Users SET City = 'Kyiv' WHERE Id = 2 X S S ... S SELECT * FROM Users WHERE City = 'Sofia' COMMIT

Demo When Deadlock Graph does not make sense

Demo Object Deadlock

Lock Escalation … >= 25% 1 2 Table S S Buffer Pool n = 5000 Memory for locks 1 S >= 25% 2 S … 3 S n S n = 5000 if lock can’t be put on table, process will try this every n + 1250 rows

Lock Types – Intent locks IS IS S

Demo Object Deadlock

Demo Range Deadlock

PHANTOM RECORDS Phantom Record ID City 1 Kyiv 2 Sofia 3 4 8 5 Tokyo 6 BEGIN TRAN SELECT * FROM Users WHERE City = 'Sofia' ID City 1 Kyiv 2 Sofia 3 4 8 5 Tokyo 6 New York 7 ID City 1 Kyiv 2 Sofia 3 4 5 Tokyo 6 New York 7 INSERT INTO Users VALUES (8, 'Sofia') S S ... S S SELECT * FROM Users WHERE City = 'Sofia' Phantom Record

SERIALIZABLE ID City 1 Kyiv 2 Sofia 3 4 8 5 Tokyo 6 New York 7 ID City BEGIN TRAN SELECT * FROM Users WHERE City = 'Sofia' ID City 1 Kyiv 2 Sofia 3 4 8 5 Tokyo 6 New York 7 ID City 1 Kyiv 2 Sofia 3 4 5 Tokyo 6 New York 7 INSERT INTO Users VALUES (8, 'Sofia') RANGE S-S ... SELECT * FROM Users WHERE City = 'Sofia' COMMIT

Demo Range Deadlock

Demo Implicit Deadlock

READ COMMITTED SNAPSHOT SELECT * FROM Users WHERE City = 'Kyiv' BEGIN TRAN UPDATE Users SET City = 'Sofia' WHERE City = 'Kyiv' ID City 1 Sofia 2 3 4 5 6 New York 7 ID City 1 Kyiv 2 Sofia 3 4 5 6 New York 7 X SELECT * FROM Users WHERE City = 'Kyiv' Version Store ID City 1 Kyiv tempdb

Demo Implicit Deadlock

Demo NOLOCK Deadlock

READ UNCOMMITTED (NOLOCK) SELECT * FROM Users WHERE City = 'Kyiv' ID City 1 Kyiv 2 Sofia 3 4 5 6 New York 7 ID City 1 Kyiv 2 Sofia 3 4 5 6 7 BEGIN TRAN UPDATE Users SET City = 'Kyiv' WHERE City = 'New York' SELECT * FROM Users WHERE City = 'Kyiv' Dirty Read ROLLBACK X X SELECT * FROM Users WHERE City = 'Kyiv'

Demo NOLOCK Deadlock

Demo Hidden Deadlock

Demo One Query Deadlock

Demo Undetectable Deadlock

Three Rules of Deadlocks If deadlock is possible, it will occur. Deadlock should not be solved, before the root cause of it wasn’t found. There are no unsolvable deadlocks. But there can be solutions, which will not suit you completely

How to Avoid? Design database so, that it will be no possibility for a deadlock occur Ha-ha-ha  Modify tables in the same order Choose appropriate Transaction Isolation Level Consider optimistic ones Keep transactions as small as possible Use stress-testing for your application or database

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Thank You! Denis Reznik Twitter: @denisreznik Email: denisreznik@gmail.com Blog: http://reznik.uneta.com.ua Facebook: https://www.facebook.com/denis.reznik.5 LinkedIn: http://ua.linkedin.com/pub/denis-reznik/3/502/234