How to Fix SQL Server Deadlocks: Causes, Detection and Real Fixes
Originally published at SQL & Cloud Lab . A deadlock happens when two sessions each hold a lock the other one needs. Neither can continue, so SQL Server picks one as the victim , rolls its transaction back and raises error 1205 . The other session carries on. Occasional deadlocks are normal in busy systems; frequent ones point to a fixable pattern in your code or indexes. What a deadlock looks…
A deadlock occurs when two sessions each hold a lock that the other needs. Neither session can continue, so SQL Server ends one transaction and raises error 1205. Occasional deadlocks are normal but frequent ones indicate a problem with code or indexes.
An example is two transactions updating the same two rows in opposite orders, creating a cycle. SQL Server records every deadlock in the system_health Extended Events session, which can be viewed as an XML graph. The graph shows the queries involved, the locks, the affected session, and the resources.
To fix deadlocks, apply the right fix. Access objects in the same order, keep transactions short, and add the right indexes. For readers, use read committed snapshot isolation. For read-then-update, use UPDLOCK to prevent hidden deadlocks. Retry error 1205 in the application with a small retry and randomised delay.
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